Crussell/backend/handlers/payments/giftcards.go

package payments

import (
	"context"
	"database/sql"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"log/slog"
	"math"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"

	"crussell/clock"
	"crussell/db"
	"crussell/internal/square"
	"crussell/internal/twofa"
	"crussell/internal/validators"
	"crussell/mw"

	"github.com/go-chi/chi/v5"
	"github.com/jackc/pgx/v5"
)

// --- Types ---

// defaultGiftCardExpiryMonths is the CMA-recommended rolling expiry window used
// whenever business_settings.gift_card_expiry_months is unset or invalid (< 1).
const defaultGiftCardExpiryMonths = 24

// GetGiftCardExpiryMonths returns the configured gift-card expiry window in
// months — the SINGLE source of truth for rolling expiry. It reads
// business_settings.gift_card_expiry_months and falls back to
// defaultGiftCardExpiryMonths when the settings row is missing (pgx.ErrNoRows)
// or the stored value is < 1 (a sub-1-month window would make every card
// effectively expired on creation). Exported so the scheduling package's
// CleanupExpiredGiftCards job and the payment handlers share one implementation.
func GetGiftCardExpiryMonths(ctx context.Context, q db.Querier) (int, error) {
	var months int
	if err := q.QueryRow(ctx, `SELECT gift_card_expiry_months FROM business_settings LIMIT 1`).Scan(&months); err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return defaultGiftCardExpiryMonths, nil
		}
		return 0, err
	}
	if months < 1 {
		return defaultGiftCardExpiryMonths, nil
	}
	return months, nil
}

type GiftCard struct {
	ID                 string     `json:"id"`
	TotalFundsAdded    float64    `json:"total_funds_added"`
	AmountRemaining    float64    `json:"amount_remaining"`
	CreatedBy          *string    `json:"created_by,omitempty"`
	CreatedAt          time.Time  `json:"created_at"`
	RedeemedAt         *time.Time `json:"redeemed_at,omitempty"`
	RedeemedBy         *string    `json:"redeemed_by,omitempty"`
	IsInventory        bool       `json:"is_inventory"`
	LastUsedAt         *time.Time `json:"last_used_at,omitempty"`
	Cancellable        bool       `json:"cancellable"`
	CancellationReason string     `json:"cancellation_reason,omitempty"`
}

type UserBalance struct {
	UserID    string    `json:"user_id"`
	Name      string    `json:"name"`
	Email     string    `json:"email"`
	Balance   float64   `json:"balance"`
	UpdatedAt time.Time `json:"updated_at"`
}

type GiftCardListResponse struct {
	TotalUnclaimed    float64       `json:"total_unclaimed"`
	TotalUserBalances float64       `json:"total_user_balances"`
	GiftCards         []GiftCard    `json:"gift_cards"`
	UserBalances      []UserBalance `json:"user_balances"`
	Total             int           `json:"total"`
	UBTotal           int           `json:"ub_total"`
	Page              int           `json:"page"`
	PerPage           int           `json:"perPage"`
	TotalPages        int           `json:"totalPages"`
	NextCursor        *string       `json:"next_cursor,omitempty"`
}

type CreateGiftCardRequest struct {
	Amount      float64 `json:"amount"`
	IsInventory bool    `json:"is_inventory,omitempty"`
}

type TopUpGiftCardRequest struct {
	Amount        float64 `json:"amount"`
	PaymentMethod string  `json:"payment_method"`
}

type TransferGiftCardRequest struct {
	ToCardID string  `json:"to_card_id"`
	Amount   float64 `json:"amount"`
}

type BuyGiftCardRequest struct {
	Amount         int64   `json:"amount"`
	RecipientType  string  `json:"recipient_type"`
	RecipientEmail string  `json:"recipient_email,omitempty"`
	CardID         *string `json:"card_id,omitempty"`
	NewCardToken   *string `json:"new_card_token,omitempty"`
	SaveCard       bool    `json:"save_card"`
	// IdempotencyKey is optional (M1): if not provided, a deterministic key is
	// generated server-side based on user_id + amount + recipient_type + card_id.
	// This ensures retries of the same logical purchase use the same key while
	// distinct purchases get different keys. Max=45 matches Square's limit.
	IdempotencyKey    string  `json:"idempotency_key,omitempty" validate:"omitempty,max=45"`
	VerificationToken *string `json:"verification_token,omitempty"`
	// VerificationCode is the customer's current 2FA one-time code (B10): an
	// enforced environment charges/persists a saved card only when this matches
	// the customer's pending code.
	VerificationCode string `json:"verification_code,omitempty"`
}

type RedeemGiftCardRequest struct {
	Code string `json:"code"`
}

// --- Admin Handlers ---

func GetGiftCards(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	// Parse query parameters
	query := r.URL.Query()
	searchTerm := query.Get("q")

	// Pagination parameters
	perPage := 10
	cursorStr := query.Get("cursor")

	if perPageStr := query.Get("per_page"); perPageStr != "" {
		if pp, err := strconv.Atoi(perPageStr); err == nil && pp > 0 && pp <= 100 {
			perPage = pp
		}
	}

	// Accept page param for backward compat (deprecated)
	page := 1
	if pageStr := query.Get("page"); pageStr != "" {
		if p, err := strconv.Atoi(pageStr); err == nil && p > 0 {
			page = p
		}
	}

	var resp GiftCardListResponse
	resp.GiftCards = []GiftCard{}
	resp.UserBalances = []UserBalance{}
	resp.Page = page
	resp.PerPage = perPage

	// --- Aggregate totals (unfiltered, unpaginated) ---

	err := db.Conn.QueryRow(ctx, `
		SELECT COALESCE(SUM(amount_remaining), 0)
		FROM gift_cards
		WHERE redeemed_by IS NULL
	`).Scan(&resp.TotalUnclaimed)
	if err != nil {
		log.Printf("Failed to calculate total unclaimed gift cards: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	err = db.Conn.QueryRow(ctx, `
		SELECT COALESCE(SUM(balance), 0)
		FROM user_giftcard_balances
	`).Scan(&resp.TotalUserBalances)
	if err != nil {
		log.Printf("Failed to calculate total user balances: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// --- Gift cards (paginated, optionally filtered) ---

	filterType := query.Get("type")
	whereClauses := []string{}
	if searchTerm != "" {
		whereClauses = append(whereClauses, "id ILIKE $1")
	}
	if filterType == "customer" {
		whereClauses = append(whereClauses, "is_inventory = FALSE")
	} else if filterType == "inventory" {
		whereClauses = append(whereClauses, "is_inventory = TRUE")
	}

	whereSQL := ""
	if len(whereClauses) > 0 {
		whereSQL = "WHERE " + strings.Join(whereClauses, " AND ")
	}

	var gcTotal int
	var gcListArgs []any

	gcListQuery := fmt.Sprintf(`
		SELECT id, total_funds_added, amount_remaining, redeemed_by, is_inventory, expiry_date, created_at
		FROM gift_cards
		%s
	`, whereSQL)

	if searchTerm != "" {
		searchPattern := "%" + searchTerm + "%"
		gcListArgs = []any{searchPattern}

		if cursorStr != "" {
			cursorCreatedAt, cursorID, err := validators.ParseCursor(cursorStr)
			if err != nil {
				http.Error(w, "invalid cursor: "+err.Error(), http.StatusBadRequest)
				return
			}
			gcListQuery += " AND (created_at, id) < ($2, $3)"
			gcListArgs = append(gcListArgs, cursorCreatedAt, cursorID)
		}
		gcListQuery += " ORDER BY created_at DESC, id DESC LIMIT $" + strconv.Itoa(len(gcListArgs)+1)
		gcListArgs = append(gcListArgs, perPage+1)
	} else {
		if cursorStr != "" {
			cursorCreatedAt, cursorID, err := validators.ParseCursor(cursorStr)
			if err != nil {
				http.Error(w, "invalid cursor: "+err.Error(), http.StatusBadRequest)
				return
			}
			gcListQuery += " WHERE (created_at, id) < ($1, $2)"
			gcListArgs = append(gcListArgs, cursorCreatedAt, cursorID)
		}
		gcListQuery += " ORDER BY created_at DESC, id DESC LIMIT $" + strconv.Itoa(len(gcListArgs)+1)
		gcListArgs = append(gcListArgs, perPage+1)
	}

	// Count query: total matching gift cards (same WHERE, without cursor/ORDER BY/LIMIT).
	// Run BEFORE the data query to avoid "conn busy" when using a per-test
	// transaction (single connection).
	gcTotal = 0
	if whereSQL != "" {
		countArgs := []any{}
		if searchTerm != "" {
			countArgs = append(countArgs, "%"+searchTerm+"%")
		}
		if err := db.Conn.QueryRow(ctx, "SELECT COUNT(*) FROM gift_cards "+whereSQL, countArgs...).Scan(&gcTotal); err != nil {
			log.Printf("Failed to scan filtered gift card count: %v", err)
		}
	} else {
		if err := db.Conn.QueryRow(ctx, "SELECT COUNT(*) FROM gift_cards").Scan(&gcTotal); err != nil {
			log.Printf("Failed to scan gift card count: %v", err)
		}
	}

	gcRows, err := db.Conn.Query(ctx, gcListQuery, gcListArgs...)

	if err != nil {
		log.Printf("Failed to query gift cards: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer gcRows.Close()

	// Drain all rows before the per-card cancellation lookups below — pgx.Tx
	// (used by the test harness via a context-stored transaction) does not
	// support concurrent queries on one connection.
	type giftCardRow struct {
		id          string
		totalFunds  float64
		remaining   float64
		redeemedBy  sql.NullString
		isInventory bool
		expiryDate  sql.NullTime
		createdAt   time.Time
	}
	var drained []giftCardRow
	for gcRows.Next() {
		var r giftCardRow
		err = gcRows.Scan(&r.id, &r.totalFunds, &r.remaining, &r.redeemedBy, &r.isInventory, &r.expiryDate, &r.createdAt)
		if err != nil {
			log.Printf("Failed to scan gift card: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		drained = append(drained, r)
	}
	gcRows.Close()
	if err := gcRows.Err(); err != nil {
		log.Printf("Gift card row iteration error: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// Surface whether each card is currently cancellable under the 14-day
	// cooling-off right (same assessment the user-facing GetMyGiftCards uses).
	// The purchase transaction is resolved per card so the admin can act on
	// behalf of the card's real purchaser; cards without an online purchase
	// (inventory/till/admin-created) simply report as not cancellable.
	for _, r := range drained {
		var gc GiftCard
		gc.ID = r.id
		gc.TotalFundsAdded = r.totalFunds
		gc.AmountRemaining = r.remaining
		gc.CreatedAt = r.createdAt
		gc.IsInventory = r.isInventory
		if r.redeemedBy.Valid {
			redeemedBy := r.redeemedBy.String
			gc.RedeemedBy = &redeemedBy
		}

		var purchaserID string
		var purchaseAmount float64
		var purchasedAt time.Time
		err := db.Conn.QueryRow(ctx, `
			SELECT amount, created_at, COALESCE(user_id, '')
			FROM gift_card_transactions
			WHERE gift_card_id = $1 AND transaction_type = 'purchase'
			  AND reference_type = 'api'
			ORDER BY created_at DESC
			LIMIT 1`, r.id).Scan(&purchaseAmount, &purchasedAt, &purchaserID)
		if err != nil && !errors.Is(err, pgx.ErrNoRows) {
			log.Printf("Failed to load purchase transaction for gift card %s: %v", r.id, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		status := assessGiftCardCancellation(ctx, db.Conn, r.id, r.totalFunds, r.remaining, r.isInventory, r.expiryDate, purchaseAmount, purchasedAt, purchaserID)
		gc.Cancellable = status.Cancellable
		gc.CancellationReason = status.CancellationReason

		resp.GiftCards = append(resp.GiftCards, gc)
	}

	// --- User balances (all, optionally filtered) ---

	var ubTotal int
	var ubListQuery string
	var ubListArgs []any

	if searchTerm != "" {
		searchPattern := "%" + searchTerm + "%"

		ubListQuery = `
			SELECT
			       b.user_id, u.n_first_name || ' ' || u.n_last_name AS name, u.email, b.balance, b.updated_at
			FROM user_giftcard_balances b
			JOIN users u ON b.user_id = u.id
			WHERE u.n_first_name ILIKE $1
			   OR u.n_last_name ILIKE $1
			   OR u.email ILIKE $1
			ORDER BY b.updated_at DESC
		`
		ubListArgs = []any{searchPattern}
	} else {
		ubListQuery = `
			SELECT
			       b.user_id, u.n_first_name || ' ' || u.n_last_name AS name, u.email, b.balance, b.updated_at
			FROM user_giftcard_balances b
			JOIN users u ON b.user_id = u.id
			ORDER BY b.updated_at DESC
		`
		ubListArgs = []any{}
	}

	ubRows, err := db.Conn.Query(ctx, ubListQuery, ubListArgs...)
	if err != nil {
		log.Printf("Failed to query user balances: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer ubRows.Close()

	// Count query for user balances.
	if searchTerm != "" {
		if err := db.Conn.QueryRow(ctx, `SELECT COUNT(*) FROM user_giftcard_balances b
			JOIN users u ON b.user_id = u.id
			WHERE u.n_first_name ILIKE $1 OR u.n_last_name ILIKE $1 OR u.email ILIKE $1`, "%"+searchTerm+"%").Scan(&ubTotal); err != nil {
			log.Printf("Failed to scan filtered user balance count: %v", err)
		}
	} else {
		if err := db.Conn.QueryRow(ctx, "SELECT COUNT(*) FROM user_giftcard_balances").Scan(&ubTotal); err != nil {
			log.Printf("Failed to scan user balance count: %v", err)
		}
	}

	for ubRows.Next() {
		var ub UserBalance
		err = ubRows.Scan(
			&ub.UserID,
			&ub.Name,
			&ub.Email,
			&ub.Balance,
			&ub.UpdatedAt,
		)
		if err != nil {
			log.Printf("Failed to scan user balance: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		resp.UserBalances = append(resp.UserBalances, ub)
	}

	resp.UBTotal = ubTotal
	resp.Total = gcTotal

	var nextCursor *string
	if len(resp.GiftCards) > perPage {
		resp.GiftCards = resp.GiftCards[:perPage]
		last := resp.GiftCards[len(resp.GiftCards)-1]
		cursor := last.CreatedAt.Format(time.RFC3339) + "|" + last.ID
		nextCursor = &cursor
	}
	resp.NextCursor = nextCursor

	totalPages := (gcTotal + perPage - 1) / perPage
	if totalPages == 0 {
		totalPages = 1
	}
	resp.TotalPages = totalPages

	if err := json.NewEncoder(w).Encode(resp); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

func CreateGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	adminID, _ := ctx.Value(mw.UserIDKey).(string)

	var req CreateGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	if req.Amount < 0 {
		http.Error(w, "Amount must not be negative", http.StatusBadRequest)
		return
	}
	if req.Amount == 0 && !req.IsInventory {
		http.Error(w, "Amount must be greater than zero", http.StatusBadRequest)
		return
	}

	// C2: cap the admin-funded amount at £250 (25,000 pence) per transaction
	// (owner decision — tighter than the £10,000 ceiling ValidateAmount
	// enforces on other payment entry points, and matching the till's cap).
	// An inventory card may still be created at £0.
	if int64(math.Round(req.Amount*100)) > maxAdminGiftCardTransactionPence {
		http.Error(w, "Amount exceeds the maximum of £250", http.StatusBadRequest)
		return
	}

	// Daily limit (owner decision): an admin may create/top-up/transfer at
	// most £5,000 of gift-card value per UTC day. Enforced before any write.
	adminValueToday, err := adminGiftCardValueToday(ctx, db.Conn, adminID)
	if err != nil {
		log.Printf("Failed to query admin gift-card value today for %s: %v", adminID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if int64(math.Round(adminValueToday*100))+int64(math.Round(req.Amount*100)) > maxAdminGiftCardDailyPence {
		http.Error(w, "You have reached your £5,000 daily gift-card value limit", http.StatusBadRequest)
		return
	}

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback transaction", "err", err)
		}
	}()

	var gc GiftCard
	var lastUsedAt sql.NullTime
	var purchaseVoucherType string
	err = tx.QueryRow(ctx, `SELECT COALESCE(voucher_type, 'SPV') FROM business_settings LIMIT 1`).Scan(&purchaseVoucherType)
	if err != nil {
		log.Printf("Failed to query voucher type: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if purchaseVoucherType == "" {
		purchaseVoucherType = "SPV"
	}
	// HMRC VAT Notice 700/7: salon-only gift cards are SPV by definition, so
	// the EFFECTIVE type is written even when the stored setting is 'MPV' —
	// recording raw 'MPV' here would make the redemption path defer VAT a
	// second time (VAT is already collected at sale via the GetVATConfig SPV
	// override).
	purchaseVoucherType = effectiveVoucherTypeForPurchase(purchaseVoucherType)
	expiryMonths, err := GetGiftCardExpiryMonths(ctx, tx)
	if err != nil {
		log.Printf("Failed to query gift card expiry months (using default %d): %v", defaultGiftCardExpiryMonths, err)
		expiryMonths = defaultGiftCardExpiryMonths
	}
	err = tx.QueryRow(ctx, `
		INSERT INTO gift_cards (total_funds_added, amount_remaining, created_by, is_inventory, last_used_at, expiry_date, voucher_type_at_purchase)
		VALUES ($1, $1, $2, $3, NOW(), NOW() + ($5 * INTERVAL '1 month'), $4)
		RETURNING id, total_funds_added, amount_remaining, created_by, created_at, is_inventory, last_used_at
	`, req.Amount, adminID, req.IsInventory, purchaseVoucherType, expiryMonths).Scan(
		&gc.ID,
		&gc.TotalFundsAdded,
		&gc.AmountRemaining,
		&gc.CreatedBy,
		&gc.CreatedAt,
		&gc.IsInventory,
		&lastUsedAt,
	)
	if err != nil {
		log.Printf("Failed to create gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if lastUsedAt.Valid {
		gc.LastUsedAt = &lastUsedAt.Time
	}

	var notes *string
	if req.IsInventory {
		s := "inventory card"
		notes = &s
	} else {
		s := "giveaway"
		notes = &s
	}
	_, err = tx.Exec(ctx, `
		INSERT INTO gift_card_transactions (gift_card_id, transaction_type, amount, reference_type, reference_id, user_id, notes)
		VALUES ($1, 'purchase', $2, 'api', NULL, $3, $4)
	`, gc.ID, req.Amount, adminID, notes)
	if err != nil {
		log.Printf("Failed to record gift card transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := tx.Commit(ctx); err != nil {
		log.Printf("Failed to commit transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	w.WriteHeader(http.StatusCreated)
	if err := json.NewEncoder(w).Encode(gc); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

func TopUpGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	adminID, _ := ctx.Value(mw.UserIDKey).(string)
	cardID := chi.URLParam(r, "id")
	if cardID == "" || !validators.IsValidID(cardID) {
		http.Error(w, "Invalid gift card ID", http.StatusBadRequest)
		return
	}

	var req TopUpGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	if req.Amount <= 0 {
		http.Error(w, "Amount must be greater than zero", http.StatusBadRequest)
		return
	}

	// C2: cap the top-up at £250 (25,000 pence) per transaction (owner
	// decision — tighter than the £10,000 ceiling ValidateAmount enforces on
	// other payment entry points, and matching the till's cap).
	if int64(math.Round(req.Amount*100)) > maxAdminGiftCardTransactionPence {
		http.Error(w, "Amount exceeds the maximum of £250", http.StatusBadRequest)
		return
	}

	// Daily limit (owner decision): an admin may create/top-up/transfer at
	// most £5,000 of gift-card value per UTC day. Enforced before any write.
	adminValueToday, err := adminGiftCardValueToday(ctx, db.Conn, adminID)
	if err != nil {
		log.Printf("Failed to query admin gift-card value today for %s: %v", adminID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if int64(math.Round(adminValueToday*100))+int64(math.Round(req.Amount*100)) > maxAdminGiftCardDailyPence {
		http.Error(w, "You have reached your £5,000 daily gift-card value limit", http.StatusBadRequest)
		return
	}

	validMethods := map[string]string{
		"cash":          "cash",
		"card_machine":  "in_person_card",
		"online_square": "online_square",
		"on_the_house":  "on_the_house",
	}
	_, ok := validMethods[req.PaymentMethod]
	if !ok {
		http.Error(w, "Invalid payment method. Must be one of: cash, card_machine, online_square, on_the_house", http.StatusBadRequest)
		return
	}

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback transaction", "err", err)
		}
	}()

	var redeemedBy sql.NullString
	var isInventory bool
	var currentTotalFunds float64
	err = tx.QueryRow(ctx, "SELECT redeemed_by, is_inventory, total_funds_added FROM gift_cards WHERE id = $1 FOR UPDATE", cardID).Scan(&redeemedBy, &isInventory, &currentTotalFunds)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			http.Error(w, "Gift card not found", http.StatusNotFound)
			return
		}
		log.Printf("Failed to check gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if redeemedBy.Valid {
		http.Error(w, "Cannot top up a card that has already been redeemed to an account", http.StatusBadRequest)
		return
	}

	expiryMonths, err := GetGiftCardExpiryMonths(ctx, tx)
	if err != nil {
		log.Printf("Failed to query gift card expiry months (using default %d): %v", defaultGiftCardExpiryMonths, err)
		expiryMonths = defaultGiftCardExpiryMonths
	}

	txType := "topup"
	var notes *string
	if isInventory && currentTotalFunds == 0 {
		txType = "purchase"
		s := "first top-up on inventory card"
		notes = &s
	}

	var gc GiftCard
	var createdBy sql.NullString
	err = tx.QueryRow(ctx, `
		UPDATE gift_cards
		SET total_funds_added = total_funds_added + $1,
		    amount_remaining = amount_remaining + $1,
		    last_used_at = NOW(),
		    expiry_date = NOW() + ($3 * INTERVAL '1 month')
		WHERE id = $2
		RETURNING id, total_funds_added, amount_remaining, created_by, created_at
	`, req.Amount, cardID, expiryMonths).Scan(
		&gc.ID,
		&gc.TotalFundsAdded,
		&gc.AmountRemaining,
		&createdBy,
		&gc.CreatedAt,
	)
	if err != nil {
		log.Printf("Failed to top up gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if createdBy.Valid {
		gc.CreatedBy = &createdBy.String
	}

	_, err = tx.Exec(ctx, `
		INSERT INTO gift_card_transactions (gift_card_id, transaction_type, amount, reference_type, reference_id, user_id, notes)
		VALUES ($1, $2, $3, 'api', NULL, $4, $5)
	`, cardID, txType, req.Amount, adminID, notes)
	if err != nil {
		log.Printf("Failed to record gift card transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := tx.Commit(ctx); err != nil {
		log.Printf("Failed to commit transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := json.NewEncoder(w).Encode(gc); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

func TransferGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	adminID, _ := ctx.Value(mw.UserIDKey).(string)
	fromCardID := chi.URLParam(r, "from")
	if fromCardID == "" || !validators.IsValidID(fromCardID) {
		http.Error(w, "Invalid source gift card ID", http.StatusBadRequest)
		return
	}

	var req TransferGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	req.ToCardID = validators.NormalizeGiftCardCode(req.ToCardID)
	if !validators.IsValidID(req.ToCardID) {
		http.Error(w, "Invalid destination gift card ID", http.StatusBadRequest)
		return
	}

	if strings.EqualFold(fromCardID, req.ToCardID) {
		http.Error(w, "Source and destination cards must be different", http.StatusBadRequest)
		return
	}

	if req.Amount <= 0 {
		http.Error(w, "Amount must be greater than zero", http.StatusBadRequest)
		return
	}

	// C2: cap the transfer at £250 (25,000 pence) per transaction (owner
	// decision — tighter than the £10,000 ceiling ValidateAmount enforces on
	// other payment entry points, and matching the till's cap).
	if int64(math.Round(req.Amount*100)) > maxAdminGiftCardTransactionPence {
		http.Error(w, "Amount exceeds the maximum of £250", http.StatusBadRequest)
		return
	}

	// Daily limit (owner decision): an admin may create/top-up/transfer at
	// most £5,000 of gift-card value per UTC day. Enforced before any write
	// (and before the advisory lock acquisition below).
	adminValueToday, err := adminGiftCardValueToday(ctx, db.Conn, adminID)
	if err != nil {
		log.Printf("Failed to query admin gift-card value today for %s: %v", adminID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if int64(math.Round(adminValueToday*100))+int64(math.Round(req.Amount*100)) > maxAdminGiftCardDailyPence {
		http.Error(w, "You have reached your £5,000 daily gift-card value limit", http.StatusBadRequest)
		return
	}

	// Serialize against a concurrent cancellation of the SOURCE card: the
	// gift-card cancel flow (CancelGiftCard) holds this session advisory lock
	// across its eligibility check AND its funding reversal, so a transfer
	// moving value OFF the source mid-cancellation would otherwise return the
	// customer's money while the transferred balance stays live (double value).
	// Taking the same lock here makes transfer-from and cancel mutually
	// exclusive (C2/F2). A transfer INTO a card is covered by the cancel
	// flow's FOR UPDATE re-verification (the destination's balance change makes
	// it ineligible), so only the source needs the lock.
	pinConn, err := db.Conn.Acquire(ctx)
	if err != nil {
		log.Printf("Failed to acquire connection for gift-card transfer lock: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer pinConn.Release()
	lockOK, err := acquireAdvisoryLock(ctx, pinConn, "crussell:giftcard-cancel:"+fromCardID)
	if err != nil {
		log.Printf("Failed to acquire gift-card cancel lock for %s: %v", fromCardID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if !lockOK {
		log.Printf("Gift-card transfer lock for %s not acquired within bound — a cancellation is in progress", fromCardID)
		http.Error(w, "This gift card is being processed — please try again in a moment", http.StatusConflict)
		return
	}
	defer releasePaymentLock(pinConn, "crussell:giftcard-cancel:"+fromCardID)

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback transaction", "err", err)
		}
	}()

	var fromRedeemedBy, toRedeemedBy sql.NullString
	var fromRemaining, toRemaining float64

	// Lock both rows FOR UPDATE in a globally deterministic order (lesser ID
	// first, then greater) so two concurrent cross-transfers (A→B and B→A)
	// acquire the locks in the same order and can never deadlock. Locking
	// "source first" is only deterministic per-request — the caller picks the
	// source, so opposite transfers would deadlock (N-5). IDs are compared
	// case-insensitively: gift card codes are 12-hex that may arrive mixed-case
	// from the URL param vs the body, and the same two cards must always sort
	// the same way for every concurrent transaction.
	lockFirstID, lockSecondID := fromCardID, req.ToCardID
	if strings.ToLower(lockFirstID) > strings.ToLower(lockSecondID) {
		lockFirstID, lockSecondID = lockSecondID, lockFirstID
	}
	lockFirstIsSource := strings.EqualFold(lockFirstID, fromCardID)

	err = tx.QueryRow(ctx, "SELECT redeemed_by, amount_remaining FROM gift_cards WHERE id = $1 FOR UPDATE", lockFirstID).Scan(&fromRedeemedBy, &fromRemaining)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			if lockFirstIsSource {
				http.Error(w, "Source gift card not found", http.StatusNotFound)
			} else {
				http.Error(w, "Destination gift card not found", http.StatusNotFound)
			}
			return
		}
		log.Printf("Failed to check gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// Lock the second row in the same order so both concurrent transactions
	// hold the same lock sequence and can never deadlock.
	err = tx.QueryRow(ctx, "SELECT redeemed_by, amount_remaining FROM gift_cards WHERE id = $1 FOR UPDATE", lockSecondID).Scan(&toRedeemedBy, &toRemaining)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			if lockFirstIsSource {
				http.Error(w, "Destination gift card not found", http.StatusNotFound)
			} else {
				http.Error(w, "Source gift card not found", http.StatusNotFound)
			}
			return
		}
		log.Printf("Failed to check gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// The lock order may differ from the source/destination roles when the
	// destination ID sorts before the source ID; swap the scanned values back
	// so the business logic below always treats fromCardID as the source.
	if !lockFirstIsSource {
		fromRedeemedBy, toRedeemedBy = toRedeemedBy, fromRedeemedBy
		fromRemaining, toRemaining = toRemaining, fromRemaining
	}

	if fromRedeemedBy.Valid || toRedeemedBy.Valid {
		http.Error(w, "Cannot transfer balance to/from cards redeemed to accounts", http.StatusBadRequest)
		return
	}

	if fromRemaining < req.Amount {
		http.Error(w, "Insufficient balance on source gift card", http.StatusBadRequest)
		return
	}

	// A transfer is a "use" of BOTH cards per the rolling-expiry terms — each
	// card's timer resets at the same moment its balance moves.
	expiryMonths, err := GetGiftCardExpiryMonths(ctx, tx)
	if err != nil {
		log.Printf("Failed to query gift card expiry months (using default %d): %v", defaultGiftCardExpiryMonths, err)
		expiryMonths = defaultGiftCardExpiryMonths
	}

	_, err = tx.Exec(ctx, `
		UPDATE gift_cards
		SET amount_remaining = amount_remaining - $1,
		    last_used_at = NOW(),
		    expiry_date = NOW() + ($3 * INTERVAL '1 month')
		WHERE id = $2
	`, req.Amount, fromCardID, expiryMonths)
	if err != nil {
		log.Printf("Failed to deduct from source: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	_, err = tx.Exec(ctx, `
		UPDATE gift_cards
		SET amount_remaining = amount_remaining + $1,
		    total_funds_added = total_funds_added + $1,
		    last_used_at = NOW(),
		    expiry_date = NOW() + ($3 * INTERVAL '1 month')
		WHERE id = $2
	`, req.Amount, req.ToCardID, expiryMonths)
	if err != nil {
		log.Printf("Failed to add to destination: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := tx.Commit(ctx); err != nil {
		log.Printf("Failed to commit transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	w.WriteHeader(http.StatusOK)
	if err := json.NewEncoder(w).Encode(map[string]string{"status": "success"}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

// --- User Handlers ---

// --- Gift-card redeem failure rate limiting (B16) ---
//
// RedeemGiftCard converts a 12-hex gift-card code into account balance. The
// code space is small enough that an attacker can brute-force plausible codes
// through repeated redeem attempts; the route-level per-user limiter
// (mw.RateLimitByUser in main.go) throttles by ACCOUNT, but a distributed probe
// (many accounts, each trying codes) would still burn a DB lookup per attempt.
// This in-memory per-CODE counter adds a second layer: after
// giftCardRedeemFailMax consecutive "invalid code" failures for the same
// normalized code within giftCardRedeemFailWindow, further redeem attempts for
// that code are rejected 429 BEFORE any advisory lock or DB work.
//
// The counter is keyed on the normalized code, is reset whenever a redeem
// attempt resolves the code to a REAL card (any found row breaks the streak of
// invalid-code failures), and entries expire after the window so a one-off
// typo'd code is never locked out forever. In-memory only (no schema change —
// matching the A5 refund-failure counter in refunds.go, manualReconcileFailures),
// best-effort: a process restart clears it, and it is NOT a replacement for the
// account-level limiter.
const (
	// giftCardRedeemFailMax is how many consecutive invalid-code redeem
	// failures against the same card code trigger the 429 lockout.
	giftCardRedeemFailMax = 5
	// giftCardRedeemFailWindow is how long a code's failure streak is
	// remembered. A card locked by repeated failures stays locked until the
	// window elapses, then a fresh attempt starts a new streak.
	giftCardRedeemFailWindow = 15 * time.Minute
	// giftCardRedeemFailMaxEntries caps the in-memory map so the counter can
	// never grow unbounded (one entry per code being probed).
	giftCardRedeemFailMaxEntries = 10_000
)

type giftCardRedeemFailState struct {
	count     int
	windowEnd time.Time
}

var (
	giftCardRedeemFailMu sync.Mutex
	giftCardRedeemFails  = make(map[string]giftCardRedeemFailState)
)

// giftCardRedeemLocked reports whether a code is currently in the 429 lockout
// (>= giftCardRedeemFailMax consecutive invalid-code failures inside the
// window). An expired entry is treated as not locked — the window has passed.
func giftCardRedeemLocked(code string) bool {
	giftCardRedeemFailMu.Lock()
	defer giftCardRedeemFailMu.Unlock()
	st, ok := giftCardRedeemFails[code]
	if !ok || clock.Now().After(st.windowEnd) {
		return false
	}
	return st.count >= giftCardRedeemFailMax
}

// giftCardRedeemFail records one more consecutive invalid-code failure for a
// code. A fresh streak (new code, or the previous streak expired) starts at 1
// with a fresh window. Bounded: expired entries are purged on insert and, if
// the map is still full, one entry is evicted so the map never exceeds
// giftCardRedeemFailMaxEntries.
func giftCardRedeemFail(code string) {
	giftCardRedeemFailMu.Lock()
	defer giftCardRedeemFailMu.Unlock()
	now := clock.Now()
	if st, ok := giftCardRedeemFails[code]; ok && !now.After(st.windowEnd) {
		st.count++
		giftCardRedeemFails[code] = st
		return
	}
	if len(giftCardRedeemFails) >= giftCardRedeemFailMaxEntries {
		for c, st := range giftCardRedeemFails {
			if now.After(st.windowEnd) {
				delete(giftCardRedeemFails, c)
			}
		}
		if len(giftCardRedeemFails) >= giftCardRedeemFailMaxEntries {
			for c := range giftCardRedeemFails {
				delete(giftCardRedeemFails, c)
				break
			}
		}
	}
	giftCardRedeemFails[code] = giftCardRedeemFailState{count: 1, windowEnd: now.Add(giftCardRedeemFailWindow)}
}

// giftCardRedeemReset clears a code's invalid-code-failure streak — called
// whenever a redeem attempt resolves the code to a real gift card, because a
// successful resolution breaks the consecutive-failure run.
func giftCardRedeemReset(code string) {
	giftCardRedeemFailMu.Lock()
	delete(giftCardRedeemFails, code)
	giftCardRedeemFailMu.Unlock()
}

func RedeemGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	userID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || userID == "" {
		http.Error(w, "Authentication required", http.StatusUnauthorized)
		return
	}

	var req RedeemGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	code := validators.NormalizeGiftCardCode(req.Code)
	if !validators.IsValidID(code) {
		http.Error(w, "Invalid gift card code format", http.StatusBadRequest)
		return
	}

	// B16: a code in the per-card lockout (>= giftCardRedeemFailMax
	// consecutive invalid-code failures inside the window) is rejected 429
	// BEFORE the advisory lock or any DB work — brute-force probing of the
	// 12-hex code space never reaches the database.
	if giftCardRedeemLocked(code) {
		log.Printf("Gift card redeem rate-limited: code %s has failed too many consecutive redeem attempts", code)
		http.Error(w, "Too many failed redeem attempts for this gift card — try again later", http.StatusTooManyRequests)
		return
	}

	// Serialize against a concurrent cancellation of this same card: the
	// gift-card cancel flow (CancelGiftCard) holds this session advisory lock
	// across its eligibility check AND its funding reversal, so redeeming the
	// balance mid-cancellation would otherwise return the customer's money
	// while the balance stays live (double value). Taking the same lock here
	// makes redeem and cancel mutually exclusive (C2/F2).
	pinConn, err := db.Conn.Acquire(ctx)
	if err != nil {
		log.Printf("Failed to acquire connection for gift-card redeem lock: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer pinConn.Release()
	lockOK, err := acquireAdvisoryLock(ctx, pinConn, "crussell:giftcard-cancel:"+code)
	if err != nil {
		log.Printf("Failed to acquire gift-card cancel lock for %s: %v", code, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if !lockOK {
		log.Printf("Gift-card redeem lock for %s not acquired within bound — a cancellation is in progress", code)
		http.Error(w, "This gift card is being processed — please try again in a moment", http.StatusConflict)
		return
	}
	defer releasePaymentLock(pinConn, "crussell:giftcard-cancel:"+code)

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback transaction", "err", err)
		}
	}()

	var amountRemaining float64
	var redeemedBy sql.NullString
	err = tx.QueryRow(ctx, `
		SELECT amount_remaining, redeemed_by
		FROM gift_cards
		WHERE id = $1
		FOR UPDATE
	`, code).Scan(&amountRemaining, &redeemedBy)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			// B16: an "invalid code" failure — count it against the per-card
			// lockout so repeated brute-force attempts on the same code get 429
			// after giftCardRedeemFailMax consecutive misses.
			giftCardRedeemFail(code)
			http.Error(w, "Invalid or expired gift card code", http.StatusNotFound)
			return
		}
		log.Printf("Failed to query gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	// B16: the code resolved to a real gift card — a found row breaks the
	// consecutive invalid-code-failure streak, so clear the per-card counter.
	giftCardRedeemReset(code)

	if redeemedBy.Valid {
		http.Error(w, "This gift card has already been redeemed", http.StatusBadRequest)
		return
	}

	if amountRemaining <= 0 {
		http.Error(w, "This gift card has no remaining balance", http.StatusBadRequest)
		return
	}

	expiryMonths, err := GetGiftCardExpiryMonths(ctx, tx)
	if err != nil {
		log.Printf("Failed to query gift card expiry months (using default %d): %v", defaultGiftCardExpiryMonths, err)
		expiryMonths = defaultGiftCardExpiryMonths
	}

	_, err = tx.Exec(ctx, `
		UPDATE gift_cards
		SET amount_remaining = 0,
		    redeemed_at = NOW(),
		    redeemed_by = $1,
		    last_used_at = NOW(),
		    expiry_date = NOW() + ($3 * INTERVAL '1 month')
		WHERE id = $2
	`, userID, code, expiryMonths)
	if err != nil {
		log.Printf("Failed to update gift card: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	_, err = tx.Exec(ctx, `
		INSERT INTO user_giftcard_balances (user_id, balance, updated_at)
		VALUES ($1, $2, NOW())
		ON CONFLICT (user_id) DO UPDATE SET
			balance = user_giftcard_balances.balance + EXCLUDED.balance,
			updated_at = NOW()
	`, userID, amountRemaining)
	if err != nil {
		log.Printf("Failed to update user gift card balance: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	_, err = tx.Exec(ctx, `
		INSERT INTO gift_card_transactions (gift_card_id, transaction_type, amount, reference_type, reference_id, user_id, notes)
		VALUES ($1, 'redeem_to_balance', $2, 'api', NULL, $3, NULL)
	`, code, amountRemaining, userID)
	if err != nil {
		log.Printf("Failed to record gift card transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := tx.Commit(ctx); err != nil {
		log.Printf("Failed to commit transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	w.WriteHeader(http.StatusOK)
	if err := json.NewEncoder(w).Encode(map[string]any{
		"status":          "success",
		"amount_redeemed": amountRemaining,
	}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

func GetGiftCardBalance(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	userID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || userID == "" {
		http.Error(w, "Authentication required", http.StatusUnauthorized)
		return
	}

	var balance float64
	err := db.Conn.QueryRow(ctx, "SELECT balance FROM user_giftcard_balances WHERE user_id = $1", userID).Scan(&balance)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			if err := json.NewEncoder(w).Encode(map[string]float64{"balance": 0.00}); err != nil {
				log.Printf("Failed to encode JSON response: %v", err)
			}
			return
		}
		log.Printf("Failed to query user balance: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := json.NewEncoder(w).Encode(map[string]float64{"balance": balance}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

// GetUserGiftCardBalanceAdmin Handler returns any user's balance for the admin.
func GetUserGiftCardBalanceAdmin(w http.ResponseWriter, r *http.Request) {
	if !isAdminRequest(r) {
		http.Error(w, "Unauthorized", http.StatusForbidden)
		return
	}

	ctx := r.Context()
	userID := chi.URLParam(r, "id")
	if userID == "" || !validators.IsValidID(userID) {
		http.Error(w, "Invalid user ID", http.StatusBadRequest)
		return
	}

	adminID, _ := ctx.Value(mw.UserIDKey).(string)

	var balance float64
	err := db.Conn.QueryRow(ctx, "SELECT balance FROM user_giftcard_balances WHERE user_id = $1", userID).Scan(&balance)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			if err := json.NewEncoder(w).Encode(map[string]float64{"balance": 0.00}); err != nil {
				log.Printf("Failed to encode JSON response: %v", err)
			}
			return
		}
		log.Printf("Failed to query user balance: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	detailsJSON := fmt.Sprintf(`{"balance": %.2f}`, balance)

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
	} else {
		defer func() {
			if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
				slog.Error("failed to rollback transaction", "err", err)
			}
		}()

		if _, err := tx.Exec(ctx, `
			INSERT INTO admin_audit_log (admin_id, action_type, target_user_id, details)
			VALUES ($1, 'balance_check', $2, $3::jsonb)
		`, adminID, userID, detailsJSON); err != nil {
			log.Printf("Failed to record admin_audit_log (non-critical): %v", err)
		}

		if err := tx.Commit(ctx); err != nil {
			log.Printf("Failed to commit transaction: %v", err)
		}
	}

	if err := json.NewEncoder(w).Encode(map[string]float64{"balance": balance}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

func BuyGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	userID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || userID == "" {
		http.Error(w, "Authentication required", http.StatusUnauthorized)
		return
	}

	var req BuyGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	// M1: idempotency_key is optional. When omitted, a deterministic fallback
	// key is derived server-side AFTER the advisory lock is acquired (see
	// deriveGiftCardIdempotencyKey): the base key (user_id + amount +
	// recipient_type + card_id) is reused while a PENDING payment row exists
	// for the same logical purchase (a lost-response retry lands on the SAME
	// key so Square dedups — the old fresh-random-suffix fallback generated a
	// NEW key per retry, missed the pending row, and charged twice), and it
	// advances deterministically past COMPLETED purchases so two genuinely
	// distinct no-key purchases never collapse onto one dedup key. Clients who
	// need full control still supply their own idempotency_key; that path is
	// unchanged.
	clientSuppliedKey := req.IdempotencyKey != ""
	var noKeyCardPart string
	if !clientSuppliedKey {
		noKeyCardPart = "new"
		if req.CardID != nil && *req.CardID != "" {
			noKeyCardPart = *req.CardID
		}
	}
	if err := validators.Validate.Struct(&req); err != nil {
		log.Printf("Failed to process request: %v", err)
		http.Error(w, "Invalid request", http.StatusBadRequest)
		return
	}

	// Product rule (security): only verified accounts may save cards. An
	// unverified/guest/affiliate user may still buy a gift card, but
	// save_card=true is rejected here — before any charge source resolution.
	if rejectSaveCardForUnverified(w, r, req.SaveCard) {
		return
	}

	allowedAmounts := map[int64]bool{1000: true, 2000: true, 5000: true}
	if !allowedAmounts[req.Amount] {
		http.Error(w, "Invalid amount. Must be £10, £20, or £50.", http.StatusBadRequest)
		return
	}

	if req.RecipientType != "self" && req.RecipientType != "friend" {
		http.Error(w, "Invalid recipient type", http.StatusBadRequest)
		return
	}

	if err := ValidateCardInfo(req.CardID, req.NewCardToken); err != nil {
		log.Printf("Failed to process request: %v", err)
		http.Error(w, "Invalid request", http.StatusBadRequest)
		return
	}

	if err := ValidateVerificationToken(req.VerificationToken); err != nil {
		log.Printf("Failed to process request: %v", err)
		http.Error(w, "Invalid request", http.StatusBadRequest)
		return
	}

	paymentService := NewPaymentService()

	// Serialize gift-card purchase attempts on the idempotency key to prevent
	// concurrent same-key retries from both reusing a pending record and both
	// executing the gift-card creation (2× value for 1 charge). Mirrors the tip
	// advisory-lock pattern (handlers.go). Lock is keyed on the idempotency key
	// so distinct purchases are unaffected; falls back to userID when absent.
	// Bounded try-lock (R6) so a contended lock never blocks the pool across
	// the Square round-trip.
	lockKey := req.IdempotencyKey
	if !clientSuppliedKey {
		// Identical logical no-key purchases must serialize on the SAME lock
		// key — the deterministic base, not a per-request random suffix — so
		// concurrent lost-response retries cannot both derive a fresh slot and
		// both charge.
		lockKey = fmt.Sprintf("gc-%s-%d-%s-%s", userID, req.Amount, req.RecipientType, noKeyCardPart)
	}
	pinConn, lockOK := acquireBookingPaymentLock(ctx, w, "crussell:giftcard:"+lockKey, "Purchase in progress, try again")
	if !lockOK {
		return
	}
	defer releaseBookingPaymentLock(pinConn, "crussell:giftcard:"+lockKey)

	// Derive the deterministic fallback key UNDER the lock so the spent-slot
	// scan races no concurrent purchase (mirrors deriveBookingPaymentIdempotencyKey
	// in handlers.go, which derives under the per-booking lock).
	if !clientSuppliedKey {
		derivedKey, dErr := deriveGiftCardIdempotencyKey(ctx, db.Conn, userID, req.Amount, req.RecipientType, noKeyCardPart)
		if dErr != nil {
			log.Printf("Failed to derive gift-card idempotency key: %v", dErr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		req.IdempotencyKey = derivedKey
	}

	// Idempotency: only short-circuit when the existing record is 'completed'.
	// A 'pending' record means the previous Square call failed — returning it
	// as 200 would show a success without ever charging. Re-attempt below with
	// the same key (Square dedups safely) and reuse the pending record.
	reusePendingID := ""
	if req.IdempotencyKey != "" {
		existing, err := paymentService.CheckIdempotencyByKey(ctx, req.IdempotencyKey)
		if err != nil {
			log.Printf("Failed to check idempotency: %v", err)
		}
		// A6: CheckIdempotencyByKey matches on the key ALONE (service.go), so a
		// client-supplied deterministic/guessable key (e.g. another user's
		// "gc-<userID>-<amount>-<type>-<cardPart>" fallback key) would resolve
		// to ANOTHER user's payment row — returning it as the caller's completed
		// purchase, reusing it for a charge, or rejecting the caller on it
		// (cross-user hijack). Never reuse or return a row the caller does not
		// own: a foreign match is treated as a fresh request.
		if existing != nil && (existing.CreatedBy == nil || *existing.CreatedBy != userID) {
			log.Printf("Gift card idempotency key %q matched payment row %s (status %s) belonging to a different user — treating as a fresh request", req.IdempotencyKey, existing.ID, existing.Status)
			if clientSuppliedKey {
				// The collided key is occupied (payments.idempotency_key is
				// UNIQUE) — derive a fresh deterministic key for THIS user so
				// the new purchase inserts a new row instead of 500ing on the
				// constraint.
				derivedKey, dErr := deriveGiftCardIdempotencyKey(ctx, db.Conn, userID, req.Amount, req.RecipientType, noKeyCardPart)
				if dErr != nil {
					log.Printf("Failed to derive fresh gift-card idempotency key after foreign-key collision: %v", dErr)
					http.Error(w, "internal server error", http.StatusInternalServerError)
					return
				}
				req.IdempotencyKey = derivedKey
			}
			existing = nil
		}
		if existing != nil {
			if existing.Status == "completed" {
				// RE-VALIDATE the matched purchase payment's refund state before
				// reporting it as success (same guard as the CreateBookingPayment
				// completed-dedup branches): a refunded purchase's money is no
				// longer live, and a same-key retry must not claim the purchase
				// succeeded when the money was already returned.
				if refunded, rErr := paymentHasLiveRefund(ctx, db.Conn, existing.ID); rErr != nil {
					log.Printf("Failed to re-validate gift-card dedup hit %s against refunds: %v", existing.ID, rErr)
					http.Error(w, "internal server error", http.StatusInternalServerError)
					return
				} else if refunded {
					log.Printf("Gift card retry rejected: purchase payment %s (key %q) was refunded — refusing to report a refunded purchase as success", existing.ID, req.IdempotencyKey)
					http.Error(w, "This payment has been refunded and can no longer be replayed", http.StatusConflict)
					return
				}
				if err := json.NewEncoder(w).Encode(existing); err != nil {
					log.Printf("Failed to encode JSON response: %v", err)
				}
				return
			}
			if existing.Status == "pending" {
				// Guard the amount: a retry with a different amount must not
				// reuse the pending record (gift card would be issued at the
				// new amount against the old charge record).
				if int64(math.Round(existing.Amount*100)) != req.Amount {
					log.Printf("Gift card retry amount mismatch: pending record %s has %.2f, request has %d pence", existing.ID, existing.Amount, req.Amount)
					http.Error(w, "Amount does not match the pending gift card payment", http.StatusBadRequest)
					return
				}
				// Defense-in-depth refund guard on the pending-reuse path too: a
				// pending purchase cannot normally carry a refund (refunds attach
				// to completed charges), but if one ever exists the money is in
				// flight/returned and re-attempting the charge must not proceed.
				if refunded, rErr := paymentHasLiveRefund(ctx, db.Conn, existing.ID); rErr != nil {
					log.Printf("Failed to re-validate pending gift-card reuse %s against refunds: %v", existing.ID, rErr)
					http.Error(w, "internal server error", http.StatusInternalServerError)
					return
				} else if refunded {
					log.Printf("Gift card retry rejected: pending purchase payment %s (key %q) was refunded — refusing to reuse a refunded payment", existing.ID, req.IdempotencyKey)
					http.Error(w, "This payment has been refunded and can no longer be replayed", http.StatusConflict)
					return
				}
				reusePendingID = existing.ID
				log.Printf("[PAYMENTS] Reusing pending payment %s for idempotent gift-card retry (key %s)", existing.ID, req.IdempotencyKey)
			}
			if existing.Status == "failed" {
				// Swept as stale (>24h) or definitively rejected — a retry would
				// risk a second Square charge. Reject cleanly (R2).
				log.Printf("Gift card retry rejected: pending record %s was marked failed", existing.ID)
				http.Error(w, "This gift card purchase previously failed and can no longer be retried", http.StatusConflict)
				return
			}
		}
	}

	// Daily limit (owner decision): a user may buy at most £500 of online
	// gift cards per UTC day. Sums only COMPLETED online purchases (the
	// gift_card_transactions rows BuyGiftCard writes) so a pending-retry of a
	// failed Square attempt is never blocked by its own un-issued value.
	spentToday, err := userGiftCardSpentToday(ctx, db.Conn, userID)
	if err != nil {
		log.Printf("Failed to query user gift-card spend today for %s: %v", userID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if int64(math.Round(spentToday*100))+req.Amount > maxUserGiftCardDailyPence {
		http.Error(w, "You have reached your £500 daily gift-card purchase limit", http.StatusBadRequest)
		return
	}

	// 2FA gating (C5): persisting or charging a card requires 2FA when the
	// feature is enforced — both paying with an existing saved card (CardID)
	// and SAVING a new card during this purchase (SaveCard), mirroring
	// CreateBookingPayment/CreateTipPayment. A one-off new-card (nonce) charge
	// that is not saved is not gated.
	// consume=false (MEDIUM-2): the code is verified here but only NULLed
	// inside the completed-charge transaction below (ConsumePendingCode), so a
	// failed/ambiguous Square charge does NOT burn the operator-relayed code
	// and a same-key retry can re-verify the SAME code.
	if (req.CardID != nil && *req.CardID != "") || req.SaveCard {
		if !requireTwoFactorForCardAccess(w, r, paymentService, userID, req.VerificationCode, false) {
			return
		}
	}

	var sourceID string
	var savedCardID *string
	var savedCardCustomerID string
	// Resolve the new-card-vs-saved-card Square source — shared with
	// CreateBookingPayment/CreateTipPayment (see resolveChargeSource for the
	// R6 rationale).
	sourceID, savedCardID, savedCardCustomerID, sourceOK := resolveChargeSource(ctx, w, paymentService, userID, req.NewCardToken, req.CardID, req.SaveCard, "Card not found")
	if !sourceOK {
		return
	}

	amountPounds := float64(req.Amount) / 100.0

	// Step 1: Insert payment with status='pending' inside a DB transaction.
	// Square is NOT called yet — if the tx fails, no harm done.
	// Gift card and balance are created AFTER payment succeeds (below).
	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback transaction", "err", err)
		}
	}()

	var buyPaymentID string
	if reusePendingID != "" {
		// Reusing the pending record from a failed prior attempt — do not
		// insert a duplicate (idempotency_key is UNIQUE). Proceed straight to
		// the Square call, which dedups on the same key.
		// Refresh square_source_id: this attempt may charge a DIFFERENT token
		// than the failed attempt (one-time cnon: nonces are spent), and the
		// sweep replays the charge from the stored source.
		// B6: keep square_request_snapshot's source_id in sync in the SAME
		// statement — the sweep replays the stored snapshot verbatim, and a
		// snapshot carrying the spent source would replay into
		// IDEMPOTENCY_KEY_REUSED, stranding the row pending forever. The
		// at-rest snapshot is AES-256-GCM-encrypted in non-mock deployments
		// (encryptSnapshot's "enc:v1:" marker), so the SourceID refresh must
		// run in Go — decrypt → set SourceID → re-encrypt — instead of the
		// legacy SQL jsonb_set, whose COALESCE(...,'{}')::jsonb cast cannot
		// parse ciphertext. A row with no stored snapshot (legacy) gets just
		// the source column refreshed, mirroring the booking reuse path
		// (handlers.go). Best-effort: any failure leaves the snapshot
		// untouched — the live square_source_id column stays authoritative
		// and the sweep overrides the replay source from it.
		var snap sql.NullString
		if err := tx.QueryRow(ctx, `SELECT square_request_snapshot FROM payments WHERE id = $1`, reusePendingID).Scan(&snap); err != nil {
			log.Printf("Failed to read square_request_snapshot for reused gift-card payment %s: %v", reusePendingID, err)
		}
		var storedSnap *string
		if snap.Valid && snap.String != "" {
			body := []byte(snap.String)
			if !IsExplicitDevOrMockEnv() {
				if dec, dErr := decryptSnapshot(body); dErr != nil {
					log.Printf("Failed to decrypt square_request_snapshot for reused gift-card payment %s: %v", reusePendingID, dErr)
				} else {
					body = dec
				}
			}
			var req square.CreatePaymentReq
			if uErr := json.Unmarshal(body, &req); uErr != nil {
				log.Printf("Failed to parse square_request_snapshot for reused gift-card payment %s: %v", reusePendingID, uErr)
			} else {
				req.SourceID = sourceID
				updated, mErr := json.Marshal(req)
				if mErr != nil {
					log.Printf("Failed to re-marshal square_request_snapshot for reused gift-card payment %s: %v", reusePendingID, mErr)
				} else {
					stored := updated
					if !IsExplicitDevOrMockEnv() {
						if enc, eErr := encryptSnapshot(updated); eErr != nil {
							log.Printf("Failed to encrypt square_request_snapshot for reused gift-card payment %s: %v", reusePendingID, eErr)
						} else {
							stored = enc
						}
					}
					s := string(stored)
					storedSnap = &s
				}
			}
		}
		if storedSnap != nil {
			if _, srcErr := tx.Exec(ctx, `
				UPDATE payments
				SET square_source_id = $1,
				    square_request_snapshot = $2
				WHERE id = $3
			`, sourceID, *storedSnap, reusePendingID); srcErr != nil {
				log.Printf("Failed to update square_source_id/square_request_snapshot on reused gift-card payment %s: %v", reusePendingID, srcErr)
			}
		} else {
			if _, srcErr := tx.Exec(ctx, `UPDATE payments SET square_source_id = $1 WHERE id = $2`, sourceID, reusePendingID); srcErr != nil {
				log.Printf("Failed to update square_source_id on reused gift-card payment %s: %v", reusePendingID, srcErr)
			}
		}
		buyPaymentID = reusePendingID
	} else {
		fees := paymentService.CalculateFees(req.Amount, "online")
		record := PaymentRecord{
			PaymentType:     "full",
			PaymentMethod:   "online_square",
			Status:          "pending",
			Amount:          amountPounds,
			SquarePaymentID: nil,
			IdempotencyKey:  &req.IdempotencyKey,
			Fees:            float64(fees) / 100.0,
			UserSavedCardID: savedCardID,
			CreatedAt:       clock.Now(),
			UpdatedAt:       clock.Now(),
			CreatedBy:       &userID,
		}
		err = tx.QueryRow(ctx, `
			INSERT INTO payments (payment_type, payment_method, status, amount, square_payment_id, idempotency_key, fees, user_saved_card_id, created_by, created_at, updated_at, square_source_id)
			VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12)
			RETURNING id
		`, record.PaymentType, record.PaymentMethod, record.Status, record.Amount, record.SquarePaymentID, record.IdempotencyKey, record.Fees, record.UserSavedCardID, record.CreatedBy, record.CreatedAt, record.UpdatedAt, sourceID).Scan(&buyPaymentID)
		if err != nil {
			log.Printf("Failed to insert payment record: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		// Apply VAT to the pending payment
		vatCfg, vatErr := GetVATConfig(ctx, tx)
		if vatErr == nil && vatAppliesToVoucher(vatCfg) {
			if _, vatExecErr := tx.Exec(ctx, "SELECT apply_vat_to_payment($1, $2)", buyPaymentID, vatCfg.DefaultVATRate); vatExecErr != nil {
				log.Printf("Failed to apply VAT to buy gift card payment %s: %v", buyPaymentID, vatExecErr)
			}
		}

		if err := tx.Commit(ctx); err != nil {
			log.Printf("Failed to commit buy transaction: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
	}

	// Commit in the reuse path too. No rows were written, but the commit is
	// required in the test harness: there the context carries an outer test tx,
	// so Begin creates a nested savepoint whose deferred rollback would
	// otherwise undo the status UPDATE executed later on the same connection.
	// In production Begin is a plain tx and this commit is a harmless no-op.
	if reusePendingID != "" {
		if err := tx.Commit(ctx); err != nil {
			log.Printf("Failed to commit buy transaction (reuse): %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
	}

	// Step 2: DB transaction committed — safe to call Square now.
	// If Square fails, the payment record stays 'pending' for manual retry.
	var buyerEmail string
	if err := db.Conn.QueryRow(ctx, `SELECT email FROM users WHERE id = $1`, userID).Scan(&buyerEmail); err != nil {
		log.Printf("[SQUARE-PROD] Failed to resolve buyer email for user %s: %v (Square receipts will not be emailed)", userID, err)
	}

	var verificationToken string
	if req.VerificationToken != nil {
		verificationToken = *req.VerificationToken
	}

	paymentReq := square.CreatePaymentReq{
		Amount:   req.Amount,
		Currency: "GBP",
		SourceID: sourceID,
		// CustomerID carries the saved-card row's Square customer id on ccof:
		// charges (save-card path); a cnon: nonce charge (one-off) needs none
		// (R6).
		CustomerID:        savedCardCustomerID,
		IdempotencyKey:    req.IdempotencyKey,
		Note:              "Gift Card Purchase",
		BuyerEmail:        buyerEmail,
		VerificationToken: verificationToken,
	}
	// C3: a card-on-file (ccof) charge — paying with an existing saved card or
	// saving a new card during this purchase — is customer-initiated: Square
	// requires customer_details on stored-credential payments. A one-off cnon:
	// nonce charge is not a stored credential and needs none.
	if req.CardID != nil || req.SaveCard {
		paymentReq.CustomerDetails = &square.CreateCustomerDetails{CustomerInitiated: true}
	}

	// M1: store the verbatim request JSON so the sweep can replay the charge
	// with an IDENTICAL body under the same key — Square compares the whole
	// request on key reuse, and a reconstructed body returns
	// IDEMPOTENCY_KEY_REUSED, leaving the row pending forever.
	//
	// The write is INTENTIONALLY unconditional (WHERE id = $2, no
	// snapshot-is-null guard like the booking/tip/terminal flows): the reuse
	// branch above (B6) already refreshed square_request_snapshot in the SAME
	// transaction as the square_source_id refresh, and this post-commit write
	// stores the fresh full body for THIS attempt. Both paths converge on a
	// correct snapshot, so a guard would either be dead (first attempt) or
	// wrongly skip this write on the reuse path when the in-tx refresh failed
	// best-effort. Do NOT "fix" this into the guarded form without reworking
	// the reuse-branch refresh.
	if snap, mErr := json.Marshal(paymentReq); mErr != nil {
		log.Printf("Failed to marshal square_request_snapshot for gift-card payment %s: %v", buyPaymentID, mErr)
	} else if stored, eErr := encryptSnapshot(snap); eErr != nil {
		log.Printf("Failed to encrypt square_request_snapshot for gift-card payment %s: %v", buyPaymentID, eErr)
	} else if _, sErr := db.Conn.Exec(ctx, `UPDATE payments SET square_request_snapshot = $1 WHERE id = $2`, string(stored), buyPaymentID); sErr != nil {
		log.Printf("Failed to store square_request_snapshot for gift-card payment %s: %v", buyPaymentID, sErr)
	}

	paymentResult, err := SquareClient.CreatePayment(ctx, paymentReq)
	if err != nil {
		log.Printf("Failed to process gift card purchase payment: %v", err)
		// Payment record intentionally left as 'pending' for manual retry.
		http.Error(w, "Payment failed", chargeFailureStatus(err))
		return
	}

	// Step 3: Square succeeded — atomically flip the payment to completed and
	// create the gift card + balance + transaction in ONE transaction. If any
	// step fails, the whole thing rolls back, the payment stays 'pending', and
	// a same-key retry re-attempts the Square charge (Square dedups) before
	// delivering the card. Previously these were separate non-transactional
	// writes: a failure after the payment-completed update left the customer
	// CHARGED but with no card, and the completed-dedup swallowed the retry.
	issueTx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin gift-card issue transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := issueTx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback gift-card issue transaction", "err", err)
		}
	}()

	_, upErr := issueTx.Exec(ctx,
		`UPDATE payments SET status = 'completed', square_payment_id = $1 WHERE id = $2`,
		paymentResult.SquarePayID, buyPaymentID,
	)
	if upErr != nil {
		log.Printf("CRITICAL: Square payment succeeded (ID=%s) but payment %s update failed: %v — manual reconciliation required", paymentResult.SquarePayID, buyPaymentID, upErr)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// MEDIUM-2: a saved-card gift-card purchase reached its terminal SUCCESS
	// state — consume the verified 2FA code now, inside the transaction that
	// records the completed charge (the gate verified without consuming, so a
	// failed/ambiguous Square charge did not burn the code and a same-key
	// retry could re-verify the SAME code).
	if (req.CardID != nil && *req.CardID != "") || req.SaveCard {
		if consErr := twofa.ConsumePendingCode(ctx, issueTx, userID); consErr != nil {
			log.Printf("CRITICAL: Square payment succeeded (ID=%s) but consuming the 2FA code for user %s failed: %v — manual reconciliation required", paymentResult.SquarePayID, userID, consErr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
	}

	var cardID string

	expiryMonths, err := GetGiftCardExpiryMonths(ctx, issueTx)
	if err != nil {
		log.Printf("Failed to query gift card expiry months (using default %d): %v", defaultGiftCardExpiryMonths, err)
		expiryMonths = defaultGiftCardExpiryMonths
	}

	if req.RecipientType == "self" {
		var purchaseVoucherType string
		err = issueTx.QueryRow(ctx, `SELECT COALESCE(voucher_type, 'SPV') FROM business_settings LIMIT 1`).Scan(&purchaseVoucherType)
		if err != nil {
			log.Printf("Failed to query voucher type: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		if purchaseVoucherType == "" {
			purchaseVoucherType = "SPV"
		}
		// HMRC VAT Notice 700/7: write the EFFECTIVE type (SPV) so
		// voucher_type_at_purchase never records 'MPV' and redemption never
		// applies deferred VAT a second time.
		purchaseVoucherType = effectiveVoucherTypeForPurchase(purchaseVoucherType)
		err = issueTx.QueryRow(ctx, `
			INSERT INTO gift_cards (total_funds_added, amount_remaining, created_by, redeemed_at, redeemed_by, is_inventory, last_used_at, expiry_date, voucher_type_at_purchase)
			VALUES ($1, 0, $2, NOW(), $2, FALSE, NOW(), NOW() + ($4 * INTERVAL '1 month'), $3)
			RETURNING id
		`, amountPounds, userID, purchaseVoucherType, expiryMonths).Scan(&cardID)
		if err != nil {
			log.Printf("CRITICAL: Square payment succeeded (ID=%s) but gift card creation failed: %v — manual reconciliation required", paymentResult.SquarePayID, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		_, err = issueTx.Exec(ctx, `
			INSERT INTO user_giftcard_balances (user_id, balance, updated_at)
			VALUES ($1, $2, NOW())
			ON CONFLICT (user_id) DO UPDATE SET
				balance = user_giftcard_balances.balance + EXCLUDED.balance,
				updated_at = NOW()
		`, userID, amountPounds)
		if err != nil {
			log.Printf("CRITICAL: Square payment succeeded (ID=%s) but balance update for user %s failed: %v — manual reconciliation required", paymentResult.SquarePayID, userID, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		_, err = issueTx.Exec(ctx, `
			INSERT INTO gift_card_transactions (gift_card_id, transaction_type, amount, reference_type, reference_id, user_id, notes)
			VALUES ($1, 'purchase', $2, 'api', NULL, $3, 'self-purchase, auto-redeemed')
		`, cardID, amountPounds, userID)
		if err != nil {
			log.Printf("CRITICAL: Square payment succeeded (ID=%s) but gift card transaction record failed: %v — manual reconciliation required", paymentResult.SquarePayID, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
	} else {
		var purchaseVoucherType string
		err = issueTx.QueryRow(ctx, `SELECT COALESCE(voucher_type, 'SPV') FROM business_settings LIMIT 1`).Scan(&purchaseVoucherType)
		if err != nil {
			log.Printf("Failed to query voucher type: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		if purchaseVoucherType == "" {
			purchaseVoucherType = "SPV"
		}
		// HMRC VAT Notice 700/7: write the EFFECTIVE type (SPV) so
		// voucher_type_at_purchase never records 'MPV' and redemption never
		// applies deferred VAT a second time.
		purchaseVoucherType = effectiveVoucherTypeForPurchase(purchaseVoucherType)
		err = issueTx.QueryRow(ctx, `
			INSERT INTO gift_cards (total_funds_added, amount_remaining, created_by, is_inventory, last_used_at, expiry_date, voucher_type_at_purchase)
			VALUES ($1, $1, $2, FALSE, NOW(), NOW() + ($4 * INTERVAL '1 month'), $3)
			RETURNING id
		`, amountPounds, userID, purchaseVoucherType, expiryMonths).Scan(&cardID)
		if err != nil {
			log.Printf("CRITICAL: Square payment succeeded (ID=%s) but gift card creation failed: %v — manual reconciliation required", paymentResult.SquarePayID, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		recipient := req.RecipientEmail
		if recipient == "" {
			var userEmail string
			err = issueTx.QueryRow(ctx, "SELECT email FROM users WHERE id = $1", userID).Scan(&userEmail)
			if err != nil {
				log.Printf("Failed to query user email: %v", err)
				http.Error(w, "internal server error", http.StatusInternalServerError)
				return
			}
			recipient = userEmail
		}
		// TODO: send gift card code via email to recipient once SMTP is wired.
		// Do NOT log the spendable gift-card code — it is a credential (12-digit
		// code anyone can redeem). Log only the value and redacted recipient for
		// audit — the full email is third-party PII with no retention coverage
		// and the raw free-text field is a log-injection vector.
		log.Printf("Gift card purchased for friend — value: £%.2f, intended for: %s (code stored in DB, not logged)", amountPounds, redactEmail(recipient))

		_, err = issueTx.Exec(ctx, `
			INSERT INTO gift_card_transactions (gift_card_id, transaction_type, amount, reference_type, reference_id, user_id, notes)
			VALUES ($1, 'purchase', $2, 'api', NULL, $3, 'purchased for friend')
		`, cardID, amountPounds, userID)
		if err != nil {
			log.Printf("CRITICAL: Square payment succeeded (ID=%s) but gift card transaction record failed: %v — manual reconciliation required", paymentResult.SquarePayID, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
	}

	if err := issueTx.Commit(ctx); err != nil {
		log.Printf("CRITICAL: Square payment succeeded (ID=%s) but gift-card issue transaction commit failed: %v — manual reconciliation required", paymentResult.SquarePayID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	w.WriteHeader(http.StatusCreated)
	if err := json.NewEncoder(w).Encode(map[string]any{
		"status": "success",
		"code":   cardID,
		"amount": amountPounds,
	}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

// deriveGiftCardIdempotencyKey returns the deterministic fallback idempotency
// key for a no-client-key gift-card purchase:
// "gc-<userID>-<amount>-<recipientType>-<cardPart>", sha256-truncated when the
// verbatim form exceeds Square's 45-char limit (the hash stays deterministic).
//
// The key must distinguish "same live purchase retried" (dedup) from "new
// purchase that happens to be identical" (new charge). The candidate is the
// base key (seq 0) then the base key with a "-<seq>" suffix (seq >= 1) until a
// slot without a COMPLETED or swept/declined FAILED purchase is found. A
// COMPLETED purchase always advances the sequence — two genuine identical
// no-key purchases (e.g. two £20 self cards) are distinct operations and must
// diverge onto distinct keys (the old random-suffix fallback's collapse fix),
// and a FAILED purchase (swept stale or definitively rejected) occupies its
// slot the same way so the customer's identical repurchase diverges onto a
// fresh key instead of being 409-rejected forever (A10). A PENDING row never
// occupies a slot: a lost-response retry re-derives the base key, the
// idempotency lookup below reuses the pending row, and Square's same-key dedup
// returns the original charge — ONE charge instead of the old double-charge.
// Must be called under the crussell:giftcard advisory lock so the spent-slot
// scan races no concurrent purchase (mirrors deriveBookingPaymentIdempotencyKey
// in handlers.go).
func deriveGiftCardIdempotencyKey(ctx context.Context, q db.Querier, userID string, amount int64, recipientType, cardPart string) (string, error) {
	baseKey := fmt.Sprintf("gc-%s-%d-%s-%s", userID, amount, recipientType, cardPart)
	for seq := 0; ; seq++ {
		candidate := nextIdempotencyCandidate(baseKey, seq)
		var occupiedID string
		err := q.QueryRow(ctx, `
			SELECT id FROM payments
			WHERE created_by = $1 AND idempotency_key = $2 AND status IN ('completed', 'failed')
		`, userID, candidate).Scan(&occupiedID)
		if errors.Is(err, pgx.ErrNoRows) {
			return candidate, nil
		}
		if err != nil {
			return "", err
		}
	}
}

// --- Helpers ---

// redactEmail masks an email for logs (PII — third-party addresses are not
// covered by retention/anonymization, and the raw free-text field is a
// log-injection vector). Mirrors the dev mock's redaction: first two chars of
// the local part plus the domain, e.g. "ja***@example.com"; malformed
// addresses fall back to "[redacted]". The full email stays in the DB row.
func redactEmail(email string) string {
	at := strings.Index(email, "@")
	if at < 2 || at+1 >= len(email) {
		return "[redacted]"
	}
	return email[:2] + "***@" + email[at+1:]
}

type ExpiredBalance struct {
	ID              string     `json:"id"`
	AccountID       *string    `json:"account_id,omitempty"`
	OriginalBalance float64    `json:"original_balance"`
	ExpiredAt       time.Time  `json:"expired_at"`
	ClaimedAt       *time.Time `json:"claimed_at,omitempty"`
	ClaimedByAdmin  *string    `json:"claimed_by_admin,omitempty"`
	Notes           *string    `json:"notes,omitempty"`
}

func GetExpiredBalances(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	rows, err := db.Conn.Query(ctx, `
		SELECT id, account_id, original_balance, expired_at, claimed_at, claimed_by_admin, notes
		FROM gift_card_expired_balances
		ORDER BY expired_at DESC
	`)
	if err != nil {
		log.Printf("Failed to query expired balances: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer rows.Close()

	var balances []ExpiredBalance
	for rows.Next() {
		var b ExpiredBalance
		var accountID, claimedByAdmin, notes sql.NullString
		var claimedAt sql.NullTime

		err = rows.Scan(&b.ID, &accountID, &b.OriginalBalance, &b.ExpiredAt, &claimedAt, &claimedByAdmin, &notes)
		if err != nil {
			log.Printf("Failed to scan expired balance: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		if accountID.Valid {
			b.AccountID = &accountID.String
		}
		if claimedAt.Valid {
			b.ClaimedAt = &claimedAt.Time
		}
		if claimedByAdmin.Valid {
			b.ClaimedByAdmin = &claimedByAdmin.String
		}
		if notes.Valid {
			b.Notes = &notes.String
		}

		balances = append(balances, b)
	}
	if err := rows.Err(); err != nil {
		log.Printf("Row iteration error: %v", err)
		http.Error(w, "Internal server error", http.StatusInternalServerError)
		return
	}

	if balances == nil {
		balances = []ExpiredBalance{}
	}

	if err := json.NewEncoder(w).Encode(map[string]any{
		"expired_balances": balances,
		"total":            len(balances),
	}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

type ClaimExpiredBalanceRequest struct {
	BalanceID string  `json:"balance_id"`
	Notes     *string `json:"notes,omitempty"`
}

func ClaimExpiredBalance(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	adminID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || adminID == "" {
		http.Error(w, "Unauthorized", http.StatusUnauthorized)
		return
	}

	var req ClaimExpiredBalanceRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "Invalid request body", http.StatusBadRequest)
		return
	}

	if req.BalanceID == "" {
		http.Error(w, "balance_id is required", http.StatusBadRequest)
		return
	}

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback transaction", "err", err)
		}
	}()

	var existingClaimedAt sql.NullTime
	err = tx.QueryRow(ctx, `
		SELECT claimed_at FROM gift_card_expired_balances WHERE id = $1 FOR UPDATE
	`, req.BalanceID).Scan(&existingClaimedAt)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			http.Error(w, "Expired balance not found", http.StatusNotFound)
			return
		}
		log.Printf("Failed to check expired balance: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if existingClaimedAt.Valid {
		http.Error(w, "Balance already claimed", http.StatusConflict)
		return
	}

	_, err = tx.Exec(ctx, `
		UPDATE gift_card_expired_balances
		SET claimed_at = NOW(), claimed_by_admin = $1, notes = $2
		WHERE id = $3
	`, adminID, req.Notes, req.BalanceID)
	if err != nil {
		log.Printf("Failed to claim expired balance: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	if err := tx.Commit(ctx); err != nil {
		log.Printf("Failed to commit transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	w.WriteHeader(http.StatusOK)
	if err := json.NewEncoder(w).Encode(map[string]string{"status": "claimed"}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

// --- 14-day cooling-off cancellation (C3) ---
//
// The Gift Card T&C is a binding UK consumer contract that grants a 14-day
// right to cancel online gift-card purchases with a refund to the original
// payment method (Consumer Contracts (Information, Cancellation and Additional
// Charges) Regulations 2013). Until this endpoint existed there was NO code
// path to execute that right: RefundPayment explicitly rejects
// gift-card-purchase payments (booking_id IS NULL) and no cancel route/UI
// existed — a legal exposure.
//
// Eligibility signal for "purchased online by this user within 14 days":
//   - A gift_card_transactions row on the card with transaction_type='purchase',
//     reference_type='api' and user_id = the authenticated caller. BuyGiftCard
//     (the only customer-facing online purchase path) writes exactly this.
//     Till/admin sales can NEVER match: till sales write reference_type
//     'till_sale' (see till.go) and admin-created cards (CreateGiftCard) write
//     reference_type='api' but with the ADMIN's user id, never the caller's.
//   - The card itself must be unredeemed (redeemed_by IS NULL — self-purchases
//     auto-redeem and are therefore not cancellable), non-inventory, not
//     expired, and still hold exactly its original purchase value
//     (total_funds_added == amount_remaining == purchase amount — a top-up,
//     transfer, or spend breaks this).
//   - The originating payments row: booking_id IS NULL (the same discriminator
//     RefundPayment and the stale-pending sweep use for gift-card purchases),
//     created_by = the buyer, payment_method='online_square', status='completed',
//     with a square_payment_id — the target of the refund.

// giftCardCoolingOffPeriod is the 14-day statutory cancellation window
// (Consumer Contracts (Information, Cancellation and Additional Charges)
// Regulations 2013, regs. 29-38).
const giftCardCoolingOffPeriod = 14 * 24 * time.Hour

// giftCardCancelRefundReason is the Square refund reason. Square's
// refund-reason limit is 192 chars; this is comfortably under it.
const giftCardCancelRefundReason = "Gift card cancelled within 14-day cooling-off period (Consumer Contracts Regulations 2013)"

type CancelGiftCardRequest struct {
	Code string `json:"code"`
	// PaymentID optionally identifies the originating purchase payment so the
	// handler skips the amount/timing match. When omitted the purchase payment
	// is located from the gift-card purchase transaction.
	PaymentID string `json:"payment_id,omitempty"`
}

// MyGiftCard is one of the caller's online-purchased (unredeemed) gift cards,
// with the expiry date (T&C: "the expiry date is displayed in your account")
// and whether the 14-day cancellation right currently applies.
type MyGiftCard struct {
	Code               string     `json:"code"`
	Amount             float64    `json:"amount"`
	PurchasedAt        time.Time  `json:"purchased_at"`
	ExpiryDate         *time.Time `json:"expiry_date,omitempty"`
	Cancellable        bool       `json:"cancellable"`
	CancellationReason string     `json:"cancellation_reason,omitempty"`
	PaymentID          string     `json:"payment_id,omitempty"`
}

type MyGiftCardsResponse struct {
	GiftCards []MyGiftCard `json:"gift_cards"`
}

// GetMyGiftCards lists the authenticated user's customer-facing online
// gift-card purchases that are still held as cards (unredeemed — a "self"
// purchase is auto-redeemed into the pooled balance and so never appears
// here). Each entry carries the rolling expiry date and whether the 14-day
// cooling-off cancellation right applies, so the account page can surface a
// "Cancel & refund" action exactly where the consumer is legally entitled to
// one.
func GetMyGiftCards(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	userID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || userID == "" {
		http.Error(w, "Authentication required", http.StatusUnauthorized)
		return
	}

	// Only gift_card_transactions rows created by BuyGiftCard match
	// (reference_type='api' + user_id = caller); till sales ('till_sale') and
	// admin-created cards (user_id = admin) never appear.
	rows, err := db.Conn.Query(ctx, `
		SELECT gc.id, gc.total_funds_added, gc.amount_remaining, gc.is_inventory,
		       gc.expiry_date, gct.amount AS purchase_amount, gct.created_at AS purchased_at
		FROM gift_card_transactions gct
		JOIN gift_cards gc ON gc.id = gct.gift_card_id
		WHERE gct.user_id = $1 AND gct.transaction_type = 'purchase'
		  AND gct.reference_type = 'api'
		  AND gc.redeemed_by IS NULL
		ORDER BY gct.created_at DESC
	`, userID)
	if err != nil {
		log.Printf("Failed to query user gift cards: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// Drain all rows before the per-card payment/refund lookups below —
	// pgx.Tx (used by the test harness via a context-stored transaction) does
	// not support concurrent queries on one connection.
	type giftCardRow struct {
		code           string
		totalFunds     float64
		remaining      float64
		isInventory    bool
		expiryDate     sql.NullTime
		purchaseAmount float64
		purchasedAt    time.Time
	}
	var drained []giftCardRow
	for rows.Next() {
		var r giftCardRow
		if err := rows.Scan(&r.code, &r.totalFunds, &r.remaining, &r.isInventory, &r.expiryDate, &r.purchaseAmount, &r.purchasedAt); err != nil {
			log.Printf("Failed to scan user gift card: %v", err)
			continue
		}
		drained = append(drained, r)
	}
	rows.Close()
	if err := rows.Err(); err != nil {
		log.Printf("User gift card row iteration error: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	out := []MyGiftCard{}
	for _, r := range drained {
		gc := MyGiftCard{
			Code:        r.code,
			Amount:      r.purchaseAmount,
			PurchasedAt: r.purchasedAt,
		}
		if r.expiryDate.Valid {
			expiry := r.expiryDate.Time
			gc.ExpiryDate = &expiry
		}

		status := assessGiftCardCancellation(ctx, db.Conn, r.code, r.totalFunds, r.remaining, r.isInventory, r.expiryDate, r.purchaseAmount, r.purchasedAt, userID)
		gc.Cancellable = status.Cancellable
		gc.CancellationReason = status.CancellationReason
		gc.PaymentID = status.PaymentID
		out = append(out, gc)
	}
	if out == nil {
		out = []MyGiftCard{}
	}

	if err := json.NewEncoder(w).Encode(MyGiftCardsResponse{GiftCards: out}); err != nil {
		log.Printf("Failed to encode JSON response: %v", err)
	}
}

// giftCardCancellationStatus captures whether a gift card is currently
// cancellable under the 14-day cooling-off right, and why not when it is not.
type giftCardCancellationStatus struct {
	Cancellable        bool
	CancellationReason string
	PaymentID          string // the originating purchase payment, when cancellable
}

// assessGiftCardCancellation evaluates a single gift card's cancellation
// eligibility under the 14-day cooling-off rules (CCR 2013 regs. 29-38) using
// the EXACT same signal set and reason strings as the user-facing
// GetMyGiftCards list, so the admin list can surface the same state.
//
// purchaserID is the owner of the card's online purchase transaction
// (transaction_type='purchase', reference_type='api'); empty means the card
// was never purchased online (inventory/till/admin-created), so it can never
// be cancellable. The caller must have drained all open rows before calling —
// q must not be an open rows cursor (pgx.Tx cannot run a second query on the
// same connection).
func assessGiftCardCancellation(ctx context.Context, q db.Querier, code string, totalFunds, remaining float64, isInventory bool, expiryDate sql.NullTime, purchaseAmount float64, purchasedAt time.Time, purchaserID string) giftCardCancellationStatus {
	var st giftCardCancellationStatus

	if purchaserID == "" {
		st.CancellationReason = "This gift card was not purchased online, so it cannot be cancelled"
		return st
	}

	// A partially spent card is only cancellable when its shortfall is
	// verified till spend (reg 34(9) refunds the UNSPENT balance); top-ups
	// and unaccounted shortfalls (transfers/clawbacks) are not.
	partialSpendOK := true
	if !approxEqual(totalFunds, purchaseAmount) {
		partialSpendOK = false
	} else if !approxEqual(remaining, purchaseAmount) {
		if remaining <= 0 {
			partialSpendOK = false
		} else {
			spentAtTill, serr := giftCardSpendAtTill(ctx, q, code)
			if serr != nil {
				log.Printf("Failed to verify till spend for gift card %s: %v", code, serr)
				partialSpendOK = false
			} else {
				partialSpendOK = approxEqual(spentAtTill+remaining, purchaseAmount)
			}
		}
	}

	switch {
	case isInventory:
		st.CancellationReason = "This card was created as shop stock, not purchased online"
	case !approxEqual(totalFunds, purchaseAmount):
		st.CancellationReason = "This gift card has been topped up, transferred, or otherwise altered"
	case !approxEqual(remaining, purchaseAmount) && !partialSpendOK:
		if remaining <= 0 {
			st.CancellationReason = "This gift card has no remaining balance to refund"
		} else {
			st.CancellationReason = "This gift card has been topped up, transferred, or partially spent in a way that cannot be verified"
		}
	case expiryDate.Valid && expiryDate.Time.Before(clock.Now()):
		st.CancellationReason = "This gift card has expired"
	case purchasedAt.Before(clock.Now().Add(-giftCardCoolingOffPeriod)):
		st.CancellationReason = "The 14-day cancellation period has expired"
	default:
		paymentID, _, paymentOK, perr := findGiftCardPurchasePayment(ctx, q, purchaserID, "", purchaseAmount, purchasedAt)
		if perr != nil {
			log.Printf("Failed to locate purchase payment for gift card %s: %v", code, perr)
			st.CancellationReason = "The original purchase payment could not be verified"
		} else if !paymentOK {
			st.CancellationReason = "The original purchase payment could not be found"
		} else {
			// A completed/pending refund row means the money already
			// returned (or is in flight) — the card cannot be cancelled a
			// second time.
			var refundStatus string
			err := q.QueryRow(ctx, `
				SELECT status FROM refunds
				WHERE payment_id = $1 AND status IN ('completed', 'pending')
				LIMIT 1`, paymentID).Scan(&refundStatus)
			switch {
			case err == nil:
				st.CancellationReason = "This gift card has already been refunded"
			case errors.Is(err, pgx.ErrNoRows):
				st.Cancellable = true
				st.PaymentID = paymentID
			default:
				log.Printf("Failed to check refund status for payment %s: %v", paymentID, err)
				st.CancellationReason = "Unable to verify the refund status of this gift card"
			}
		}
	}
	return st
}

// CancelGiftCard resolves the authenticated caller as the cancellation actor
// and delegates to the shared cancellation core (see cancelGiftCardForUser for
// the full money-safety contract).
func CancelGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	userID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || userID == "" {
		http.Error(w, "Authentication required", http.StatusUnauthorized)
		return
	}

	var req CancelGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	cancelGiftCardForUser(ctx, w, r, userID, req)
}

// AdminCancelGiftCard is the admin-side cancellation surface for the 14-day
// cooling-off right: it lets a partially spent online-purchased card be
// managed from the admin Gift Card Management screen. The admin route group
// (mw.RequireAdmin in main.go) guarantees the role; the admin's user id is the
// ACTOR (recorded on the refunds row and the 'cancelled' audit transaction),
// while the cancellation core still verifies the card's purchase transaction
// against its ACTUAL purchaser, so an admin can act on behalf of the real
// owner without weakening the money-safety invariants.
func AdminCancelGiftCard(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	adminID, ok := ctx.Value(mw.UserIDKey).(string)
	if !ok || adminID == "" {
		http.Error(w, "Authentication required", http.StatusUnauthorized)
		return
	}
	if !isAdminRequest(r) {
		http.Error(w, "Admin access required", http.StatusForbidden)
		return
	}

	var req CancelGiftCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	cancelGiftCardForUser(ctx, w, r, adminID, req)
}

// cancelGiftCardForUser executes the statutory 14-day right to cancel an
// online gift-card purchase: it issues a Square refund to the original payment
// method — the FULL purchase value for an unspent card, or the UNSPENT
// remainder for a card whose shortfall is verified till spend (reg 34(9)) —
// then reverses the gift-card funding (amount_remaining zeroed and the
// card expired so the balance can never be spent). Registered under
// mw.RequireAuth + mw.RequireNonGuest (guests cannot buy gift cards); the
// admin surface (AdminCancelGiftCard) reuses this same core with the admin as
// the actor.
//
// Failure safety: the refunds row is inserted (pending) BEFORE the Square call
// with a deterministic idempotency key; if Square fails, the row stays pending
// for a same-key retry (Square dedups) and the gift card is NOT touched — the
// customer keeps the card and the request returns 5xx for a retry. Only once
// Square accepts the refund is the card reversed, so a refund can never be
// issued without the card value being neutralised, and the card can never be
// cancelled while the money is still with the salon.
func cancelGiftCardForUser(ctx context.Context, w http.ResponseWriter, r *http.Request, userID string, req CancelGiftCardRequest) {
	code := validators.NormalizeGiftCardCode(req.Code)
	if !validators.IsValidID(code) {
		http.Error(w, "Invalid gift card code format", http.StatusBadRequest)
		return
	}
	if req.PaymentID != "" && !validators.IsValidID(req.PaymentID) {
		http.Error(w, "Invalid payment id", http.StatusBadRequest)
		return
	}

	// Serialize cancellation attempts per gift card (bounded try-lock,
	// mirroring the RefundPayment handler) so two concurrent cancels of the
	// same card cannot both pass the eligibility + refunds-row guards.
	pinConn, err := db.Conn.Acquire(ctx)
	if err != nil {
		log.Printf("Failed to acquire connection for gift-card cancel lock: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer pinConn.Release()
	lockOK, err := acquireAdvisoryLock(ctx, pinConn, "crussell:giftcard-cancel:"+code)
	if err != nil {
		log.Printf("Failed to acquire gift-card cancel lock for %s: %v", code, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if !lockOK {
		log.Printf("Gift-card cancel lock for %s not acquired within bound — a cancellation is already in progress", code)
		http.Error(w, "A gift-card cancellation is already in progress, try again", http.StatusConflict)
		return
	}
	defer releasePaymentLock(pinConn, "crussell:giftcard-cancel:"+code)

	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		log.Printf("Failed to begin gift-card cancel transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback gift-card cancel transaction", "err", err)
		}
	}()

	// Lock the card FOR UPDATE so concurrent cancels / top-ups / transfers
	// serialize on the eligibility read below.
	var totalFunds, remaining float64
	var redeemedBy sql.NullString
	var isInventory bool
	var expiryDate sql.NullTime
	err = tx.QueryRow(ctx, `
		SELECT total_funds_added, amount_remaining, redeemed_by, is_inventory, expiry_date
		FROM gift_cards WHERE id = $1 FOR UPDATE`, code).
		Scan(&totalFunds, &remaining, &redeemedBy, &isInventory, &expiryDate)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			http.Error(w, "Gift card not found", http.StatusNotFound)
			return
		}
		log.Printf("Failed to load gift card %s for cancellation: %v", code, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// --- Eligibility (C3) ---
	if isInventory {
		http.Error(w, "This gift card was created as shop stock, not purchased online — it cannot be cancelled", http.StatusBadRequest)
		return
	}
	if redeemedBy.Valid {
		http.Error(w, "This gift card has already been redeemed to an account balance and cannot be cancelled", http.StatusBadRequest)
		return
	}

	// The card must be traceable to a customer-facing online purchase (see the
	// signal documentation above). The purchase row is resolved WITHOUT
	// filtering on the actor's user id so the card's TRUE purchaser is found:
	// in the user flow the actor must BE the purchaser (enforced below), while
	// an admin acts on behalf of the card's real owner.
	var purchaseAmount float64
	var purchasedAt time.Time
	var purchaserID string
	err = tx.QueryRow(ctx, `
		SELECT amount, created_at, COALESCE(user_id, '')
		FROM gift_card_transactions
		WHERE gift_card_id = $1 AND transaction_type = 'purchase'
		  AND reference_type = 'api'
		ORDER BY created_at DESC
		LIMIT 1`, code).Scan(&purchaseAmount, &purchasedAt, &purchaserID)
	if errors.Is(err, pgx.ErrNoRows) {
		if isAdminRequest(r) {
			http.Error(w, "This gift card was not purchased online, so it cannot be cancelled", http.StatusBadRequest)
		} else {
			http.Error(w, "This gift card was not purchased online by your account, so it cannot be cancelled here", http.StatusBadRequest)
		}
		return
	}
	if err != nil {
		log.Printf("Failed to load gift-card purchase transaction for %s: %v", code, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if purchaserID != userID && !isAdminRequest(r) {
		http.Error(w, "This gift card was not purchased online by your account, so it cannot be cancelled here", http.StatusBadRequest)
		return
	}

	if purchasedAt.Before(clock.Now().Add(-giftCardCoolingOffPeriod)) {
		http.Error(w, "The 14-day cancellation period for this gift card has expired", http.StatusBadRequest)
		return
	}

	// Eligibility (C3): a full-refund cancellation is only valid while the
	// card still holds exactly its original purchase value. A partially spent
	// card remains cancellable when the shortfall is verified till spend —
	// reg 34(9) entitles the consumer to the UNSPENT balance back while the
	// value already consumed on services stays with the salon. Top-ups,
	// redeemed cards, and unaccounted shortfalls (transfers/clawbacks) are
	// rejected outright.
	refundAmount := purchaseAmount
	partialRefund := false
	var spentAtTill float64
	if !approxEqual(totalFunds, purchaseAmount) {
		http.Error(w, "This gift card has been topped up, transferred, or partially spent and can no longer be cancelled for a full refund", http.StatusBadRequest)
		return
	}
	if !approxEqual(remaining, purchaseAmount) {
		if remaining <= 0 {
			http.Error(w, "This gift card has no remaining balance to refund", http.StatusBadRequest)
			return
		}
		spentAtTill, err = giftCardSpendAtTill(ctx, tx, code)
		if err != nil {
			log.Printf("Failed to verify till spend for gift card %s: %v", code, err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		if !approxEqual(spentAtTill+remaining, purchaseAmount) {
			http.Error(w, "This gift card has been partially spent, transferred, or otherwise altered and can no longer be cancelled; the remaining balance can be spent or redeemed to your account balance", http.StatusBadRequest)
			return
		}
		refundAmount = remaining
		partialRefund = true
	}

	if expiryDate.Valid && expiryDate.Time.Before(clock.Now()) {
		http.Error(w, "This gift card has already expired and cannot be cancelled", http.StatusBadRequest)
		return
	}

	// --- Locate the originating purchase payment (Square charge) ---
	// The payment is matched against the card's TRUE purchaser, never the
	// actor: for the user flow purchaserID == userID (enforced above), and for
	// an admin cancellation the card's owner is used.
	paymentID, squarePaymentID, paymentOK, perr := findGiftCardPurchasePayment(ctx, tx, purchaserID, req.PaymentID, purchaseAmount, purchasedAt)
	if perr != nil {
		log.Printf("Failed to locate purchase payment for gift card %s: %v", code, perr)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if !paymentOK {
		http.Error(w, "The original purchase payment for this gift card could not be found, so it cannot be refunded", http.StatusBadRequest)
		return
	}

	refundKey := paymentID + "-gccancel-" + strconv.FormatInt(int64(math.Round(refundAmount*100)), 10)
	refundID := ""
	resumingRefund := false
	var priorRefundID, priorStatus, priorKey string
	var priorAmount float64
	err = tx.QueryRow(ctx, `
		SELECT id, status, amount, COALESCE(idempotency_key, '')
		FROM refunds WHERE payment_id = $1
		ORDER BY created_at DESC, id DESC LIMIT 1`, paymentID).
		Scan(&priorRefundID, &priorStatus, &priorAmount, &priorKey)
	switch {
	case errors.Is(err, pgx.ErrNoRows):
		// Fresh cancellation — insert the pending refund row below.
	case err != nil:
		log.Printf("Failed to check prior refund for payment %s: %v", paymentID, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	default:
		switch priorStatus {
		case "completed":
			if priorAmount >= refundAmount-0.005 {
				// The full cancellation entitlement (the full purchase value,
				// or the unspent remainder of a partially spent card) was
				// already refunded. Neutralise the card so the value cannot
				// be spent on top of the returned money, then report.
				if remaining > 0 {
					if cerr := cancelGiftCardFunding(ctx, tx, code, userID, priorRefundID, refundAmount); cerr != nil {
						log.Printf("CRITICAL: gift card %s was already refunded but funding reversal failed: %v — MANUAL RECONCILIATION REQUIRED", code, cerr)
						http.Error(w, "internal server error", http.StatusInternalServerError)
						return
					}
				}
				if cerr := tx.Commit(ctx); cerr != nil {
					log.Printf("Failed to commit gift-card cancel transaction: %v", cerr)
					http.Error(w, "internal server error", http.StatusInternalServerError)
					return
				}
				w.WriteHeader(http.StatusOK)
				if err := json.NewEncoder(w).Encode(map[string]any{
					"status":          "success",
					"message":         "This gift card was already refunded and has now been cancelled.",
					"amount_refunded": refundAmount,
				}); err != nil {
					log.Printf("Failed to encode JSON response: %v", err)
				}
				return
			}
			// Partial prior refund (C6/F8): the card still holds its full
			// cancellation entitlement (the full purchase value, or the unspent
			// remainder of a partially spent card) but only part of it was
			// returned. Refund the DIFFERENCE via Square with a fresh
			// deterministic key before neutralising, so the un-refunded remainder
			// is never silently swallowed.
			refundAmount = refundAmount - priorAmount
			refundKey = paymentID + "-gccancel-diff-" + strconv.FormatInt(int64(math.Round(refundAmount*100)), 10)
			log.Printf("Gift-card purchase %s has a prior partial refund of £%.2f — issuing the £%.2f remainder", paymentID, priorAmount, refundAmount)
		case "pending":
			// In-flight refund for this payment. Only resume if the pending
			// row is OURS; a foreign pending row must not be re-issued.
			if !strings.HasPrefix(priorKey, paymentID+"-gccancel-") {
				log.Printf("Gift-card cancel rejected: payment %s has a pending refund row %s from another flow", paymentID, priorRefundID)
				http.Error(w, "A refund for this gift card purchase is already being processed — please try again later", http.StatusConflict)
				return
			}
			refundID = priorRefundID
			refundKey = priorKey
			refundAmount = priorAmount
			resumingRefund = true
		case "failed":
			// Money provably never moved — safe to re-issue with the stored
			// key (or persist the deterministic key when a legacy row has
			// none, mirroring ensureRefundKey).
			refundID = priorRefundID
			refundAmount = priorAmount
			resumingRefund = true
			if priorKey != "" {
				refundKey = priorKey
			} else {
				if _, uErr := tx.Exec(ctx, `
					UPDATE refunds SET idempotency_key = $1
					WHERE id = $2 AND idempotency_key IS NULL`, refundKey, refundID); uErr != nil {
					log.Printf("Failed to persist refund idempotency key for %s: %v", refundID, uErr)
					http.Error(w, "internal server error", http.StatusInternalServerError)
					return
				}
			}
		}
	}

	if refundID == "" {
		// Insert the pending refund row. booking_id stays NULL for
		// gift-card purchases (the payments row has no booking).
		err = tx.QueryRow(ctx, `
			INSERT INTO refunds (payment_id, booking_id, amount, status, reason, idempotency_key, created_by, created_at, origin)
			VALUES ($1, NULL, $2, 'pending', $3, $4, $5, $6, 'giftcard_cancel')
			ON CONFLICT (idempotency_key) DO NOTHING
			RETURNING id`,
			paymentID, refundAmount, giftCardCancelRefundReason, refundKey, userID, clock.Now()).Scan(&refundID)
		if errors.Is(err, pgx.ErrNoRows) {
			// A same-key row exists from a concurrent attempt (unreachable
			// under the advisory lock, but stay safe): treat it as ours.
			if rErr := tx.QueryRow(ctx, `SELECT id FROM refunds WHERE idempotency_key = $1`, refundKey).Scan(&refundID); rErr != nil {
				log.Printf("Failed to re-read concurrent refund row for key %s: %v", refundKey, rErr)
				http.Error(w, "internal server error", http.StatusInternalServerError)
				return
			}
		} else if err != nil {
			log.Printf("Failed to insert gift-card cancel refund record: %v", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
	}

	if err := tx.Commit(ctx); err != nil {
		log.Printf("Failed to commit gift-card cancel transaction: %v", err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// resolveRefundRow updates the committed refunds row to a definitive
	// outcome on the FAILURE paths below, where the reversal transaction has
	// already been rolled back (no money moved / declined / ambiguous). On the
	// success paths the row is instead resolved INSIDE the reversal
	// transaction, so 'completed' commits atomically with the funding
	// reversal (C2/F4) — a completed refund can never coexist with a live
	// card.
	resolveRefundRow := func(status, sqRefundID string) {
		_, upErr := db.Conn.Exec(ctx, `UPDATE refunds SET status = $1, square_refund_id = $2 WHERE id = $3`, status, sqRefundID, refundID)
		if upErr != nil {
			slog.Error("CRITICAL: Square refund resolved but refund row update failed — manual reconciliation required", "refund_id", refundID, "err", upErr)
		}
	}

	// --- Reversal + refund transaction (C2/F2, C2/F4) ---
	// The eligibility transaction above committed with the card still holding
	// its full purchase value. Between that commit and the reversal the card
	// must not be redeemable or transferable — the crussell:giftcard-cancel
	// advisory lock (held for this whole handler, and now also taken by
	// RedeemGiftCard and TransferGiftCard) serializes those operations against
	// this cancellation. The reversal transaction below re-verifies under
	// FOR UPDATE as a second line of defence, and the Square refund is issued
	// only AFTER that re-verification passes.
	rtx, rerr := db.Conn.Begin(ctx)
	if rerr != nil {
		slog.Error("CRITICAL: no transaction available to re-verify and reverse gift card", "gift_card_id", code, "refund_id", refundID, "err", rerr)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	defer func() {
		if err := rtx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback gift-card reversal transaction", "err", err)
		}
	}()

	var revTotalFunds, revRemaining float64
	var revRedeemedBy sql.NullString
	var revIsInventory bool
	var revExpiry sql.NullTime
	err = rtx.QueryRow(ctx, `
		SELECT total_funds_added, amount_remaining, redeemed_by, is_inventory, expiry_date
		FROM gift_cards WHERE id = $1 FOR UPDATE`, code).
		Scan(&revTotalFunds, &revRemaining, &revRedeemedBy, &revIsInventory, &revExpiry)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			http.Error(w, "Gift card not found", http.StatusNotFound)
			return
		}
		log.Printf("Failed to re-load gift card %s for cancellation reversal: %v", code, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}

	// A partially spent card must re-verify against the SAME remaining value
	// that was spend-verified and funded at eligibility; a full-value card
	// re-verifies against the full purchase amount.
	expectRemaining := purchaseAmount
	if partialRefund {
		expectRemaining = remaining
	}
	if revIsInventory || revRedeemedBy.Valid || !approxEqual(revTotalFunds, purchaseAmount) || !approxEqual(revRemaining, expectRemaining) || (revExpiry.Valid && revExpiry.Time.Before(clock.Now())) {
		// The card changed between the eligibility commit and the reversal —
		// redeemed, spent, transferred, or expired. Do NOT refund on top of
		// the live balance (double value). The reversal tx has no writes and
		// rolls back via defer, so the card is untouched.
		if resumingRefund {
			// Resumed a prior pending/failed row — Square money state is
			// unknown, so leave the row for the sweep to reconcile.
			slog.Error("CRITICAL: gift card %s changed state while a gift-card-cancel refund was pending (refund %s) — refund row left pending for sweep reconciliation", "gift_card_id", code, "refund_id", refundID)
		} else {
			// Freshly-inserted row in THIS request — Square was never called,
			// so no money moved: mark it failed so it never blocks a retry.
			// Roll back the reversal tx first so the failed-status write is
			// not undone by the deferred rollback.
			if rbErr := rtx.Rollback(ctx); rbErr != nil && !errors.Is(rbErr, pgx.ErrTxClosed) {
				log.Printf("Failed to rollback gift-card reversal transaction before marking refund failed: %v", rbErr)
			}
			log.Printf("Gift-card cancel aborted for %s: card state changed since eligibility — refund row %s marked failed, Square never called", code, refundID)
			resolveRefundRow("failed", "")
		}
		http.Error(w, "This gift card has been redeemed, spent, or transferred since the cancellation was requested and can no longer be refunded", http.StatusConflict)
		return
	}

	// Re-verified still eligible — issue the Square refund now. The refunds
	// row is already committed (pending) with a deterministic key, so a
	// crash/response-loss retry re-reads the row and resumes with the SAME
	// key — Square dedups and the card is never double-refunded.
	refundResult, sqErr := SquareClient.RefundPayment(ctx, square.RefundPaymentReq{
		PaymentID:      squarePaymentID,
		Amount:         int64(math.Round(refundAmount * 100)),
		IdempotencyKey: refundKey,
		Reason:         giftCardCancelRefundReason,
	})

	switch {
	case sqErr == nil:
		status := "completed"
		if refundResult.Status == "PENDING" {
			// Square accepted the refund; the money is in flight. The card is
			// still neutralised immediately — leaving the balance spendable
			// while the refund is on its way would create value from nothing.
			// The pending row stays for the sweep to reconcile.
			status = "pending"
			log.Printf("Square refund %s for gift-card purchase payment %s is PENDING — refund row left pending for reconciliation", refundResult.ID, paymentID)
		} else if refundResult.Status == "FAILED" || refundResult.Status == "REJECTED" {
			status = "failed"
			log.Printf("Square refund %s for gift-card purchase payment %s FAILED — refund row marked failed", refundResult.ID, paymentID)
		}
		if status == "failed" {
			// Roll back the reversal tx first so the failed-status write is
			// not undone by the deferred rollback.
			if rbErr := rtx.Rollback(ctx); rbErr != nil && !errors.Is(rbErr, pgx.ErrTxClosed) {
				log.Printf("Failed to rollback gift-card reversal transaction before marking refund failed: %v", rbErr)
			}
			resolveRefundRow("failed", refundResult.ID)
			http.Error(w, "Refund failed", http.StatusInternalServerError)
			return
		}

		// Money accepted (COMPLETED) or in flight (PENDING) — reverse the
		// gift-card funding and resolve the refunds row in THIS SAME
		// transaction. On a COMPLETED refund the row flips to 'completed' only
		// when this transaction commits, so a completed refund can never
		// coexist with a live card (C2/F4). A PENDING Square refund stays
		// 'pending' for the sweep. Any failure below rolls the whole
		// transaction back: the card stays live and the row stays pending.
		if cerr := cancelGiftCardFunding(ctx, rtx, code, userID, refundID, refundAmount); cerr != nil {
			slog.Error("CRITICAL: Square refund accepted but gift card %s funding reversal failed — refund row left pending for sweep", "gift_card_id", code, "refund_id", refundID, "err", cerr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		if status == "completed" {
			if _, upErr := rtx.Exec(ctx, `UPDATE refunds SET status = 'completed', square_refund_id = $1 WHERE id = $2`, refundResult.ID, refundID); upErr != nil {
				slog.Error("CRITICAL: Square refund accepted but gift card %s refund row could not be resolved in the reversal transaction — row left pending", "gift_card_id", code, "refund_id", refundID, "err", upErr)
				http.Error(w, "internal server error", http.StatusInternalServerError)
				return
			}
		} else if _, upErr := rtx.Exec(ctx, `UPDATE refunds SET square_refund_id = $1 WHERE id = $2`, refundResult.ID, refundID); upErr != nil {
			slog.Error("CRITICAL: Square refund PENDING but gift card %s refund row could not record square_refund_id — row left pending", "gift_card_id", code, "refund_id", refundID, "err", upErr)
		}
		if cerr := rtx.Commit(ctx); cerr != nil {
			slog.Error("CRITICAL: Square refund accepted but gift card %s reversal commit failed — refund row left pending, card left live", "gift_card_id", code, "refund_id", refundID, "err", cerr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}

		message := "Gift card cancelled and the full amount refunded to your original payment method."
		if partialRefund {
			message = fmt.Sprintf("Gift card cancelled. £%.2f (the unspent portion) was refunded to your original payment method; the £%.2f already spent on salon services is not refundable.", refundAmount, spentAtTill)
		}
		w.WriteHeader(http.StatusOK)
		if err := json.NewEncoder(w).Encode(map[string]any{
			"status":          "success",
			"message":         message,
			"refund_id":       refundID,
			"amount_refunded": refundAmount,
		}); err != nil {
			log.Printf("Failed to encode JSON response: %v", err)
		}
		return

	case errors.Is(sqErr, square.ErrRefundAlreadyProcessed):
		// PAYMENT_ALREADY_REFUNDED — the money already moved at Square (a
		// prior same-key attempt of this flow refunded it). The card was
		// re-verified above and still holds its full value: reverse the
		// funding and resolve the row 'completed' in the SAME transaction
		// before returning success.
		if cerr := cancelGiftCardFunding(ctx, rtx, code, userID, refundID, refundAmount); cerr != nil {
			slog.Error("CRITICAL: Square refund already processed but gift card %s funding reversal failed — manual reconciliation required", "gift_card_id", code, "err", cerr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		if _, upErr := rtx.Exec(ctx, `UPDATE refunds SET status = 'completed' WHERE id = $1`, refundID); upErr != nil {
			slog.Error("CRITICAL: Square refund already processed but gift card %s refund row could not be resolved — manual reconciliation required", "gift_card_id", code, "err", upErr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		if cerr := rtx.Commit(ctx); cerr != nil {
			slog.Error("CRITICAL: Square refund already processed but gift card %s reversal commit failed — manual reconciliation required", "gift_card_id", code, "err", cerr)
			http.Error(w, "internal server error", http.StatusInternalServerError)
			return
		}
		log.Printf("Gift-card purchase %s already refunded at Square — refund %s marked completed, card %s cancelled", paymentID, refundID, code)
		w.WriteHeader(http.StatusOK)
		if err := json.NewEncoder(w).Encode(map[string]any{
			"status":          "success",
			"message":         "This gift card had already been refunded and has now been cancelled.",
			"refund_id":       refundID,
			"amount_refunded": refundAmount,
		}); err != nil {
			log.Printf("Failed to encode JSON response: %v", err)
		}
		return

	case errors.Is(sqErr, square.ErrRefundDeclined):
		// Definitive rejection — the money will never move. Mark the refund
		// failed so it is never retried and never blocks the amount, and DO
		// NOT cancel the card (the customer keeps it — the reversal tx has no
		// writes and rolls back below).
		if rbErr := rtx.Rollback(ctx); rbErr != nil && !errors.Is(rbErr, pgx.ErrTxClosed) {
			log.Printf("Failed to rollback gift-card reversal transaction before marking refund failed: %v", rbErr)
		}
		resolveRefundRow("failed", "")
		log.Printf("Gift-card purchase refund %s definitively declined by Square: %v", refundID, sqErr)
		http.Error(w, "Refund failed", http.StatusInternalServerError)
		return

	default:
		// Ambiguous — Square may or may not have processed the refund. The
		// reversal tx is rolled back (no writes) so the card stays live, the
		// refunds row stays 'pending' for a same-key retry (Square dedups),
		// and the gift card is NOT cancelled, per the failure-safety
		// requirement.
		log.Printf("Gift-card refund %s left pending after ambiguous Square error: %v", refundID, sqErr)
		http.Error(w, "Refund failed — please try again", http.StatusInternalServerError)
		return
	}
}

// giftCardSpendAtTill returns the total value of completed payments made
// against the gift card at the till. Till spend (handlers.go) decrements
// amount_remaining and records a payments row with payment_method='giftcard',
// status='completed' and gift_card_id = the card's code; transfers and
// top-ups deliberately do NOT create such rows. A shortfall between
// total_funds_added and amount_remaining that is not explained by this sum
// therefore signals an unverifiable alteration (a transfer, clawback, or
// partial refund), in which case the 14-day cancellation right cannot be
// exercised on the balance (reg 34(9) permits deducting only the value
// genuinely consumed on services and refunding the unspent remainder).
func giftCardSpendAtTill(ctx context.Context, q db.Querier, code string) (float64, error) {
	var spentAtTill float64
	err := q.QueryRow(ctx, `
		SELECT COALESCE(SUM(amount), 0) FROM payments
		WHERE gift_card_id = $1 AND status = 'completed'`, code).Scan(&spentAtTill)
	if err != nil {
		return 0, err
	}
	return spentAtTill, nil
}

// findGiftCardPurchasePayment resolves the originating payments row for an
// online gift-card purchase (BuyGiftCard): booking_id IS NULL (the same
// discriminator RefundPayment and the stale-pending sweep use to identify
// gift-card purchases), created_by = the buyer, payment_method='online_square',
// status='completed' with a square_payment_id. When the client supplies the
// payment id it is verified against the same invariants instead of being
// matched by amount/timing. The payment row is created just before the Square
// charge and the gift card a moment later, so the auto-match uses a 15-minute
// window that ends at the purchase transaction's timestamp.
func findGiftCardPurchasePayment(ctx context.Context, q db.Querier, userID, suppliedPaymentID string, purchaseAmount float64, purchasedAt time.Time) (paymentID, squarePaymentID string, ok bool, err error) {
	if suppliedPaymentID != "" {
		var bookingID sql.NullString
		var createdBy, status, method string
		var sqID sql.NullString
		var amount float64
		err := q.QueryRow(ctx, `
			SELECT id, booking_id, created_by, status, payment_method, square_payment_id, amount
			FROM payments WHERE id = $1`, suppliedPaymentID).
			Scan(&paymentID, &bookingID, &createdBy, &status, &method, &sqID, &amount)
		if errors.Is(err, pgx.ErrNoRows) {
			return "", "", false, nil
		}
		if err != nil {
			return "", "", false, err
		}
		if bookingID.Valid || createdBy != userID || status != "completed" || method != "online_square" {
			return "", "", false, nil
		}
		if !sqID.Valid || !approxEqual(amount, purchaseAmount) {
			return "", "", false, nil
		}
		return paymentID, sqID.String, true, nil
	}

	err = q.QueryRow(ctx, `
		SELECT id, square_payment_id
		FROM payments
		WHERE booking_id IS NULL AND created_by = $1
		  AND payment_method = 'online_square' AND status = 'completed'
		  AND square_payment_id IS NOT NULL
		  AND ABS(amount - $2) < 0.005
		  AND created_at BETWEEN $3::timestamptz - INTERVAL '15 minutes' AND $3::timestamptz + INTERVAL '15 minutes'
		ORDER BY created_at DESC
		LIMIT 1`, userID, purchaseAmount, purchasedAt).
		Scan(&paymentID, &squarePaymentID)
	if errors.Is(err, pgx.ErrNoRows) {
		return "", "", false, nil
	}
	if err != nil {
		return "", "", false, err
	}
	return paymentID, squarePaymentID, true, nil
}

// cancelGiftCardFunding reverses an online-purchased gift card after its
// refund has been accepted: the balance is zeroed and the card expired so it
// can never be redeemed or spent, and a 'cancelled' gift_card_transactions row
// records the reversal against the refund. Runs inside the caller's
// transaction.
func cancelGiftCardFunding(ctx context.Context, tx pgx.Tx, code, userID, refundID string, amount float64) error {
	if _, err := tx.Exec(ctx, `
		UPDATE gift_cards
		SET amount_remaining = 0, expiry_date = NOW(), last_used_at = NOW()
		WHERE id = $1`, code); err != nil {
		return fmt.Errorf("failed to zero gift card %s: %w", code, err)
	}
	if _, err := tx.Exec(ctx, `
		INSERT INTO gift_card_transactions (gift_card_id, transaction_type, amount, reference_type, reference_id, user_id, notes)
		VALUES ($1, 'cancelled', $2, 'refund', $3, $4, '14-day cooling-off cancellation — refunded to original payment method')
	`, code, amount, refundID, userID); err != nil {
		return fmt.Errorf("failed to record gift card cancellation transaction for %s: %w", code, err)
	}
	return nil
}

// approxEqual reports whether two currency amounts are equal to the nearest
// penny (float64 scans of NUMERIC can carry tiny representation error).
func approxEqual(a, b float64) bool {
	return math.Abs(a-b) < 0.005
}