Crussell/backend/handlers/payments/discounts.go

package payments

import (
	"context"
	"errors"
	"log"
	"math"
	"time"

	"crussell/db"

	"github.com/jackc/pgx/v5"
)

// EligibleDiscount describes a single discount that is currently eligible for a
// booking, computed identically for the discount preview and the
// apply-at-payment path so the preview shows exactly what payment will apply.
// Amount is the discounted value in pounds.
type EligibleDiscount struct {
	Source        string // "campaign" or "referral"
	Name          string
	Percent       float64
	Amount        float64
	SourceID      string  // discount_campaigns.id or referral_discounts.id
	CampaignType  string  // "time_based", "milestone", or "" for referral
	MilestoneType *string // "per_user_booking_count", "anniversary", "global_booking_count", or nil
	IsReferral    bool
}

// ComputeEligibleDiscounts returns every campaign/referral discount currently
// eligible for the booking, using the same queries the apply-at-payment path
// runs (including the global in-person milestone discount that was previously
// only computed at payment time). It is read-only: it never writes
// booking_discounts, payments, or campaign counters. Callers pass the querier
// that matches their context — db.Conn for the preview, the payment
// transaction for the apply path.
//
// Existing booking_discounts for the booking are collected in ONE query up
// front and checked in-memory, replacing the previous per-campaign
// "SELECT 1 FROM booking_discounts WHERE booking_id=$1 AND source_id=$2" that
// produced an N+1 inside the anniversary loop.
func ComputeEligibleDiscounts(ctx context.Context, q db.Querier, bookingID, userID string, bookingTotal float64) []EligibleDiscount {
	// The apply path refuses to apply NEW discounts once a booking has 2+
	// completed real payments (the customer has already paid) — mirror that
	// here so the preview does not promise a discount apply will refuse.
	var existingPayment int
	if err := q.QueryRow(ctx, `
		SELECT COUNT(*) FROM payments
		WHERE booking_id = $1 AND status = 'completed' AND payment_method NOT IN ('discount', 'on_the_house')
	`, bookingID).Scan(&existingPayment); err != nil {
		log.Printf("Failed to scan existing payment count: %v", err)
	}
	if existingPayment >= 2 {
		return nil
	}

	if bookingTotal <= 0 {
		return nil
	}

	// Existing booking_discounts for THIS booking, keyed by source so a
	// campaign id can never collide with a referral id. Single query replaces
	// the N+1 per-campaign existence checks (both files).
	existingSources := map[string]bool{}
	{
		rows, err := q.Query(ctx, `
			SELECT COALESCE(discount_source, ''), COALESCE(source_id, '')
			FROM booking_discounts WHERE booking_id = $1
		`, bookingID)
		if err == nil {
			for rows.Next() {
				var src, sid string
				if rows.Scan(&src, &sid) == nil {
					existingSources[src+"|"+sid] = true
				}
			}
			rows.Close()
		} else {
			log.Printf("Failed to query existing booking discounts for booking %s: %v", bookingID, err)
		}
	}

	var discounts []EligibleDiscount

	// Time-based campaign: the highest-percent active time_based campaign.
	var campaignID, campaignName string
	var campaignPercent float64
	if err := q.QueryRow(ctx, `
		SELECT id, discount_percent, name FROM discount_campaigns
		WHERE status = 'active' AND campaign_type = 'time_based'
		AND start_date <= NOW() AND end_date >= NOW()
		AND (max_redemptions IS NULL OR times_redeemed < max_redemptions)
		ORDER BY discount_percent DESC LIMIT 1
	`).Scan(&campaignID, &campaignPercent, &campaignName); err == nil && campaignID != "" {
		if !existingSources["campaign|"+campaignID] {
			discounts = append(discounts, EligibleDiscount{
				Source:       "campaign",
				Name:         campaignName,
				Percent:      campaignPercent,
				Amount:       roundTo2(bookingTotal * campaignPercent / 100),
				SourceID:     campaignID,
				CampaignType: "time_based",
			})
		}
	}

	// Per-user booking-count milestone: the campaign matching the user's
	// completed-booking count that has not yet been used for this user.
	var userBookingCount int
	if err := q.QueryRow(ctx, `
		SELECT COUNT(*) FROM bookings WHERE user_id = $1 AND status = 'completed'
	`, userID).Scan(&userBookingCount); err != nil {
		log.Printf("Failed to scan user completed booking count: %v", err)
	}

	var milestoneCampaignID, milestoneName string
	var milestonePercent float64
	if err := q.QueryRow(ctx, `
		SELECT id, discount_percent, name FROM discount_campaigns
		WHERE status = 'active' AND campaign_type = 'milestone' AND milestone_type = 'per_user_booking_count'
		AND milestone_value = $1
		AND NOT EXISTS (SELECT 1 FROM booking_discounts WHERE user_id = $2 AND source_id = discount_campaigns.id)
	`, userBookingCount, userID).Scan(&milestoneCampaignID, &milestonePercent, &milestoneName); err != nil {
		log.Printf("Failed to query milestone campaign for user %s, count %d: %v", userID, userBookingCount, err)
	}

	if milestoneCampaignID != "" && !existingSources["campaign|"+milestoneCampaignID] {
		mt := "per_user_booking_count"
		discounts = append(discounts, EligibleDiscount{
			Source:        "campaign",
			Name:          milestoneName,
			Percent:       milestonePercent,
			Amount:        roundTo2(bookingTotal * milestonePercent / 100),
			SourceID:      milestoneCampaignID,
			CampaignType:  "milestone",
			MilestoneType: &mt,
		})
	}

	// Anniversary milestone: the first qualifying campaign for the user's
	// first visit, matched by elapsed time. Only the FIRST match is applied
	// (the apply path historically broke after one anniversary discount).
	var firstVisitDate time.Time
	if err := q.QueryRow(ctx, `
		SELECT MIN(start_time) FROM bookings WHERE user_id = $1 AND status = 'completed'
	`, userID).Scan(&firstVisitDate); err != nil {
		log.Printf("Failed to scan first visit date: %v", err)
	}
	if !firstVisitDate.IsZero() {
		type annCamp struct {
			id    string
			pct   float64
			value int
			unit  string
			name  string
		}
		annRows, err := q.Query(ctx, `
			SELECT id, discount_percent, milestone_value, milestone_unit, name FROM discount_campaigns
			WHERE status = 'active' AND campaign_type = 'milestone' AND milestone_type = 'anniversary'
			AND NOT EXISTS (SELECT 1 FROM booking_discounts WHERE user_id = $1 AND source_id = discount_campaigns.id AND milestone_type = 'anniversary')
		`, userID)
		if err == nil {
			var campaigns []annCamp
			for annRows.Next() {
				var c annCamp
				if annRows.Scan(&c.id, &c.pct, &c.value, &c.unit, &c.name) == nil {
					campaigns = append(campaigns, c)
				}
			}
			annRows.Close()

			for _, c := range campaigns {
				if existingSources["campaign|"+c.id] {
					continue
				}

				var matches bool
				elapsed := time.Since(firstVisitDate)
				switch c.unit {
				case "months":
					matches = int(elapsed.Hours()/(30*24)) >= c.value
				case "years":
					matches = int(elapsed.Hours()/(365.25*24)) >= c.value
				}
				if matches {
					mt := "anniversary"
					discounts = append(discounts, EligibleDiscount{
						Source:        "campaign",
						Name:          c.name,
						Percent:       c.pct,
						Amount:        roundTo2(bookingTotal * c.pct / 100),
						SourceID:      c.id,
						CampaignType:  "milestone",
						MilestoneType: &mt,
					})
					break
				}
			}
		} else {
			log.Printf("Failed to query anniversary campaigns: %v", err)
		}
	}

	// Global booking-count milestone — only applies when the booking's first
	// real payment was taken in person (in_person_card).
	var firstPaymentMethod string
	if err := q.QueryRow(ctx, `
		SELECT payment_method FROM payments WHERE booking_id = $1 AND payment_method NOT IN ('discount', 'on_the_house') ORDER BY created_at ASC LIMIT 1
	`, bookingID).Scan(&firstPaymentMethod); err == nil && firstPaymentMethod == "in_person_card" {
		var globalCount int
		if err := q.QueryRow(ctx, `
			SELECT COUNT(*) FROM bookings WHERE status = 'completed'
		`).Scan(&globalCount); err != nil {
			log.Printf("Failed to scan global completed booking count: %v", err)
		}

		var globalCampaignID, globalName string
		var globalPercent float64
		if err := q.QueryRow(ctx, `
			SELECT id, discount_percent, name FROM discount_campaigns
			WHERE status = 'active' AND campaign_type = 'milestone' AND milestone_type = 'global_booking_count'
			AND milestone_value <= $1
			AND (max_redemptions IS NULL OR times_redeemed < max_redemptions)
			AND NOT EXISTS (SELECT 1 FROM booking_discounts WHERE source_id = discount_campaigns.id AND booking_id = $2)
			ORDER BY milestone_value DESC LIMIT 1
		`, globalCount, bookingID).Scan(&globalCampaignID, &globalPercent, &globalName); err != nil {
			log.Printf("Failed to query global milestone campaign: %v", err)
		}

		if globalCampaignID != "" && !existingSources["campaign|"+globalCampaignID] {
			mt := "global_booking_count"
			discounts = append(discounts, EligibleDiscount{
				Source:        "campaign",
				Name:          globalName,
				Percent:       globalPercent,
				Amount:        roundTo2(bookingTotal * globalPercent / 100),
				SourceID:      globalCampaignID,
				CampaignType:  "milestone",
				MilestoneType: &mt,
			})
		}
	}

	// Referrer's unused referral discount.
	var rdID string
	var rdPercent float64
	if err := q.QueryRow(ctx, `
		SELECT id, discount_percent FROM referral_discounts
		WHERE user_id = $1 AND used = FALSE
		LIMIT 1
	`, userID).Scan(&rdID, &rdPercent); err == nil && rdID != "" {
		if !existingSources["referral|"+rdID] {
			discounts = append(discounts, EligibleDiscount{
				Source:     "referral",
				Name:       "Referral Discount (10%)",
				Percent:    rdPercent,
				Amount:     roundTo2(bookingTotal * rdPercent / 100),
				SourceID:   rdID,
				IsReferral: true,
			})
		}
	}

	return discounts
}

// ApplyEligibleDiscount persists a single eligible discount for the booking:
// the booking_discounts row, the discount payment record, and the campaign
// redemption counter (or the referral used flag). The caller holds the payment
// transaction so these writes commit atomically with the payment. It is
// idempotent per booking because ComputeEligibleDiscounts excludes discounts
// whose source_id is already recorded for the booking.
//
// For campaign discounts the redemption counter is incremented FIRST, as an
// ATOMIC CONDITIONAL UPDATE guarded by max_redemptions (B13): two concurrent
// payments on different bookings for the same campaign can both pass the
// caller's unlocked "is it exhausted?" read, but only the first conditional
// increment matches — the loser's UPDATE affects zero rows (a 0-row result is
// returned) and this function returns a *campaignExhaustedAtApplyError with
// NOTHING written, so the caller can surface campaign_fully_redeemed. Doing the
// reservation before the booking_discounts/payment inserts keeps the
// transaction clean when a campaign is exhausted at apply time: no discount
// rows are minted for a redemption that never happened.
func ApplyEligibleDiscount(ctx context.Context, q db.Querier, bookingID, userID string, bookingTotal float64, d EligibleDiscount) error {
	if d.IsReferral {
		if _, err := q.Exec(ctx, `
			INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
			VALUES ($1, $2, 'referral', $3, NULL, NULL, $4, $5, $6)
		`, bookingID, userID, d.SourceID, d.Percent, bookingTotal, d.Amount); err != nil {
			log.Printf("Failed to insert referral discount: %v", err)
			return nil
		}
		if _, err := q.Exec(ctx, `
			INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
			VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
		`, bookingID, d.Amount, userID); err != nil {
			// The booking_discounts row was already inserted in this tx, so the
			// referral discount WAS redeemed — the used flag must still be set
			// below. Log ALERT and fall through to the UPDATE instead of
			// returning early (a lost used-flag would let the same referral
			// discount apply to a future booking).
			log.Printf("ALERT: failed to insert discount payment record for referral %s, booking %s: %v", d.SourceID, bookingID, err)
		}
		if _, err := q.Exec(ctx, `
			UPDATE referral_discounts SET used = TRUE, used_at = NOW() WHERE id = $1
		`, d.SourceID); err != nil {
			log.Printf("ALERT: failed to mark referral discount as used, booking %s: %v", bookingID, err)
		}
		return nil
	}

	var milestoneType any
	if d.MilestoneType != nil {
		milestoneType = *d.MilestoneType
	}

	// B13: atomic conditional reservation. The UPDATE increments the counter
	// ONLY while the campaign still has headroom; PostgreSQL makes this safe
	// under READ COMMITTED — a concurrent same-row UPDATE blocks, then
	// re-evaluates this WHERE against the post-increment row, so the loser
	// matches zero rows instead of over-redeeming past max_redemptions. Zero
	// rows means a concurrent redemption on another booking exhausted the
	// campaign between the caller's preview computation and this apply-time
	// re-check; nothing has been written yet, so the caller surfaces the
	// campaign_fully_redeemed path (B13).
	var reservedID string
	if err := q.QueryRow(ctx, `
		UPDATE discount_campaigns SET times_redeemed = times_redeemed + 1
		WHERE id = $1 AND (max_redemptions IS NULL OR times_redeemed < max_redemptions)
		RETURNING id
	`, d.SourceID).Scan(&reservedID); err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return &campaignExhaustedAtApplyError{campaignID: d.SourceID, lostPence: int64(math.Round(d.Amount * 100))}
		}
		log.Printf("ALERT: failed to reserve redemption for campaign %s, booking %s: %v — discount NOT applied", d.SourceID, bookingID, err)
		return nil
	}

	if _, err := q.Exec(ctx, `
		INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
		VALUES ($1, $2, 'campaign', $3, $4, $5, $6, $7, $8)
	`, bookingID, userID, d.SourceID, d.CampaignType, milestoneType, d.Percent, bookingTotal, d.Amount); err != nil {
		// The reservation (counter increment) already stands in this tx, so the
		// redemption was consumed. Log ALERT and skip the payment record — a
		// discount payment row without a booking_discounts row would be a
		// ledger anomaly. max_redemptions bounds the lost reservation: the next
		// eligible booking finds the campaign with one fewer redemption.
		log.Printf("ALERT: failed to insert campaign discount for campaign %s, booking %s: %v", d.SourceID, bookingID, err)
		return nil
	}
	if _, err := q.Exec(ctx, `
		INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
		VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
	`, bookingID, d.Amount, userID); err != nil {
		// The booking_discounts row was already inserted in this tx, so the
		// campaign WAS redeemed and the reservation already stands. Log ALERT
		// and return (a lost record would hide the discount from the ledger).
		log.Printf("ALERT: failed to insert discount payment record for campaign %s, booking %s: %v", d.SourceID, bookingID, err)
	}
	return nil
}