package clock

import (
	"testing"
	"time"
)

func TestNow_ReturnsUTC(t *testing.T) {
	t.Parallel()
	now := Now()
	if now.Location() != time.UTC {
		t.Errorf("clock.Now() returned time in %v, expected UTC", now.Location())
	}
	// UTC is a fixed-offset zone: the zone abbreviation and offset must both be
	// the canonical UTC values so SQL TIMESTAMPTZ comparisons and duration math
	// never drift.
	name, off := now.Zone()
	if name != "UTC" || off != 0 {
		t.Errorf("clock.Now() returned zone %q offset %d, expected UTC offset 0", name, off)
	}
}

// TestLondon_Location_IsEuropeLondon proves the single London location used for
// wall-clock business decisions is the IANA Europe/London zone (which carries
// the full DST rule table for GMT<->BST).
func TestLondon_Location_IsEuropeLondon(t *testing.T) {
	t.Parallel()
	if got := London.String(); got != "Europe/London" {
		t.Errorf("clock.London.String() = %q, expected %q", got, "Europe/London")
	}
}

// TestLondon_DSTTransitions pins the two 2026 Europe/London transitions:
//
//	Spring forward (2026-03-29 01:00 GMT -> 02:00 BST):
//	  - 2026-03-29 00:30 UTC = 01:30 GMT, offset +0 (still winter time)
//	  - 2026-03-29 01:00 UTC = 02:00 BST, offset +1 (the transition instant)
//	Fall back (2026-10-25 02:00 BST -> 01:00 GMT):
//	  - 2026-10-25 00:30 UTC = 01:30 BST, offset +1 (still summer time)
//	  - 2026-10-25 01:00 UTC = 01:00 GMT, offset +0 (the ambiguous hour, which
//	    Go resolves to the SECOND occurrence)
//
// These instants are the boundary of every London wall-clock decision the app
// makes (closing hours, default-hours effective dates, today summaries), so
// clock.London must get them exactly right.
func TestLondon_DSTTransitions(t *testing.T) {
	t.Parallel()
	cases := []struct {
		name        string
		utc         time.Time
		wantWall    string // London wall-clock, "2006-01-02 15:04"
		wantZone    string // "GMT" or "BST"
		wantOffsetH int
	}{
		{"spring before", time.Date(2026, 3, 29, 0, 30, 0, 0, time.UTC), "2026-03-29 00:30", "GMT", 0},
		{"spring transition", time.Date(2026, 3, 29, 1, 0, 0, 0, time.UTC), "2026-03-29 02:00", "BST", 1},
		{"autumn before", time.Date(2026, 10, 25, 0, 30, 0, 0, time.UTC), "2026-10-25 01:30", "BST", 1},
		{"autumn ambiguous->GMT", time.Date(2026, 10, 25, 1, 0, 0, 0, time.UTC), "2026-10-25 01:00", "GMT", 0},
	}
	for _, tc := range cases {
		t.Run(tc.name, func(t *testing.T) {
			l := tc.utc.In(London)
			if got := l.Format("2006-01-02 15:04"); got != tc.wantWall {
				t.Errorf("%s UTC in London = %q, expected wall-clock %q", tc.utc.Format(time.RFC3339), got, tc.wantWall)
			}
			zone, off := l.Zone()
			if zone != tc.wantZone {
				t.Errorf("%s UTC in London zone = %q, expected %q", tc.utc.Format(time.RFC3339), zone, tc.wantZone)
			}
			if off != tc.wantOffsetH*3600 {
				t.Errorf("%s UTC in London offset = %ds, expected %dh", tc.utc.Format(time.RFC3339), off, tc.wantOffsetH)
			}
		})
	}

	// Sanity: the two wall-clock times on the autumn transition day share the
	// same 01:xx wall hour (the ambiguous hour) but at different UTC instants —
	// proving Go resolves the fold to the second occurrence when converting the
	// UTC instant back to London.
	fold1 := time.Date(2026, 10, 25, 0, 30, 0, 0, time.UTC).In(London).Format("15:04") // 01:30 BST
	fold2 := time.Date(2026, 10, 25, 1, 30, 0, 0, time.UTC).In(London).Format("15:04") // 01:30 GMT
	if fold1 != "01:30" || fold2 != "01:30" {
		t.Errorf("expected both sides of the autumn fold to show 01:30, got %q and %q", fold1, fold2)
	}
}

func TestNow_IsReasonable(t *testing.T) {
	t.Parallel()
	now := Now()
	// Check it's within the last minute (reasonable)
	since := time.Since(now)
	if since < 0 || since > time.Minute {
		t.Errorf("clock.Now() returned unreasonable time: %v ago", since)
	}
}