//go:build test

package scheduling

// Strict timezone/DST/midnight-boundary tests for the scheduling package's
// London-date derivation. All assertions use fixed time.Date instants.

import (
	"testing"
	"time"

	"crussell/clock"
)

// TestScheduling_DefaultHours_DST_Midnight proves ApplyScheduledDefaultHours's
// "today" date (derived via LondonDateString) is the LONDON calendar date, not
// the UTC date. During BST, London dates begin at 23:00 UTC the previous day:
// both 2026-06-14 23:30 UTC (= 2026-06-15 00:30 BST) and 2026-06-15 00:30 UTC
// (= 2026-06-15 01:30 BST) fall on London date 2026-06-15 — a naive UTC date
// would report 2026-06-14 for the first instant and misdate the effective day
// of a staged default-hours change.
func TestScheduling_DefaultHours_DST_Midnight(t *testing.T) {
	t.Parallel()
	cases := []struct {
		name string
		utc  time.Time
		want string
	}{
		{"00:30 BST window", time.Date(2026, 6, 14, 23, 30, 0, 0, time.UTC), "2026-06-15"},
		{"01:30 BST same day", time.Date(2026, 6, 15, 0, 30, 0, 0, time.UTC), "2026-06-15"},
		{"GMT midnight is UTC midnight", time.Date(2026, 1, 14, 23, 30, 0, 0, time.UTC), "2026-01-14"},
		{"GMT next day", time.Date(2026, 1, 15, 0, 30, 0, 0, time.UTC), "2026-01-15"},
		{"autumn BST->GMT transition day", time.Date(2026, 10, 25, 0, 30, 0, 0, time.UTC), "2026-10-25"},
	}
	for _, tc := range cases {
		t.Run(tc.name, func(t *testing.T) {
			if got := LondonDateString(tc.utc); got != tc.want {
				t.Errorf("LondonDateString(%s UTC) = %q, expected London date %q", tc.utc.Format(time.RFC3339), got, tc.want)
			}
		})
	}

	// Direct proof of the DST window: the UTC date of the first instant differs
	// from its London date — exactly the drift the helper must absorb.
	boundary := time.Date(2026, 6, 14, 23, 30, 0, 0, time.UTC)
	if boundary.Format("2006-01-02") == LondonDateString(boundary) {
		t.Fatal("test setup invariant: 2026-06-14 23:30 UTC must straddle the London midnight boundary")
	}

	// The helper must be consistent with clock.London's own wall-clock for the
	// same instant (the production seam ApplyScheduledDefaultHours now uses).
	nowish := time.Date(2026, 6, 14, 23, 45, 0, 0, time.UTC)
	if LondonDateString(nowish) != nowish.In(clock.London).Format("2006-01-02") {
		t.Errorf("LondonDateString disagrees with the direct London conversion for %s", nowish.Format(time.RFC3339))
	}
}