import { describe, expect, it } from "vitest";
import { Weekday } from "@/generated/prisma/enums";
import {
  addDays,
  calendarDateOf,
  compareCalendarDates,
  eachCalendarDate,
  isAfterCalendarDate,
  isBeforeCalendarDate,
  minutesFromMidnight,
  todayIn,
  wallClockToInstant,
  weekdayOf,
} from "../business-time";

describe("weekdayOf", () => {
  it("maps calendar dates to the correct Weekday", () => {
    // 2026-08-03 is a Monday.
    expect(weekdayOf("2026-08-03")).toBe(Weekday.MONDAY);
    expect(weekdayOf("2026-08-04")).toBe(Weekday.TUESDAY);
    expect(weekdayOf("2026-08-08")).toBe(Weekday.SATURDAY);
    expect(weekdayOf("2026-08-09")).toBe(Weekday.SUNDAY);
  });
});

describe("wallClockToInstant / minutesFromMidnight round-trip", () => {
  it("round-trips a plain wall-clock time in a fixed-offset zone", () => {
    const instant = wallClockToInstant("2026-01-15", 9 * 60 + 30, "Europe/London");
    expect(minutesFromMidnight(instant, "Europe/London")).toBe(9 * 60 + 30);
    expect(calendarDateOf(instant, "Europe/London")).toBe("2026-01-15");
  });

  it("produces the documented UTC instant for a known GMT (winter) date", () => {
    // 2026-01-15 is outside BST — Europe/London is UTC+0.
    const instant = wallClockToInstant("2026-01-15", 10 * 60, "Europe/London");
    expect(instant.toISOString()).toBe("2026-01-15T10:00:00.000Z");
  });
});

describe("DST correctness (Europe/London BST transition, last Sunday of March 2026)", () => {
  it("the same wall-clock time yields UTC instants shifted by the DST offset change", () => {
    // Before the 2026-03-29 transition: Europe/London is GMT (UTC+0).
    const beforeTransition = wallClockToInstant("2026-03-28", 10 * 60, "Europe/London");
    expect(beforeTransition.toISOString()).toBe("2026-03-28T10:00:00.000Z");

    // After the transition: Europe/London is BST (UTC+1).
    const afterTransition = wallClockToInstant("2026-03-30", 10 * 60, "Europe/London");
    expect(afterTransition.toISOString()).toBe("2026-03-30T09:00:00.000Z");

    // A naive "always UTC+0" implementation would put these exactly 48h
    // apart; DST-correct math puts them 47h apart because a wall-clock
    // hour was skipped.
    const diffHours = (afterTransition.getTime() - beforeTransition.getTime()) / 3_600_000;
    expect(diffHours).toBe(47);
  });

  it("minutesFromMidnight recovers the same local wall-clock time on both sides of the transition", () => {
    const before = wallClockToInstant("2026-03-28", 10 * 60, "Europe/London");
    const after = wallClockToInstant("2026-03-30", 10 * 60, "Europe/London");
    expect(minutesFromMidnight(before, "Europe/London")).toBe(10 * 60);
    expect(minutesFromMidnight(after, "Europe/London")).toBe(10 * 60);
  });
});

describe("calendar date arithmetic", () => {
  it("addDays is pure calendar-date math, independent of any zone", () => {
    expect(addDays("2026-02-27", 1)).toBe("2026-02-28");
    expect(addDays("2026-02-28", 1)).toBe("2026-03-01"); // 2026 is not a leap year
    expect(addDays("2026-12-31", 1)).toBe("2027-01-01");
  });

  it("compares calendar dates correctly", () => {
    expect(compareCalendarDates("2026-01-01", "2026-01-02")).toBeLessThan(0);
    expect(compareCalendarDates("2026-01-02", "2026-01-01")).toBeGreaterThan(0);
    expect(compareCalendarDates("2026-01-01", "2026-01-01")).toBe(0);
    expect(isBeforeCalendarDate("2026-01-01", "2026-01-02")).toBe(true);
    expect(isAfterCalendarDate("2026-01-02", "2026-01-01")).toBe(true);
  });

  it("eachCalendarDate yields an inclusive range and nothing when from > to", () => {
    expect([...eachCalendarDate("2026-01-01", "2026-01-03")]).toEqual([
      "2026-01-01",
      "2026-01-02",
      "2026-01-03",
    ]);
    expect([...eachCalendarDate("2026-01-03", "2026-01-01")]).toEqual([]);
  });

  it("eachCalendarDate guards against absurdly large ranges", () => {
    expect(() => [...eachCalendarDate("2026-01-01", "2030-01-01")]).toThrow();
  });
});

describe("todayIn", () => {
  it("returns a well-formed calendar date", () => {
    expect(todayIn("Europe/London")).toMatch(/^\d{4}-\d{2}-\d{2}$/);
  });
});