import { describe, expect, it } from "vitest";
import {
  buildDstOffsetList,
  buildTimeZoneList,
  formatUtcOffset,
  isValidTimeZone,
  nextOffsetTransition,
  standardOffsetSeconds,
  utcOffsetSeconds,
} from "../src/timezone.js";

// All builder timestamps are Unix-epoch microseconds (matter.js TlvEpochUs
// convention); "since the beginning of time" is Matter epoch zero in Unix us.
const MATTER_ZERO = 946_684_800_000_000n;
const FALL_BACK_2026 = BigInt(Date.parse("2026-11-01T07:00:00Z")) * 1_000n;
const SPRING_FORWARD_2027 = BigInt(Date.parse("2027-03-14T08:00:00Z")) * 1_000n;

describe("isValidTimeZone", () => {
  it("accepts IANA names", () => {
    expect(isValidTimeZone("America/Chicago")).toBe(true);
    expect(isValidTimeZone("Europe/Berlin")).toBe(true);
    expect(isValidTimeZone("UTC")).toBe(true);
  });

  it("rejects invalid names", () => {
    expect(isValidTimeZone("Central Time")).toBe(false);
    expect(isValidTimeZone("")).toBe(false);
    expect(isValidTimeZone("America/Springfield")).toBe(false);
  });
});

describe("utcOffsetSeconds", () => {
  it("returns CST offset in January", () => {
    expect(utcOffsetSeconds("America/Chicago", new Date("2026-01-15T12:00:00Z"))).toBe(-6 * 3600);
  });

  it("returns CDT offset in July", () => {
    expect(utcOffsetSeconds("America/Chicago", new Date("2026-07-15T12:00:00Z"))).toBe(-5 * 3600);
  });

  it("returns 0 for UTC", () => {
    expect(utcOffsetSeconds("UTC", new Date("2026-07-15T12:00:00Z"))).toBe(0);
  });

  it("handles half-hour offsets", () => {
    expect(utcOffsetSeconds("Asia/Kolkata", new Date("2026-07-15T12:00:00Z"))).toBe(5.5 * 3600);
  });
});

describe("formatUtcOffset", () => {
  it("formats negative, positive, and zero offsets", () => {
    expect(formatUtcOffset(-6 * 3600)).toBe("UTC-06:00");
    expect(formatUtcOffset(5.5 * 3600)).toBe("UTC+05:30");
    expect(formatUtcOffset(0)).toBe("UTC+00:00");
  });
});

describe("nextOffsetTransition", () => {
  it("finds the exact spring-forward instant for Chicago", () => {
    const transition = nextOffsetTransition("America/Chicago", new Date("2026-01-15T00:00:00Z"));
    expect(transition).not.toBeNull();
    // 2026-03-08 02:00 CST (UTC-6) -> 03:00 CDT (UTC-5): 08:00:00 UTC.
    expect(transition!.at.toISOString()).toBe("2026-03-08T08:00:00.000Z");
    expect(transition!.offsetBeforeSeconds).toBe(-6 * 3600);
    expect(transition!.offsetAfterSeconds).toBe(-5 * 3600);
  });

  it("finds the exact fall-back instant for Chicago", () => {
    const transition = nextOffsetTransition("America/Chicago", new Date("2026-07-15T00:00:00Z"));
    expect(transition).not.toBeNull();
    // 2026-11-01 02:00 CDT (UTC-5) -> 01:00 CST (UTC-6): 07:00:00 UTC.
    expect(transition!.at.toISOString()).toBe("2026-11-01T07:00:00.000Z");
    expect(transition!.offsetBeforeSeconds).toBe(-5 * 3600);
    expect(transition!.offsetAfterSeconds).toBe(-6 * 3600);
  });

  it("returns null for fixed-offset zones", () => {
    expect(nextOffsetTransition("UTC", new Date("2026-01-15T00:00:00Z"))).toBeNull();
  });
});

describe("standardOffsetSeconds", () => {
  it("returns the non-DST offset regardless of season", () => {
    expect(standardOffsetSeconds("America/Chicago", new Date("2026-07-15T00:00:00Z"))).toBe(-6 * 3600);
    expect(standardOffsetSeconds("America/Chicago", new Date("2026-01-15T00:00:00Z"))).toBe(-6 * 3600);
  });

  it("handles the southern hemisphere", () => {
    // Sydney: AEST UTC+10 standard, AEDT UTC+11 in southern summer.
    expect(standardOffsetSeconds("Australia/Sydney", new Date("2026-01-15T00:00:00Z"))).toBe(10 * 3600);
  });

  it("handles fixed-offset zones", () => {
    expect(standardOffsetSeconds("UTC", new Date("2026-07-15T00:00:00Z"))).toBe(0);
    expect(standardOffsetSeconds("Asia/Kolkata", new Date("2026-07-15T00:00:00Z"))).toBe(5.5 * 3600);
  });
});

describe("buildTimeZoneList", () => {
  it("builds a single always-valid entry with the standard offset and IANA name", () => {
    const list = buildTimeZoneList("America/Chicago", new Date("2026-07-26T00:00:00Z"));
    expect(list).toEqual([{ offset: -6 * 3600, validAt: MATTER_ZERO, name: "America/Chicago" }]);
  });
});

describe("buildDstOffsetList", () => {
  const from = new Date("2026-07-26T00:00:00Z");

  it("builds the active DST period plus the following one (max 2, as on ALPSTUGA)", () => {
    expect(buildDstOffsetList("America/Chicago", 2, from)).toEqual([
      { offset: 3600, validStarting: MATTER_ZERO, validUntil: FALL_BACK_2026 },
      { offset: 0, validStarting: FALL_BACK_2026, validUntil: SPRING_FORWARD_2027 },
    ]);
  });

  it("caps the list at the device's maximum", () => {
    expect(buildDstOffsetList("America/Chicago", 1, from)).toEqual([
      { offset: 3600, validStarting: MATTER_ZERO, validUntil: FALL_BACK_2026 },
    ]);
  });

  it("emits a single open-ended zero entry for zones without DST", () => {
    expect(buildDstOffsetList("UTC", 2, from)).toEqual([
      { offset: 0, validStarting: MATTER_ZERO, validUntil: null },
    ]);
    expect(buildDstOffsetList("Asia/Kolkata", 5, from)).toEqual([
      { offset: 0, validStarting: MATTER_ZERO, validUntil: null },
    ]);
  });

  it("starts from standard time when DST is not in effect", () => {
    const winter = new Date("2026-01-15T00:00:00Z");
    const springForward2026 = BigInt(Date.parse("2026-03-08T08:00:00Z")) * 1_000n;
    const list = buildDstOffsetList("America/Chicago", 2, winter);
    expect(list[0]).toEqual({ offset: 0, validStarting: MATTER_ZERO, validUntil: springForward2026 });
    expect(list[1]!.offset).toBe(3600);
    expect(list[1]!.validStarting).toBe(springForward2026);
  });
});