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

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();
  });
});