import { describe, expect, it } from "vitest";
import {
  applyThemeTo,
  resolveTheme,
  systemDarkQuery,
  themeLocalStorageKey,
  type ThemeDocument,
} from "./theme.js";

describe("resolveTheme", () => {
  it("light pref wins over system", () => {
    expect(resolveTheme("light", true)).toBe("light");
  });
  it("dark pref wins over system", () => {
    expect(resolveTheme("dark", false)).toBe("dark");
  });
  it("system follows OS (dark)", () => {
    expect(resolveTheme("system", true)).toBe("dark");
  });
  it("system follows OS (light)", () => {
    expect(resolveTheme("system", false)).toBe("light");
  });
});

describe("constants", () => {
  it("storage key + media query", () => {
    expect(themeLocalStorageKey()).toBe("factory.theme");
    expect(systemDarkQuery()).toBe("(prefers-color-scheme: dark)");
  });
});

interface FakeWindow {
  matchMedia: (q: string) => {
    matches: boolean;
    listeners: Array<() => void>;
    addEventListener(type: string, h: () => void): void;
    removeEventListener(type: string, h: () => void): void;
    /** Legacy (Safari <14) API; `applyThemeTo` also supports it. */
    addListener(h: () => void): void;
    removeListener(h: () => void): void;
  };
  calls: string[];
}

function makeWindow(matches = true): FakeWindow {
  // One shared media object per window (like real matchMedia for the same query).
  const media: ReturnType<FakeWindow["matchMedia"]> = {
    matches,
    listeners: [],
    addEventListener(_t, h) {
      media.listeners.push(h);
    },
    removeEventListener(_t, h) {
      const i = media.listeners.indexOf(h);
      if (i >= 0) media.listeners.splice(i, 1);
    },
    addListener(h) {
      media.listeners.push(h);
    },
    removeListener(h) {
      const i = media.listeners.indexOf(h);
      if (i >= 0) media.listeners.splice(i, 1);
    },
  };
  return {
    calls: [],
    matchMedia: (q) => {
      expect(q).toBe(systemDarkQuery());
      return media;
    },
  };
}

function makeDocument(): ThemeDocument {
  return { dataset: {} };
}

describe("applyThemeTo", () => {
  it("sets data-theme for explicit light with a dark system", () => {
    const doc = makeDocument();
    const win = makeWindow(true);
    const resolved = applyThemeTo(doc, "light", win);
    expect(resolved).toBe("light");
    expect(doc.dataset.theme).toBe("light");
  });

  it("sets data-theme for system pref (dark system)", () => {
    const doc = makeDocument();
    const resolved = applyThemeTo(doc, "system", makeWindow(true));
    expect(resolved).toBe("dark");
    expect(doc.dataset.theme).toBe("dark");
  });

  it("registers a matchMedia change listener for system pref", () => {
    const doc = makeDocument();
    const win = makeWindow(true);
    applyThemeTo(doc, "system", win);
    const media = win.matchMedia(systemDarkQuery());
    expect(media.listeners).toHaveLength(1);
    // simulate OS switch to light
    media.matches = false;
    media.listeners[0]?.();
    expect(doc.dataset.theme).toBe("light");
  });

  it("does not register a listener for explicit prefs", () => {
    const doc = makeDocument();
    const win = makeWindow(true);
    applyThemeTo(doc, "dark", win);
    expect(win.matchMedia(systemDarkQuery()).listeners).toHaveLength(0);
  });

  it("unregisters the previous listener when re-applied (idempotent)", () => {
    const doc = makeDocument();
    const win = makeWindow(true);
    applyThemeTo(doc, "system", win);
    applyThemeTo(doc, "system", win);
    expect(win.matchMedia(systemDarkQuery()).listeners).toHaveLength(1);
    applyThemeTo(doc, "light", win);
    expect(win.matchMedia(systemDarkQuery()).listeners).toHaveLength(0);
  });

  it("works without matchMedia (SSR guard): resolves from default, no crash", () => {
    const doc = makeDocument();
    const resolved = applyThemeTo(doc, "system", undefined);
    expect(resolved).toBe("light"); // no window → treated as light system
    expect(doc.dataset.theme).toBe("light");
  });
});