import { useCallback, useEffect, useRef, useState, useSyncExternalStore } from "react";
import type { AuthUser } from "@stc/contracts";
import type { CoursePayload, LibraryPayload } from "../shared/types";
import { learnerCss } from "./styles";

/** Injects the learner stylesheet once (the module graph stays importable on the server). */
export function ensureStyles(): void {
  if (typeof document === "undefined" || document.getElementById("stc-learner-css")) return;
  const el = document.createElement("style");
  el.id = "stc-learner-css";
  el.textContent = learnerCss;
  document.head.append(el);
}
export { learnerCss };

export class HttpError extends Error {
  constructor(public status: number, message: string) { super(message); }
}

export async function api<T>(path: string, init: { method?: string; body?: unknown } = {}): Promise<T> {
  const r = await fetch(path, {
    method: init.method ?? "GET",
    credentials: "same-origin",
    headers: init.body !== undefined ? { "content-type": "application/json" } : undefined,
    body: init.body !== undefined ? JSON.stringify(init.body) : undefined,
  });
  const text = await r.text();
  let data: unknown = null;
  try { data = text ? JSON.parse(text) : null; } catch { data = text; }
  if (!r.ok) {
    const msg = (data && typeof data === "object" && ("error" in data || "message" in data))
      ? String((data as { error?: string; message?: string }).error ?? (data as { message?: string }).message)
      : r.status === 404 ? "Not found" : `Request failed (${r.status})`;
    throw new HttpError(r.status, msg);
  }
  return data as T;
}

/** Loads `path` (null skips). `reload()` refetches without clearing the current data. */
export function useApi<T>(path: string | null): { data: T | null; error: HttpError | null; loading: boolean; reload: () => void; set: (v: T) => void } {
  const [state, setState] = useState<{ path: string | null; data: T | null; error: HttpError | null; loading: boolean }>({ path, data: null, error: null, loading: !!path });
  const [n, setN] = useState(0);
  const current = state.path === path ? state : { path, data: null, error: null, loading: !!path };
  useEffect(() => {
    if (!path) return;
    let live = true;
    setState((s) => (s.path === path ? { ...s, loading: true } : { path, data: null, error: null, loading: true }));
    api<T>(path).then(
      (data) => live && setState({ path, data, error: null, loading: false }),
      (e: unknown) => live && setState({ path, data: null, error: e instanceof HttpError ? e : new HttpError(0, String(e)), loading: false }),
    );
    return () => { live = false; };
  }, [path, n]);
  const reload = useCallback(() => setN((x) => x + 1), []);
  const set = useCallback((v: T) => setState((s) => ({ ...s, data: v })), []);
  return { data: current.data, error: current.error, loading: current.loading, reload, set };
}

/* ---------- tiny shared stores ---------- */

function store<T>(initial: T) {
  let value = initial;
  const subs = new Set<() => void>();
  return {
    get: () => value,
    set(v: T) { if (v !== value) { value = v; subs.forEach((f) => f()); } },
    subscribe(f: () => void) { subs.add(f); return () => { subs.delete(f); }; },
  };
}
function useStore<T>(s: ReturnType<typeof store<T>>): T {
  return useSyncExternalStore(s.subscribe, s.get, s.get);
}

const ls = {
  get(k: string): string | null { try { return localStorage.getItem(k); } catch { return null; } },
  set(k: string, v: string) { try { localStorage.setItem(k, v); } catch { /* private mode */ } },
};

export type Look = "reading" | "work";
const lookStore = store<Look>((typeof window !== "undefined" && ls.get("stc.look") === "work") ? "work" : "reading");
export function useLook(): [Look, (l: Look) => void] {
  const v = useStore(lookStore);
  return [v, (l) => { ls.set("stc.look", l); lookStore.set(l); }];
}

const searchStore = store("");
export function useSearch(): [string, (q: string) => void] {
  return [useStore(searchStore), searchStore.set];
}

const trackStore = store(0);
export function useTrack(courseId: string, tracks: { id: string }[]): [string | null, (t: string) => void] {
  useStore(trackStore);
  const saved = typeof window !== "undefined" ? ls.get(`stc.track.${courseId}`) : null;
  const value = tracks.find((t) => t.id === saved)?.id ?? tracks[0]?.id ?? null;
  return [value, (t) => { ls.set(`stc.track.${courseId}`, t); trackStore.set(trackStore.get() + 1); }];
}

/* ---------- session and caches ---------- */

const meStore = store<{ loaded: boolean; user: AuthUser | null }>({ loaded: false, user: null });
let meLoading: Promise<void> | null = null;
export function refreshMe(): Promise<void> {
  meLoading = api<{ user: AuthUser | null }>("/api/me").then(
    (r) => meStore.set({ loaded: true, user: r.user }),
    () => meStore.set({ loaded: true, user: null }),
  );
  return meLoading;
}
export function useMe(): { loaded: boolean; user: AuthUser | null } {
  const v = useStore(meStore);
  useEffect(() => { if (!v.loaded && !meLoading) void refreshMe(); }, [v.loaded]);
  return v;
}

const libraryStore = store<LibraryPayload | null>(null);
let libraryLoading: Promise<void> | null = null;
export function refreshLibrary(): Promise<void> {
  libraryLoading = api<LibraryPayload>("/api/library").then((l) => libraryStore.set(l), () => { /* keep stale */ });
  return libraryLoading;
}
export function useLibrary(): LibraryPayload | null {
  const v = useStore(libraryStore);
  useEffect(() => { if (!libraryLoading) void refreshLibrary(); }, []);
  return v;
}

const courseCache = new Map<string, CoursePayload>();
const courseVersion = store(0);
/** The course outline with progress. Cached across route changes; `reload` refetches. */
export function useCourse(courseId: string): { course: CoursePayload | null; error: HttpError | null; reload: () => Promise<void> } {
  useStore(courseVersion);
  const [error, setError] = useState<HttpError | null>(null);
  const live = useRef(true);
  const reload = useCallback(async () => {
    try {
      const c = await api<CoursePayload>(`/api/courses/${encodeURIComponent(courseId)}`);
      courseCache.set(courseId, c);
      if (live.current) setError(null);
      courseVersion.set(courseVersion.get() + 1);
    } catch (e) {
      if (live.current) setError(e instanceof HttpError ? e : new HttpError(0, String(e)));
    }
  }, [courseId]);
  useEffect(() => {
    live.current = true;
    setError(null);
    void reload();
    return () => { live.current = false; };
  }, [reload]);
  return { course: courseCache.get(courseId) ?? null, error, reload };
}
export function clearCourseCache(): void { courseCache.clear(); courseVersion.set(courseVersion.get() + 1); }

/* ---------- toast ---------- */

const toastStore = store<string | null>(null);
let toastTimer: ReturnType<typeof setTimeout> | undefined;
export function toast(msg: string): void {
  toastStore.set(msg);
  clearTimeout(toastTimer);
  toastTimer = setTimeout(() => toastStore.set(null), 2600);
}
export function useToast(): string | null { return useStore(toastStore); }

/* ---------- formatting ---------- */

export function fmtTime(s: number): string {
  const t = Math.max(0, Math.floor(s));
  const h = Math.floor(t / 3600), m = Math.floor((t % 3600) / 60), sec = t % 60;
  return h ? `${h}:${String(m).padStart(2, "0")}:${String(sec).padStart(2, "0")}` : `${m}:${String(sec).padStart(2, "0")}`;
}
export function fmtMinutes(min: number | null | undefined): string {
  if (!min) return "";
  if (min < 60) return `${Math.round(min)} min`;
  const h = Math.floor(min / 60), m = Math.round(min % 60);
  return m ? `${h} h ${m} min` : `${h} h`;
}
export function fmtSize(n: number | null | undefined): string {
  if (n === null || n === undefined) return "";
  if (n < 1024) return `${n} B`;
  if (n < 1024 * 1024) return `${Math.round(n / 1024)} KB`;
  return `${(n / 1024 / 1024).toFixed(1)} MB`;
}
export const lessonHref = (courseId: string, lessonId: string) => `/c/${encodeURIComponent(courseId)}/${encodeURIComponent(lessonId)}`;
export const courseHref = (courseId: string) => `/c/${encodeURIComponent(courseId)}`;

export async function copyText(text: string): Promise<boolean> {
  try { await navigator.clipboard.writeText(text); return true; } catch {
    const t = document.createElement("textarea");
    t.value = text; document.body.append(t); t.select();
    let ok = false;
    try { ok = document.execCommand("copy"); } catch { /* unsupported */ }
    t.remove();
    return ok;
  }
}