"use client";

/**
 * Local bookkeeping for opt-in encrypted sync: whether it is on, the random
 * sync ID, when this device last synced and whether local data changed since.
 * The passphrase itself is never stored here — the derived key lives in
 * IndexedDB as a non-extractable CryptoKey (lib/sync-key.ts) and nowhere else.
 */

const STORAGE_KEY = "freezmo.sync.v1";

export type SyncRecord = {
  v: 1;
  enabled: boolean;
  /** Random 128-bit id — the only handle the server ever sees. */
  id: string;
  /** KDF salt (not secret; travels in the envelope too). */
  salt: string;
  lastSyncedAt: number | null;
  /** Set whenever local data changes; cleared after a successful sync. */
  dirtyAt: number | null;
  /** True when a sync found both sides changed (the UI asks what to keep). */
  conflict: boolean;
};

const EMPTY: SyncRecord = {
  v: 1,
  enabled: false,
  id: "",
  salt: "",
  lastSyncedAt: null,
  dirtyAt: null,
  conflict: false,
};

type Listener = () => void;

let record: SyncRecord | null = null;
let cached: SyncRecord = EMPTY;
const listeners = new Set<Listener>();
let suppressDirty = false;

function read(): SyncRecord {
  if (typeof window === "undefined") return EMPTY;
  try {
    const raw = window.localStorage.getItem(STORAGE_KEY);
    if (!raw) return EMPTY;
    const parsed = JSON.parse(raw) as Partial<SyncRecord>;
    if (parsed.v !== 1) return EMPTY;
    return {
      v: 1,
      enabled: parsed.enabled === true,
      id: typeof parsed.id === "string" ? parsed.id : "",
      salt: typeof parsed.salt === "string" ? parsed.salt : "",
      lastSyncedAt: typeof parsed.lastSyncedAt === "number" ? parsed.lastSyncedAt : null,
      dirtyAt: typeof parsed.dirtyAt === "number" ? parsed.dirtyAt : null,
      conflict: parsed.conflict === true,
    };
  } catch {
    return EMPTY;
  }
}

function ensureLoaded(): void {
  if (record !== null) return;
  record = read();
  cached = record;
}

function write(next: SyncRecord): void {
  record = next;
  cached = next;
  try {
    window.localStorage.setItem(STORAGE_KEY, JSON.stringify(next));
  } catch {
    /* private mode */
  }
  listeners.forEach((listener) => listener());
}

export const syncStore = {
  subscribe(listener: Listener): () => void {
    ensureLoaded();
    listeners.add(listener);
    const onStorage = (event: StorageEvent) => {
      if (event.key === STORAGE_KEY) {
        record = null;
        ensureLoaded();
        listeners.forEach((l) => l());
      }
    };
    window.addEventListener("storage", onStorage);
    return () => {
      listeners.delete(listener);
      window.removeEventListener("storage", onStorage);
    };
  },
  getSnapshot(): SyncRecord {
    ensureLoaded();
    return cached;
  },
  getServerSnapshot(): SyncRecord {
    return EMPTY;
  },
};

export function setSyncRecord(next: Partial<SyncRecord>): void {
  ensureLoaded();
  write({ ...syncStore.getSnapshot(), ...next });
}

/** Called by every data store's write(); no-op when sync is off. */
export function markSyncDirty(): void {
  if (suppressDirty) return;
  ensureLoaded();
  const current = syncStore.getSnapshot();
  if (!current.enabled || current.dirtyAt !== null) return;
  write({ ...current, dirtyAt: Date.now() });
}

/** Applying a pulled payload must not mark the local data dirty again. */
export function withoutDirtyTracking<T>(fn: () => T): T {
  suppressDirty = true;
  try {
    return fn();
  } finally {
    suppressDirty = false;
  }
}

/** 16 random bytes as hex = 128 bits, unguessable. */
export function newSyncId(): string {
  const bytes = new Uint8Array(16);
  crypto.getRandomValues(bytes);
  return Array.from(bytes, (byte) => byte.toString(16).padStart(2, "0")).join("");
}