// Persistence in browser cookies.
//
// A cookie holds ~4 KB, so a save is base64url-encoded and split in chunks:
//   ic_save_<theme>_n   -> number of chunks
//   ic_save_<theme>_0.. -> the chunks
// Each theme has its own save slot. Preferences (theme, language) are stored
// in a single small cookie.

import { readCookie, removeCookie, writeCookie } from './cookies.js';

const CHUNK_SIZE = 3000;
const MAX_CHUNKS = 6;
const PREFS_KEY = 'ic_prefs';
const saveKey = (themeId) => `ic_save_${themeId}`;

// ------------------------------------------------------------------ encoding

export function encode(text) {
  const bytes = new TextEncoder().encode(text);
  let binary = '';
  for (const b of bytes) binary += String.fromCharCode(b);
  return btoa(binary).replaceAll('+', '-').replaceAll('/', '_').replace(/=+$/, '');
}

export function decode(encoded) {
  const base64 = encoded.replaceAll('-', '+').replaceAll('_', '/');
  const binary = atob(base64 + '='.repeat((4 - (base64.length % 4)) % 4));
  return new TextDecoder().decode(Uint8Array.from(binary, (c) => c.charCodeAt(0)));
}

// ------------------------------------------------------------------- chunked

function writeChunked(key, text) {
  const encoded = encode(text);
  const chunks = [];
  for (let i = 0; i < encoded.length; i += CHUNK_SIZE) chunks.push(encoded.slice(i, i + CHUNK_SIZE));
  if (chunks.length > MAX_CHUNKS) throw new Error('save_too_large');

  const previous = Number(readCookie(`${key}_n`)) || 0;
  chunks.forEach((chunk, i) => writeCookie(`${key}_${i}`, chunk));
  writeCookie(`${key}_n`, String(chunks.length));
  for (let i = chunks.length; i < previous; i++) removeCookie(`${key}_${i}`);
}

function readChunked(key) {
  const count = Number(readCookie(`${key}_n`));
  if (!Number.isInteger(count) || count < 1 || count > MAX_CHUNKS) return null;
  let encoded = '';
  for (let i = 0; i < count; i++) {
    const chunk = readCookie(`${key}_${i}`);
    if (chunk == null) return null;
    encoded += chunk;
  }
  try {
    return decode(encoded);
  } catch {
    return null;
  }
}

function removeChunked(key) {
  const count = Number(readCookie(`${key}_n`)) || 0;
  for (let i = 0; i < count; i++) removeCookie(`${key}_${i}`);
  removeCookie(`${key}_n`);
}

// -------------------------------------------------------------------- public

/** False when the browser refuses cookies (private mode, blocked, ...). */
export function cookiesAvailable() {
  try {
    writeCookie('ic_probe', '1');
    const ok = readCookie('ic_probe') === '1';
    removeCookie('ic_probe');
    return ok;
  } catch {
    return false;
  }
}

/** Throws if the save does not fit in cookies. */
export function saveGameText(themeId, text) {
  writeChunked(saveKey(themeId), text);
}

export function loadGameText(themeId) {
  return readChunked(saveKey(themeId));
}

export function deleteGame(themeId) {
  removeChunked(saveKey(themeId));
}

export function savePrefs(prefs) {
  writeCookie(PREFS_KEY, encode(JSON.stringify(prefs)));
}

export function loadPrefs() {
  try {
    const raw = readCookie(PREFS_KEY);
    const prefs = raw && JSON.parse(decode(raw));
    return prefs && typeof prefs === 'object' ? prefs : {};
  } catch {
    return {};
  }
}