// Saving and loading.
//
// Local storage is the live save; the file export exists so a cleared cache or
// a crashed browser does not take a career with it. A save is the whole game
// state and nothing else — the engine keeps no state outside it, so this is a
// complete record by construction.
//
// There is one slot per pack. A single global slot was fine while there was
// one setting; the moment a second existed, switching settings would have
// silently thrown away the other run — the guard in the app read the packId
// off whatever was in the slot and started a new game when it did not match.

const PREFIX = 'theladder.save.v1';
// The slot as it was before packs were selectable: one save, no pack in the
// key. Read once, for Corporate Ladder only, so a run in progress survives
// this change. Never written.
const LEGACY_KEY = PREFIX;
const LEGACY_PACK = 'corporateladder';
const LAST_PACK_KEY = 'theladder.pack.v1';
const FORMAT = 1;

const slotFor = (packId) => `${PREFIX}.${packId}`;

/** Local storage throws in private windows and when site data is blocked. */
function safely(action, fallback = null) {
  try {
    return action();
  } catch {
    return fallback;
  }
}

export function saveLocal(packId, state) {
  return safely(() => {
    localStorage.setItem(
      slotFor(packId),
      JSON.stringify({ format: FORMAT, savedAt: new Date().toISOString(), state }),
    );
    return true;
  }, false);
}

/**
 * The saved run for one pack, or null. Falls back to the pre-packs slot for
 * Corporate Ladder, and only when that slot really holds a Corporate Ladder
 * save — a stale slot from some other pack is not a reason to load a run into
 * a game whose events do not match it.
 */
export function loadLocal(packId) {
  return safely(() => {
    const raw = localStorage.getItem(slotFor(packId));
    if (raw) return JSON.parse(raw)?.state ?? null;
    if (packId !== LEGACY_PACK) return null;
    const legacy = localStorage.getItem(LEGACY_KEY);
    if (!legacy) return null;
    const state = JSON.parse(legacy)?.state ?? null;
    return state?.meta?.packId === LEGACY_PACK ? state : null;
  });
}

export function clearLocal(packId) {
  return safely(() => {
    localStorage.removeItem(slotFor(packId));
    if (packId === LEGACY_PACK) localStorage.removeItem(LEGACY_KEY);
    return true;
  }, false);
}

/** Which pack was last played, so a returning player lands back in it. */
export function lastPlayed() {
  return safely(() => localStorage.getItem(LAST_PACK_KEY));
}

export function rememberPack(packId) {
  return safely(() => { localStorage.setItem(LAST_PACK_KEY, packId); return true; }, false);
}

/** True if the browser will actually remember anything. */
export function storageAvailable() {
  return safely(() => {
    const probe = '__theladder_probe__';
    localStorage.setItem(probe, '1');
    localStorage.removeItem(probe);
    return true;
  }, false);
}

/** A save file's contents, as a string ready to be downloaded. */
export function serializeSave(state) {
  return JSON.stringify({ format: FORMAT, savedAt: new Date().toISOString(), state }, null, 2);
}

/**
 * Parse an imported save, refusing anything that is not one. A file from a
 * different pack is rejected by name rather than loaded into a game whose
 * events and characters do not match it.
 */
export function parseSave(text, expectedPackId) {
  let parsed;
  try {
    parsed = JSON.parse(text);
  } catch {
    throw new Error('That file is not valid JSON.');
  }
  const state = parsed?.state;
  if (!state || typeof state !== 'object' || !state.meta || typeof state.turn !== 'number') {
    throw new Error('That does not look like a saved game.');
  }
  if (expectedPackId && state.meta.packId !== expectedPackId) {
    throw new Error(
      `That save is from "${state.meta.packId}", but this is "${expectedPackId}".`,
    );
  }
  return state;
}