// A user's synced display preferences: theme, default Reader translation, verse layout, the
// commentary they last chose, and where they left off in the Reader. Each preference carries
// its own timestamp ({ v, at }) so the most recent change to each one wins, device by device.
const ENUMS = {
  theme: ['paper', 'sepia', 'dark'],
  translation: ['BSB', 'ESV', 'NLT', 'CSB', 'NASB', 'NET'],
  layout: ['flow', 'verses'],
  commentarySource: ['matthew-henry', 'john-gill', 'jamieson-fausset-brown', 'tyndale', 'adam-clarke', 'john-calvin', 'keil-delitzsch'],
};
const DAY_MS = 86_400_000;

// A cleaned value, or undefined when it isn't acceptable for that preference.
export function cleanPrefValue(key, value) {
  if (ENUMS[key]) return ENUMS[key].includes(value) ? value : undefined;
  if (key === 'position') {
    if (!value || typeof value !== 'object') return undefined;
    const book = String(value.book ?? '').toUpperCase();
    if (!/^[1-3A-Z]{3}$/.test(book) || !Number.isInteger(value.chapter) || value.chapter < 1 || value.chapter > 200) return undefined;
    return { book, chapter: value.chapter };
  }
  return undefined;
}

// Body -> { key: { v, at } } with only known keys, valid values and sane timestamps.
export function cleanPrefsPatch(body, now = Date.now()) {
  const out = {};
  const prefs = body?.prefs;
  if (!prefs || typeof prefs !== 'object' || Array.isArray(prefs)) return out;
  for (const [key, entry] of Object.entries(prefs)) {
    const v = cleanPrefValue(key, entry?.v);
    const at = entry?.at;
    if (v === undefined || !Number.isFinite(at) || at < 0 || at > now + DAY_MS) continue;
    out[key] = { v, at };
  }
  return out;
}

// Per key, the entry with the newer timestamp wins (the stored one on a tie).
export function mergePrefs(existing = {}, patch = {}) {
  const merged = { ...existing };
  for (const [key, entry] of Object.entries(patch)) {
    if (!merged[key] || entry.at > merged[key].at) merged[key] = entry;
  }
  return merged;
}