// What a person has read in the Reader: which chapters (how many times, and when last) and on which days.
// Merging two copies never loses anything, so two devices can both add to it: each chapter keeps the larger
// count and the later date, and the days are combined. The data comes from the browser, so it is rebuilt field by field.
const KEY = /^[1-3A-Z]{3}\.\d{1,3}$/;
const DATE = /^\d{4}-\d{2}-\d{2}$/;
const MAX_DAYS = 5000;

export const emptyHistory = () => ({ chapters: {}, days: [] });

export function cleanHistory(raw) {
  const chapters = {};
  if (raw?.chapters && typeof raw.chapters === 'object' && !Array.isArray(raw.chapters)) {
    for (const [key, v] of Object.entries(raw.chapters).slice(0, 1300)) {
      if (!KEY.test(key) || !v || typeof v !== 'object') continue;
      const n = Number.isInteger(v.n) ? Math.min(9999, Math.max(1, v.n)) : 1;
      chapters[key] = { n, last: DATE.test(v.last ?? '') ? v.last : '' };
    }
  }
  const days = Array.isArray(raw?.days) ? [...new Set(raw.days.filter((d) => typeof d === 'string' && DATE.test(d)))].sort().slice(-MAX_DAYS) : [];
  return { chapters, days };
}

export function mergeHistory(a, b) {
  const x = cleanHistory(a);
  const y = cleanHistory(b);
  const chapters = { ...x.chapters };
  for (const [key, v] of Object.entries(y.chapters)) {
    const mine = chapters[key];
    chapters[key] = mine ? { n: Math.max(mine.n, v.n), last: mine.last > v.last ? mine.last : v.last } : v;
  }
  return { chapters, days: [...new Set([...x.days, ...y.days])].sort().slice(-MAX_DAYS) };
}