// Combines two copies of one study (this device's and the other device's) chunk by chunk, so a
// conflict doesn't force a choice between them. Chunks are matched by id; each chunk carries the time it
// was last edited here (editedAt). Nothing the person wrote is dropped: where both copies changed the
// same note, the newer one stays and the older one is appended under a marker.
const TEXT_FIELDS = ['observation', 'interpretation', 'application', 'generalNotes', 'finalScript', 'episodeNumber', 'episodeTitle', 'teachBack', 'attendees', 'groupNotes'];
const LIST_FIELDS = ['tags', 'crossReferences', 'greekWords'];
export const MERGE_MARKER = '— from your other device —';

const same = (a, b) => JSON.stringify(a) === JSON.stringify(b);
const blank = (s) => !(s ?? '').toString().trim();

function mergeText(newer, older) {
  if (same(newer, older) || blank(older)) return newer ?? '';
  if (blank(newer)) return older;
  if (newer.includes(older)) return newer;
  if (older.includes(newer)) return older;
  return `${newer.replace(/\s+$/, '')}\n\n${MERGE_MARKER}\n${older}`;
}

function union(a = [], b = []) {
  const seen = new Set();
  return [...a, ...b].filter((item) => { const k = JSON.stringify(item); if (seen.has(k)) return false; seen.add(k); return true; });
}

// Follow-ups are matched by id; the newer copy's wording and done-state win, nothing is dropped.
function mergeById(newer = [], older = []) {
  const seen = new Set(newer.map((i) => i.id));
  return [...newer, ...older.filter((i) => !seen.has(i.id))];
}

export function mergeChunk(mine, theirs) {
  if (same(mine, theirs)) return { chunk: mine, changed: false };
  const mineNewer = (mine.editedAt ?? 0) >= (theirs.editedAt ?? 0);
  const [newer, older] = mineNewer ? [mine, theirs] : [theirs, mine];
  const out = { ...older, ...newer };
  TEXT_FIELDS.forEach((f) => { out[f] = mergeText(newer[f], older[f]); });
  LIST_FIELDS.forEach((f) => { out[f] = union(newer[f], older[f]); });
  // Pastes follow their text: keep the union per field so merged notes still show what was pasted.
  out.aiPastes = Object.fromEntries(['observation', 'interpretation', 'application'].map((f) => [f, union(newer.aiPastes?.[f], older.aiPastes?.[f])]));
  // Verse notes are kept per verse (the newer wording wins, an older different one is appended); the earliest first draft is kept.
  const noteKeys = new Set([...Object.keys(newer.verseNotes ?? {}), ...Object.keys(older.verseNotes ?? {})]);
  out.verseNotes = Object.fromEntries([...noteKeys].map((k) => [k, mergeText(newer.verseNotes?.[k], older.verseNotes?.[k])]));
  out.firstDraft = [newer.firstDraft, older.firstDraft].filter(Boolean).sort((a, b) => a.at - b.at)[0];
  out.fiveMinutes = union(newer.fiveMinutes, older.fiveMinutes);
  out.followUps = mergeById(newer.followUps, older.followUps);
  out.recalls = union(newer.recalls, older.recalls);
  out.editedAt = Math.max(mine.editedAt ?? 0, theirs.editedAt ?? 0) || undefined;
  return { chunk: out, changed: !same(out, mine) };
}

const chapterKey = (ch) => `${ch.bookAbbrev}|${ch.chapter}`;

// -> { project, stats: { added, merged } } where `added` counts chunks that only the other copy had and
// `merged` counts chunks both copies had that differed. Settings such as the title and the open chunk stay mine.
export function mergeStudies(mine, theirs) {
  const stats = { added: 0, merged: 0 };
  const theirChapters = new Map((theirs.chapters ?? []).map((ch) => [chapterKey(ch), ch]));
  const chapters = (mine.chapters ?? []).map((ch) => {
    const other = theirChapters.get(chapterKey(ch));
    theirChapters.delete(chapterKey(ch));
    if (!other) return ch;
    const theirChunks = new Map((other.chunks ?? []).map((c) => [c.id, c]));
    const chunks = (ch.chunks ?? []).map((c) => {
      const o = theirChunks.get(c.id);
      theirChunks.delete(c.id);
      if (!o) return c;
      const { chunk, changed } = mergeChunk(c, o);
      if (changed) stats.merged += 1;
      return chunk;
    });
    const extra = [...theirChunks.values()];
    stats.added += extra.length;
    return { ...ch, chunks: [...chunks, ...extra] };
  });
  for (const ch of theirChapters.values()) {
    stats.added += (ch.chunks ?? []).length;
    chapters.push(ch);
  }
  const ids = new Set(chapters.flatMap((c) => c.chunks.map((k) => k.id)));
  return {
    project: { ...mine, chapters, ...(mine.readingList || theirs.readingList ? { readingList: mergeById(mine.readingList, theirs.readingList) } : {}), selectedChunkId: ids.has(mine.selectedChunkId) ? mine.selectedChunkId : [...ids][0] ?? null },
    stats,
  };
}