const MAX_CELLS = 400000;
const MAX_DIFF_CHARS = 6000;
const MAX_SEGMENTS = 80;

const FIELD_LABELS = {
  student_name: 'ФИО ученика',
  group_id: 'Группа',
  module_id: 'Тема модуля',
  description: 'Текст работы'
};

const SIMPLE_FIELDS = ['student_name', 'group_id', 'module_id'];

const NAME_FIELDS = { group_id: 'group_name', module_id: 'module_name' };

function asText(v) {
  if (v === null || v === undefined) return '';
  return typeof v === 'string' ? v : String(v);
}

function tokenize(text) {
  return asText(text).split(/(\s+)/).filter(t => t.length > 0);
}

function compact(segments) {
  const out = [];
  for (const seg of segments) {
    if (!seg.text) continue;
    const last = out[out.length - 1];
    if (last && last.type === seg.type) last.text += seg.text;
    else out.push({ type: seg.type, text: seg.text });
  }
  return out;
}

function lcsSegments(a, b) {
  const n = a.length;
  const m = b.length;
  if (!n) return m ? [{ type: 'add', text: b.join('') }] : [];
  if (!m) return [{ type: 'del', text: a.join('') }];
  const w = m + 1;
  const dp = new Int32Array((n + 1) * w);
  for (let i = n - 1; i >= 0; i--) {
    const rowBase = i * w;
    const nextBase = (i + 1) * w;
    for (let j = m - 1; j >= 0; j--) {
      dp[rowBase + j] = a[i] === b[j]
        ? dp[nextBase + j + 1] + 1
        : Math.max(dp[nextBase + j], dp[rowBase + j + 1]);
    }
  }
  const out = [];
  let i = 0;
  let j = 0;
  while (i < n && j < m) {
    if (a[i] === b[j]) { out.push({ type: 'eq', text: a[i] }); i++; j++; }
    else if (dp[(i + 1) * w + j] >= dp[i * w + j + 1]) { out.push({ type: 'del', text: a[i] }); i++; }
    else { out.push({ type: 'add', text: b[j] }); j++; }
  }
  while (i < n) { out.push({ type: 'del', text: a[i] }); i++; }
  while (j < m) { out.push({ type: 'add', text: b[j] }); j++; }
  return out;
}

function anchoredDiff(a, b) {
  let head = 0;
  while (head < a.length && head < b.length && a[head] === b[head]) head++;
  let tailA = a.length;
  let tailB = b.length;
  while (tailA > head && tailB > head && a[tailA - 1] === b[tailB - 1]) { tailA--; tailB--; }
  const out = head ? [{ type: 'eq', text: a.slice(0, head).join('') }] : [];
  const midA = a.slice(head, tailA);
  const midB = b.slice(head, tailB);
  if (midA.length * midB.length <= MAX_CELLS) {
    out.push(...lcsSegments(midA, midB));
  } else {
    if (midA.length) out.push({ type: 'del', text: midA.join('') });
    if (midB.length) out.push({ type: 'add', text: midB.join('') });
  }
  if (tailA < a.length) out.push({ type: 'eq', text: a.slice(tailA).join('') });
  return out;
}

function capSegments(segments) {
  const out = [];
  let chars = 0;
  let truncated = false;
  for (const seg of segments) {
    const room = MAX_DIFF_CHARS - chars;
    if (out.length >= MAX_SEGMENTS || room <= 0) { truncated = true; break; }
    if (seg.text.length > room) {
      out.push({ type: seg.type, text: seg.text.slice(0, room) });
      chars += room;
      truncated = true;
      break;
    }
    out.push({ type: seg.type, text: seg.text });
    chars += seg.text.length;
  }
  if (truncated) out.push({ type: 'eq', text: '…' });
  return { segments: out, truncated };
}

function countWords(text) {
  const t = text.trim();
  return t ? t.split(/\s+/).length : 0;
}

function diffStats(segments, before, after) {
  let addedChars = 0;
  let removedChars = 0;
  let addedWords = 0;
  let removedWords = 0;
  for (const seg of segments) {
    if (seg.type === 'add') { addedChars += seg.text.length; addedWords += countWords(seg.text); }
    else if (seg.type === 'del') { removedChars += seg.text.length; removedWords += countWords(seg.text); }
  }
  return {
    added_chars: addedChars,
    removed_chars: removedChars,
    added_words: addedWords,
    removed_words: removedWords,
    chars_before: before.length,
    chars_after: after.length
  };
}

function textDiff(beforeRaw, afterRaw) {
  const before = asText(beforeRaw);
  const after = asText(afterRaw);
  if (before === after) {
    return { changed: false, segments: [], truncated: false, stats: diffStats([], before, after) };
  }
  const a = tokenize(before);
  const b = tokenize(after);
  const raw = a.length * b.length <= MAX_CELLS ? lcsSegments(a, b) : anchoredDiff(a, b);
  const full = compact(raw);
  const capped = capSegments(full);
  return {
    changed: true,
    segments: capped.segments,
    truncated: capped.truncated,
    stats: diffStats(full, before, after)
  };
}

function displayValue(row, field) {
  if (!row) return null;
  const value = row[field];
  const name = row[NAME_FIELDS[field]];
  if (value === null || value === undefined || value === '') return name ? `— (${name})` : null;
  if (name) return `${value} · ${name}`;
  return String(value);
}

function buildEntryDiff(before, after) {
  const changes = [];
  for (const field of SIMPLE_FIELDS) {
    const prev = displayValue(before, field);
    const next = displayValue(after, field);
    if (prev !== next) changes.push({ field, label: FIELD_LABELS[field], before: prev, after: next });
  }
  const beforeText = asText(before && before.description);
  const afterText = asText(after && after.description);
  if (beforeText !== afterText) {
    const d = textDiff(beforeText, afterText);
    changes.push({
      field: 'description',
      label: FIELD_LABELS.description,
      stats: d.stats,
      diff: d.segments,
      truncated: d.truncated
    });
  }
  return changes;
}

function normalizeEditSource(raw, before, after) {
  const value = typeof raw === 'string' ? raw.trim() : '';
  const beforeText = asText(before && before.description);
  const afterText = asText(after && after.description);
  const aiText = asText(after && after.description_ai);
  const textChanged = beforeText !== afterText;
  if (!textChanged) return 'manual';
  if (value === 'ai' || value === 'ai_manual') return value;
  if (aiText && afterText === aiText) return 'ai';
  return 'manual';
}

function summarizeChanges(changes) {
  if (!Array.isArray(changes)) return [];
  return changes.map(change => {
    const item = { field: change.field, label: change.label || change.field };
    if ('before' in change) item.before = change.before;
    if ('after' in change) item.after = change.after;
    if (change.stats) item.stats = change.stats;
    if (change.truncated) item.truncated = true;
    return item;
  });
}

function stripDiffs(target) {
  if (!target || typeof target !== 'object' || Array.isArray(target)) return target;
  if (!Array.isArray(target.changes)) return target;
  return { ...target, changes: summarizeChanges(target.changes) };
}

module.exports = {
  FIELD_LABELS,
  textDiff,
  buildEntryDiff,
  normalizeEditSource,
  summarizeChanges,
  stripDiffs
};