// Line diff for "pull" commands (live store text vs the local files). Pure, no dependency: longest common subsequence
// over lines, printed like a unified diff without hunks (" " same, "-" only local, "+" only on the store).

export function lineDiff(before: string, after: string): string[] {
  const a = before.split('\n'), b = after.split('\n');
  if (a.length * b.length > 4_000_000) return [...a.map(l => `-${l}`), ...b.map(l => `+${l}`)];
  const n = a.length, m = b.length;
  const lcs: Uint32Array[] = Array.from({ length: n + 1 }, () => new Uint32Array(m + 1));
  for (let i = n - 1; i >= 0; i--) for (let j = m - 1; j >= 0; j--) lcs[i]![j] = a[i] === b[j] ? lcs[i + 1]![j + 1]! + 1 : Math.max(lcs[i + 1]![j]!, lcs[i]![j + 1]!);
  const out: string[] = [];
  let i = 0, j = 0;
  while (i < n && j < m) {
    if (a[i] === b[j]) { out.push(` ${a[i]}`); i++; j++; }
    else if (lcs[i + 1]![j]! >= lcs[i]![j + 1]!) out.push(`-${a[i++]}`);
    else out.push(`+${b[j++]}`);
  }
  while (i < n) out.push(`-${a[i++]}`);
  while (j < m) out.push(`+${b[j++]}`);
  return out;
}

/** Only the changed lines with `context` unchanged lines around them ("..." between gaps). Empty when equal. */
export function compactDiff(before: string, after: string, context = 2): string[] {
  const d = lineDiff(before, after);
  if (!d.some(l => l[0] !== ' ')) return [];
  const keep = new Set<number>();
  d.forEach((l, i) => { if (l[0] !== ' ') for (let k = i - context; k <= i + context; k++) keep.add(k); });
  const out: string[] = [];
  let last = -1;
  d.forEach((l, i) => { if (!keep.has(i)) return; if (last >= 0 && i > last + 1) out.push('...'); out.push(l); last = i; });
  return out;
}