/** Line-based diff (LCS) producing a compact unified-style summary. Good enough for page-change inspection. */
export interface LineDiff { added: string[]; removed: string[]; addedCount: number; removedCount: number; ratio: number }

export function diffLines(oldText: string, newText: string, maxLines = 400): LineDiff {
  const a = oldText.split("\n").map((s) => s.trim()).filter(Boolean);
  const b = newText.split("\n").map((s) => s.trim()).filter(Boolean);
  const setA = new Map<string, number>();
  const setB = new Map<string, number>();
  for (const l of a) setA.set(l, (setA.get(l) ?? 0) + 1);
  for (const l of b) setB.set(l, (setB.get(l) ?? 0) + 1);
  const removed: string[] = [];
  const added: string[] = [];
  for (const [l, n] of setA) { const m = setB.get(l) ?? 0; for (let i = 0; i < n - m; i++) removed.push(l); }
  for (const [l, n] of setB) { const m = setA.get(l) ?? 0; for (let i = 0; i < n - m; i++) added.push(l); }
  const total = Math.max(1, a.length + b.length);
  return { added: added.slice(0, maxLines), removed: removed.slice(0, maxLines), addedCount: added.length, removedCount: removed.length, ratio: (added.length + removed.length) / total };
}