// Minimal, dependency-free unified line diff (LCS based). Config snapshots are
// modest in size (hundreds of lines), so the O(n*m) table is fine here.

export type DiffRowType = 'add' | 'del' | 'ctx';
export interface DiffRow {
  type: DiffRowType;
  text: string;
}

export function lineDiff(oldText: string, newText: string): DiffRow[] {
  const a = oldText.split('\n');
  const b = newText.split('\n');
  const n = a.length;
  const m = b.length;

  // dp[i][j] = length of LCS of a[i..] and b[j..]
  const dp: number[][] = Array.from({ length: n + 1 }, () => new Array<number>(m + 1).fill(0));
  for (let i = n - 1; i >= 0; i--) {
    for (let j = m - 1; j >= 0; j--) {
      dp[i][j] = a[i] === b[j] ? dp[i + 1][j + 1] + 1 : Math.max(dp[i + 1][j], dp[i][j + 1]);
    }
  }

  const rows: DiffRow[] = [];
  let i = 0;
  let j = 0;
  while (i < n && j < m) {
    if (a[i] === b[j]) {
      rows.push({ type: 'ctx', text: a[i] });
      i++;
      j++;
    } else if (dp[i + 1][j] >= dp[i][j + 1]) {
      rows.push({ type: 'del', text: a[i] });
      i++;
    } else {
      rows.push({ type: 'add', text: b[j] });
      j++;
    }
  }
  while (i < n) rows.push({ type: 'del', text: a[i++] });
  while (j < m) rows.push({ type: 'add', text: b[j++] });
  return rows;
}