All files / src/utils diff.ts

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export interface DiffLine {
  type: 'del' | 'add' | 'same';
  text: string;
}
 
export function computeLineDiff(originalText: string, newText: string): DiffLine[] {
  const a = originalText.split('\n');
  const b = newText.split('\n');

  const n = a.length;
  const m = b.length;
  const lcs: 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--) {
      if (a[i] === b[j]) {
        lcs[i][j] = lcs[i + 1][j + 1] + 1;
      } else {
        lcs[i][j] = Math.max(lcs[i + 1][j], lcs[i][j + 1]);
      }
    }
  }

  const out: DiffLine[] = [];
  let i = 0;
  let j = 0;
  while (i < n && j < m) {
    if (a[i] === b[j]) {
      out.push({ type: 'same', text: a[i] });
      i++;
      j++;
    } else if (lcs[i + 1][j] >= lcs[i][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;
}