Function formatMinimalDiff

packages/walkthroughgen/src/cli.ts:148–207

(filePath: string, oldContent: string, newContent: string)

}

function formatMinimalDiff(filePath: string, oldContent: string, newContent: string): string | null {
  // Normalize line endings in both inputs
  const normalize = (str: string) => str.replace(/\r\n/g, '\n').replace(/\r/g, '\n');
  const normalizedOld = normalize(oldContent);
  const normalizedNew = normalize(newContent);

  if (normalizedOld === normalizedNew) {
    return null;
  }

  // Using context: 2 to show some surrounding lines
  const patch = Diff.createPatch(filePath, normalizedOld, normalizedNew, '', '', { context: 2 });
  const patchLines = patch.split('\n');
  const effectiveChangeLines: string[] = [];

  let i = 0;
  while (i < patchLines.length) {
    const line = patchLines[i];

    // Skip standard patch headers and hunk metadata
    if (line.startsWith('---') || line.startsWith('+++') || line.startsWith('@@')) {
      i++;
      continue;
    }

    // Check for identical remove/add pairs (which means no effective change for these two lines)
    if (line.startsWith('-')) {
      let nextDiffLineIndex = i + 1;
      // Skip empty lines AND "No newline" markers AND context lines to find the next actual diff line
      while (nextDiffLineIndex < patchLines.length &&
             (patchLines[nextDiffLineIndex].trim() === '' ||
              patchLines[nextDiffLineIndex].startsWith('\\') ||
              patchLines[nextDiffLineIndex].startsWith(' '))) {
        nextDiffLineIndex++;
      }

      if (nextDiffLineIndex < patchLines.length && patchLines[nextDiffLineIndex].startsWith('+')) {
        const removedText = line.substring(1).trim();
        const addedText = patchLines[nextDiffLineIndex].substring(1).trim();
        if (removedText === addedText) {
          // Advance i past the current line, any skipped empty lines, and the matched added line
          i = nextDiffLineIndex + 1;
          continue;
        }
      }
    }

    // If the line starts with +, -, or space (context), it's a line to be included
    if (line.startsWith('+') || line.startsWith('-') || line.startsWith(' ')) {
      effectiveChangeLines.push(line);
    }
    
    i++;
  }

  if (effectiveChangeLines.length > 0) {
    return `\`\`\`diff\n${filePath}\n${effectiveChangeLines.join('\n')}\n\`\`\`\n\n`;
  }