Function getSnippetForTwoFileDiff

src/tools/FileEditTool/utils.ts:362–406

(
  fileAContents: string,
  fileBContents: string,
)

 * TODO: Unify this with the other snippet logic.
 */
export function getSnippetForTwoFileDiff(
  fileAContents: string,
  fileBContents: string,
): string {
  const patch = structuredPatch(
    'file.txt',
    'file.txt',
    fileAContents,
    fileBContents,
    undefined,
    undefined,
    {
      context: 8,
      timeout: DIFF_TIMEOUT_MS,
    },
  )

  if (!patch) {
    return ''
  }

  const full = patch.hunks
    .map(_ => ({
      startLine: _.oldStart,
      content: _.lines
        // Filter out deleted lines AND diff metadata lines
        .filter(_ => !_.startsWith('-') && !_.startsWith('\\'))
        .map(_ => _.slice(1))
        .join('\n'),
    }))
    .map(addLineNumbers)
    .join('\n...\n')

  if (full.length <= DIFF_SNIPPET_MAX_BYTES) {
    return full
  }

  // Truncate at the last line boundary that fits within the cap.
  // Marker format matches BashTool/utils.ts.
  const cutoff = full.lastIndexOf('\n', DIFF_SNIPPET_MAX_BYTES)
  const kept =
    cutoff > 0 ? full.slice(0, cutoff) : full.slice(0, DIFF_SNIPPET_MAX_BYTES)
  const remaining = countCharInString(full, '\n', kept.length) + 1
  return `${kept}\n\n... [${remaining} lines truncated] ...`
}

const CONTEXT_LINES = 4