Function fetchSingleFileGitDiff

source code/utils/gitDiff.ts:407–443

(
  absoluteFilePath: string,
)

 * Returns null if not in a git repo or if git commands fail.
 */
export async function fetchSingleFileGitDiff(
  absoluteFilePath: string,
): Promise<ToolUseDiff | null> {
  const gitRoot = findGitRoot(dirname(absoluteFilePath))
  if (!gitRoot) return null

  const gitPath = relative(gitRoot, absoluteFilePath).split(sep).join('/')
  const repository = getCachedRepository()

  // Check if the file is tracked by git
  const { code: lsFilesCode } = await execFileNoThrowWithCwd(
    gitExe(),
    ['--no-optional-locks', 'ls-files', '--error-unmatch', gitPath],
    { cwd: gitRoot, timeout: SINGLE_FILE_DIFF_TIMEOUT_MS },
  )

  if (lsFilesCode === 0) {
    // File is tracked - diff against merge base for PR-like view
    const diffRef = await getDiffRef(gitRoot)
    const { stdout, code } = await execFileNoThrowWithCwd(
      gitExe(),
      ['--no-optional-locks', 'diff', diffRef, '--', gitPath],
      { cwd: gitRoot, timeout: SINGLE_FILE_DIFF_TIMEOUT_MS },
    )
    if (code !== 0) return null
    if (!stdout) return null
    return {
      ...parseRawDiffToToolUseDiff(gitPath, stdout, 'modified'),
      repository,
    }
  }

  // File is untracked - generate synthetic diff
  const syntheticDiff = await generateSyntheticDiff(gitPath, absoluteFilePath)
  if (!syntheticDiff) return null
  return { ...syntheticDiff, repository }
}

/**
 * Parse raw unified diff output into the structured ToolUseDiff format.