[jrosiek]

Summary

Renamed files were not shown correctly, for both git and jj:

• git, edited rename: the old side was read at the new path, so it came up
  empty. Rendering then crashed with an out-of-range end_col, because the
  highlights still pointed into lines that were never loaded.
• git, single revision (:Difft <rev>): rename detection compared the
  revision with the working tree instead of with its parent. It missed the
  revision's own renames and picked up unrelated renames from the working tree.
• git, line counts: --numstat reports a rename as old => new (or
  dir/{old => new}), so the lookup by the new path failed and renamed files
  showed +0/-0.
• git and jj, pure rename (no edits): difft reports the file as unchanged
  with no line alignment, so the view was empty.
• jj, any rename: jj diff --tool difft gives difft the two revisions as
  whole directory trees. A renamed file reaches difft as a deletion of the old
  path and a creation of the new one, so its contents were never compared. The
  file showed as entirely added, with an empty old side.

Changes

• The rename map is built before file contents are loaded, so a renamed file's
  old content is read from its old path. For a single revision, the rename map
  now uses the same rev^..rev range as the content diff.
• --numstat entries for renames are keyed by their new path.
• A file with identical content and no alignment is aligned line by line, so a
  pure rename shows its content, with no hunks.
• For jj, each rename that jj diff --summary reports is diffed by running difft
  on the old and new contents directly. The two temporary files keep the new
  file's name, so difft detects the same language. If that run fails, the entry
  is kept as jj reported it.
• tests/minimal_init.lua now adds the plugin to the runtime path by its
  absolute path. With a relative path, the library loader linked the built
  library through a broken relative symlink and silently fell back to a
  previously downloaded library.

Testing

• Rust unit tests cover parsing --numstat renames, the jj summary brace
  forms ({a => b}, dir/{a => b}, {a => sub/a}), ignoring copies, rename-map
  arguments for a single revision, aligning a pure rename, and pairing jj
  renames. The tests that run difft skip when it is not installed.
• New tests/rename_spec.lua runs the native library against real scratch
  repositories:
  • git: single commit, range, staged changes, moves within and across
    directories, a deletion next to an unrelated addition, and a working-tree
    rename that must not leak into a commit diff.
  • jj, both colocated with git and not: single revision, range, working copy,
    several renames in one revision, highlighting only the edited line, language
    detection, and copies kept as additions.
  • Each suite is skipped when the native library, difft or the VCS binary is
    missing.
• Run against the library built from the current main, 26 of the 32
  end-to-end cases fail. The 6 that pass are guards that unrelated files are
  not paired.

Known limitations

• Line counts for jj come from git, so they are available only in a repository
  colocated with git. This is unchanged.
• jj copies (C in --summary) are still shown as plain additions.

🤖 Generated with Claude Code

[jrosiek]

Disclaimer: shamelessly vibecoded. Manually tested with git only.

I’d appreciate it if you could let me know whether you accept AI-assisted contributions, including bug fixes and/or new features. I’m already making some changes for my own use on my fork, but I’d be happy to prepare them as contributions to the official repository as well (possible followup jrosiek#2)