diffofdiffs

diffofdiffs compares two versions of a patch side by side. Use it to review a backport, compare revisions of a fix, or see what changed during a rebase. It shows differences between the original edits and separately compares the surrounding source in the resulting files.

• Compare patch files or commits in a local Git repository
• Keep each patch's original additions, removals, and line numbers
• Find changed arguments, identifiers, and indentation with inline highlights
• Share standalone HTML reports with themes, navigation, and viewing controls

diffofdiffs has two input modes. Patch mode compares patch files using only the source lines quoted in them. Tree mode compares commits in a Git repository using the complete files before and after each change.

Use tree mode whenever the commits are available. It provides a more accurate and complete comparison: diffofdiffs can resolve matches that are ambiguous in patch excerpts and show relevant surrounding differences that neither patch quotes. Patch mode cannot recover that missing source.

Why compare patches?

A clean Git cherry-pick doesn't guarantee that the new commit makes the same edits as the original patch. Git's three-way merge accounts for changes already on your branch, so it can treat part of a patch as already applied.

For example, a patch moves a line of code by deleting it from one place and adding it in another. If your branch has already moved that line elsewhere, Git can treat the deletion as already done and apply only the addition. The cherry-pick succeeds without a conflict, but the line is now duplicated instead of moved.

This is one of several ways a three-way merge can produce an unintended result without reporting a conflict. diffofdiffs uses a two-way comparison of the patches to show differences in their additions, deletions, and surrounding code. In this example, it shows that the cherry-picked patch is missing the original deletion.

Examples

The images below are screenshots of diffofdiffs' actual standalone HTML output. Each report is a single file that embeds its styles, scripts, and viewing controls, so it works directly in a browser without a server or network connection.

Changed arguments and shared edits. The extra state. in the upstream calls is highlighted. Both patches add the same generation increment, shown in yellow; the guard that exists only in the upstream source appears opposite a hatched gap.

HTML report | Input patches

Indentation, tabs, and spaces. The first block has the same text under different indentation: nested on one side, after an early return on the other. The second example adds a line with eight spaces of indentation on one side and a tab on the other, with separate color blocks for the spaces.

HTML report | Input patches

Download either HTML file and open it in a browser to try the controls. The sample notes give the report generation commands and screenshot settings.

Usage

Patch mode

Compare two patch files:

diffofdiffs patch1.patch patch2.patch

Patch mode uses only the source lines quoted in the patches. Either operand may be - for standard input, but not both.

Tree mode

Compare two commits using the complete source files before and after each commit:

diffofdiffs --git-tree=/path/to/repository commit1 commit2

Tree mode reads Git objects without checking out revisions or changing the index.

Output options

These options apply to both patch mode and tree mode. Add --backport-labels to name the left side Backport and the right side Upstream.

Write a standalone HTML report:

diffofdiffs --html -o report.html patch1.patch patch2.patch

Open the report directly in a browser. Its styles, scripts, and controls are embedded; it needs no server or network connection.

Use -o FILE or --output=FILE to save either text or HTML output. Without this option, output goes to standard output; -o - also selects standard output.

Terminal color is enabled automatically and disabled when output is redirected. To keep color and highlight individual character changes:

diffofdiffs --color=always --highlight=characters patch1.patch patch2.patch

When writing to a terminal, diffofdiffs uses the available width and expands source tabs at eight-column stops measured from each source line's start. Set --max-column-width=N to cap each source column or --tab-width=N to choose a positive tab width for terminal and HTML display. Saved text and HTML source keep literal tabs.

The user guide walks through a sample report and explains line numbers, context, highlighting, tabs, and comparison limits. Run diffofdiffs --help for all options.

Installation

Building from source requires:

• A compiler with GNU C17 support
• GNU make and Python 3.6 or newer
• libgit2 1.9.1 or newer, including its development headers

Older libgit2 releases can fail to read repositories with nested alternate object stores; 1.9.1 includes the upstream fix.

Python embeds the HTML assets at build time. The executable needs libgit2 at runtime; Python isn't a runtime dependency. The Userspace RCU (liburcu) list headers are bundled, so liburcu doesn't need to be installed. Use CC and PYTHON to select a compiler and Python interpreter. Set CPPFLAGS, CFLAGS, and LDFLAGS to supply additional build flags.

From the source directory:

make
sudo make install

Use make V=1 to show the full build commands.

The default destination is /usr/local/bin/diffofdiffs. To install for the current user without root access:

make install prefix="$HOME/.local"

The Makefile supports prefix, exec_prefix, and bindir. By default, exec_prefix follows prefix, and bindir is $(exec_prefix)/bin. Packagers can use DESTDIR to stage an installation without changing its final prefix:

make install DESTDIR="$PWD/stage" prefix=/usr

This writes stage/usr/bin/diffofdiffs. To remove an installation, run make uninstall with the same directory settings and permissions used for installation. For example:

make uninstall prefix="$HOME/.local"

Development

Checks require Python 3.9 or newer, Git, GNU patch, GNU diffutils, and coreutils in addition to the build dependencies. make check-asan also needs the compiler's AddressSanitizer and UndefinedBehaviorSanitizer runtime libraries. Style checks use clang-format from PATH when it is available. The current formatting baseline was produced with 22.1.8; no version is enforced. Other versions may report formatting differences. See CODING_STYLE.md for the formatting rules.

make checkstyle
make check
make check-asan

See tests/README.md for test coverage and optional corpus validation. Read CODING_STYLE.md before contributing code; DESIGN.md describes the comparison engine, and CODE_DESIGN.md records the design requirements.

License and origin

diffofdiffs is licensed under GPL-2.0-only. It grew out of interdiff --fuzzy in Sultan Alsawaf's fork of patchutils.

The bundled Userspace RCU list headers retain their LGPL-2.1-or-later license. See the vendoring notes for their source and local changes.