Skip to content

pr-git-1452/john-cai/jc/attr-diff-algo-v4

When a repository contains different kinds of files, it may be desirable to
use different algorithms based on file type. This is currently not feasible
through the command line or using git configs. However, we can leverage the
fact that gitattributes are path aware.

Teach the diff machinery to check gitattributes when diffing files by using
the existing diff. scheme, and add an "algorithm" type to the external
driver config.

Change since V3:

 * cleaned up documentation, typos
 * minor cleanup such as if statement ordering, and overly long lines

Changes since V2:

 * minor clean up and variable renaming
 * avoid parsing attribute files for the driver if the diff algorithm is set
   through the command line

Changes since V1:

 * utilize the existing diff.<driver>.* scheme where the driver is defined
   in gitattributes, but the algorithm is defined in the gitconfig.

To address some of the performance concerns in the previous series, a
benchmark shows that now only a minor performance penalty is incurred, now
that we are no longer adding an additional attributes parsing call:

$ echo "*.[ch] diff=other" >> .gitattributes $ hyperfine -r 10 -L a
git-bin-wrapper,git '{a} -c diff.other.algorithm=myers diff v2.0.0 v2.28.0'
Benchmark 1: git-bin-wrapper -c diff.other.algorithm=myers diff v2.0.0
v2.28.0 Time (mean ± σ): 716.3 ms ± 3.8 ms [User: 660.2 ms, System: 50.8 ms]
Range (min … max): 709.8 ms … 720.6 ms 10 runs

Benchmark 2: git -c diff.other.algorithm=myers diff v2.0.0 v2.28.0 Time
(mean ± σ): 704.3 ms ± 2.9 ms [User: 656.6 ms, System: 44.3 ms] Range (min …
max): 700.1 ms … 708.6 ms 10 runs

Summary 'git -c diff.other.algorithm=myers diff v2.0.0 v2.28.0' ran 1.02 ±
0.01 times faster than 'git-bin-wrapper -c diff.other.algorithm=myers diff
v2.0.0 v2.28.0'

John Cai (2):
  diff: consolidate diff algorithm option parsing
  diff: teach diff to read algorithm from diff driver

 Documentation/gitattributes.txt | 31 ++++++++++++
 diff.c                          | 90 ++++++++++++++++++++++++---------
 diff.h                          |  1 +
 t/lib-diff-alternative.sh       | 38 +++++++++++++-
 userdiff.c                      |  4 +-
 userdiff.h                      |  1 +
 6 files changed, 140 insertions(+), 25 deletions(-)

base-commit: c867e4fa180bec4750e9b54eb10f459030dbebfd

Submitted-As: https://lore.kernel.org/git/pull.1452.v4.git.git.1676927082.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1452.git.git.1675568781.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1452.v2.git.git.1676410819.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1452.v3.git.git.1676665285.gitgitgadget@gmail.com
Assets 2