gitdiffcore - Tweaking diff output
git diff *
The diff commands git diff-index, git diff-files, and git diff-tree can
be told to manipulate differences they find in unconventional ways
before showing diff output. The manipulation is collectively called
"diffcore transformation". This short note describes what they are and
how to use them to produce diff output that is easier to understand
than the conventional kind.
THE CHAIN OF OPERATION
The git diff-* family works by first comparing two sets of files:
· git diff-index compares contents of a "tree" object and the working
directory (when --cached flag is not used) or a "tree" object and
the index file (when --cached flag is used);
· git diff-files compares contents of the index file and the working
· git diff-tree compares contents of two "tree" objects;
In all of these cases, the commands themselves first optionally limit
the two sets of files by any pathspecs given on their command-lines,
and compare corresponding paths in the two resulting sets of files.
The pathspecs are used to limit the world diff operates in. They remove
the filepairs outside the specified sets of pathnames. E.g. If the
input set of filepairs included:
:100644 100644 bcd1234... 0123456... M junkfile
but the command invocation was git diff-files myfile, then the junkfile
entry would be removed from the list because only "myfile" is under
The result of comparison is passed from these commands to what is
internally called "diffcore", in a format similar to what is output
when the -p option is not used. E.g.
in-place edit :100644 100644 bcd1234... 0123456... M file0
create :000000 100644 0000000... 1234567... A file4
delete :100644 000000 1234567... 0000000... D file5
unmerged :000000 000000 0000000... 0000000... U file6
The diffcore mechanism is fed a list of such comparison results (each
of which is called "filepair", although at this point each of them
talks about a single file), and transforms such a list into another
list. There are currently 5 such transformations:
These are applied in sequence. The set of filepairs git diff-* commands
find are used as the input to diffcore-break, and the output from
diffcore-break is used as the input to the next transformation. The
final result is then passed to the output routine and generates either
diff-raw format (see Output format sections of the manual for git
diff-* commands) or diff-patch format.
DIFFCORE-BREAK: FOR SPLITTING UP COMPLETE REWRITES""
The second transformation in the chain is diffcore-break, and is
controlled by the -B option to the git diff-* commands. This is used to
detect a filepair that represents "complete rewrite" and break such
filepair into two filepairs that represent delete and create. E.g. If
the input contained this filepair:
:100644 100644 bcd1234... 0123456... M file0
and if it detects that the file "file0" is completely rewritten, it
changes it to:
:100644 000000 bcd1234... 0000000... D file0
:000000 100644 0000000... 0123456... A file0
For the purpose of breaking a filepair, diffcore-break examines the
extent of changes between the contents of the files before and after
modification (i.e. the contents that have "bcd1234..." and "0123456..."
as their SHA-1 content ID, in the above example). The amount of
deletion of original contents and insertion of new material are added
together, and if it exceeds the "break score", the filepair is broken
into two. The break score defaults to 50% of the size of the smaller of
the original and the result (i.e. if the edit shrinks the file, the
size of the result is used; if the edit lengthens the file, the size of
the original is used), and can be customized by giving a number after
"-B" option (e.g. "-B75" to tell it to use 75%).
DIFFCORE-RENAME: FOR DETECTION RENAMES AND COPIES
This transformation is used to detect renames and copies, and is
controlled by the -M option (to detect renames) and the -C option (to
detect copies as well) to the git diff-* commands. If the input
contained these filepairs:
:100644 000000 0123456... 0000000... D fileX
:000000 100644 0000000... 0123456... A file0
and the contents of the deleted file fileX is similar enough to the
contents of the created file file0, then rename detection merges these
filepairs and creates:
:100644 100644 0123456... 0123456... R100 fileX file0
When the "-C" option is used, the original contents of modified files,
and deleted files (and also unmodified files, if the
"--find-copies-harder" option is used) are considered as candidates of
the source files in rename/copy operation. If the input were like these
filepairs, that talk about a modified file fileY and a newly created
:100644 100644 0123456... 1234567... M fileY
:000000 100644 0000000... bcd3456... A file0
the original contents of fileY and the resulting contents of file0 are
compared, and if they are similar enough, they are changed to:
:100644 100644 0123456... 1234567... M fileY
:100644 100644 0123456... bcd3456... C100 fileY file0
In both rename and copy detection, the same "extent of changes"
algorithm used in diffcore-break is used to determine if two files are
"similar enough", and can be customized to use a similarity score
different from the default of 50% by giving a number after the "-M" or
"-C" option (e.g. "-M8" to tell it to use 8/10 = 80%).
Note. When the "-C" option is used with --find-copies-harder option,
git diff-* commands feed unmodified filepairs to diffcore mechanism as
well as modified ones. This lets the copy detector consider unmodified
files as copy source candidates at the expense of making it slower.
Without --find-copies-harder, git diff-* commands can detect copies
only if the file that was copied happened to have been modified in the
DIFFCORE-MERGE-BROKEN: FOR PUTTING COMPLETE REWRITES" BACK TOGETHER"
This transformation is used to merge filepairs broken by
diffcore-break, and not transformed into rename/copy by
diffcore-rename, back into a single modification. This always runs when
diffcore-break is used.
For the purpose of merging broken filepairs back, it uses a different
"extent of changes" computation from the ones used by diffcore-break
and diffcore-rename. It counts only the deletion from the original, and
does not count insertion. If you removed only 10 lines from a 100-line
document, even if you added 910 new lines to make a new 1000-line
document, you did not do a complete rewrite. diffcore-break breaks such
a case in order to help diffcore-rename to consider such filepairs as
candidate of rename/copy detection, but if filepairs broken that way
were not matched with other filepairs to create rename/copy, then this
transformation merges them back into the original "modification".
The "extent of changes" parameter can be tweaked from the default 80%
(that is, unless more than 80% of the original material is deleted, the
broken pairs are merged back into a single modification) by giving a
second number to -B option, like these:
· -B50/60 (give 50% "break score" to diffcore-break, use 60% for
· -B/60 (the same as above, since diffcore-break defaults to 50%).
Note that earlier implementation left a broken pair as a separate
creation and deletion patches. This was an unnecessary hack and the
latest implementation always merges all the broken pairs back into
modifications, but the resulting patch output is formatted differently
for easier review in case of such a complete rewrite by showing the
entire contents of old version prefixed with -, followed by the entire
contents of new version prefixed with +.
DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING
This transformation limits the set of filepairs to those that change
specified strings between the preimage and the postimage in a certain
way. -S<block of text> and -G<regular expression> options are used to
specify different ways these strings are sought.
"-S<block of text>" detects filepairs whose preimage and postimage have
different number of occurrences of the specified block of text. By
definition, it will not detect in-file moves. Also, when a changeset
moves a file wholesale without affecting the interesting string,
diffcore-rename kicks in as usual, and -S omits the filepair (since the
number of occurrences of that string didn’t change in that
rename-detected filepair). When used with --pickaxe-regex, treat the
<block of text> as an extended POSIX regular expression to match,
instead of a literal string.
"-G<regular expression>" (mnemonic: grep) detects filepairs whose
textual diff has an added or a deleted line that matches the given
regular expression. This means that it will detect in-file (or what
rename-detection considers the same file) moves, which is noise. The
implementation runs diff twice and greps, and this can be quite
When -S or -G are used without --pickaxe-all, only filepairs that match
their respective criterion are kept in the output. When --pickaxe-all
is used, if even one filepair matches their respective criterion in a
changeset, the entire changeset is kept. This behavior is designed to
make reviewing changes in the context of the whole changeset easier.
DIFFCORE-ORDER: FOR SORTING THE OUTPUT BASED ON FILENAMES
This is used to reorder the filepairs according to the user’s (or
project’s) taste, and is controlled by the -O option to the git diff-*
This takes a text file each of whose lines is a shell glob pattern.
Filepairs that match a glob pattern on an earlier line in the file are
output before ones that match a later line, and filepairs that do not
match any glob pattern are output last.
As an example, a typical orderfile for the core Git probably would look
git-diff(1), git-diff-files(1), git-diff-index(1), git-diff-tree(1),
Part of the git(1) suite.
1. The Git User’s Manual