GITDIFFCORE(7)GITDIFFCORE(7)NAMEgitdiffcore - Tweaking diff output
SYNOPSIS
git diff *
DESCRIPTION
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:
o 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);
o git diff-files compares contents of the index file and the working
directory;
o 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:
.ft C
:100644 100644 bcd1234... 0123456... M junkfile
.ft
but the command invocation was git diff-files myfile, then the junkfile
entry would be removed from the list because only "myfile" is under
consideration.
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GITDIFFCORE(7)GITDIFFCORE(7)
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.
.ft C
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
.ft
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:
o diffcore-break
o diffcore-rename
o diffcore-merge-broken
o diffcore-pickaxe
o diffcore-order
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 diff-
core-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 con-
trolled 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:
.ft C
:100644 100644 bcd1234... 0123456... M file0
.ft
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GITDIFFCORE(7)GITDIFFCORE(7)
and if it detects that the file "file0" is completely rewritten, it
changes it to:
.ft C
:100644 000000 bcd1234... 0000000... D file0
:000000 100644 0000000... 0123456... A file0
.ft
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 SHA1 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 orig-
inal 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 origi-
nal 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 con-
trolled by the -M option (to detect renames) and the -C option (to
detect copies as well) to the git diff-* commands. If the input con-
tained these filepairs:
.ft C
:100644 000000 0123456... 0000000... D fileX
:000000 100644 0000000... 0123456... A file0
.ft
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:
.ft C
:100644 100644 0123456... 0123456... R100 fileX file0
.ft
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GITDIFFCORE(7)GITDIFFCORE(7)
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
file file0:
.ft C
:100644 100644 0123456... 1234567... M fileY
:000000 100644 0000000... bcd3456... A file0
.ft
the original contents of fileY and the resulting contents of file0 are
compared, and if they are similar enough, they are changed to:
.ft C
:100644 100644 0123456... 1234567... M fileY
:100644 100644 0123456... bcd3456... C100 fileY file0
.ft
In both rename and copy detection, the same "extent of changes" algo-
rithm used in diffcore-break is used to determine if two files are
"similar enough", and can be customized to use a similarity score dif-
ferent 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
same changeset.
DIFFCORE-MERGE-BROKEN: FOR PUTTING COMPLETE REWRITES" BACK TOGETHER"
This transformation is used to merge filepairs broken by diff-
core-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
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GITDIFFCORE(7)GITDIFFCORE(7)
"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 docu-
ment, 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:
o -B50/60 (give 50% "break score" to diffcore-break, use 60% for diff-
core-merge-broken).
o -B/60 (the same as above, since diffcore-break defaults to 50%).
Note that earlier implementation left a broken pair as a separate cre-
ation and deletion patches. This was an unnecessary hack and the latest
implementation always merges all the broken pairs back into modifica-
tions, but the resulting patch output is formatted differently for eas-
ier review in case of such a complete rewrite by showing the entire
contents of old version prefixed with -, followed by the entire con-
tents of new version prefixed with +.
DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING
This transformation is used to find filepairs that represent changes
that touch a specified string, and is controlled by the -S option and
the \--pickaxe-all option to the git diff-* commands.
When diffcore-pickaxe is in use, it checks if there are filepairs whose
"result" side has the specified string and whose "origin" side does
not. Such a filepair represents "the string appeared in this change-
set". It also checks for the opposite case that loses the specified
string.
When \--pickaxe-all is not in effect, diffcore-pickaxe leaves only such
filepairs that touch the specified string in its output. When \--pick-
axe-all is used, diffcore-pickaxe leaves all filepairs intact if there
is such a filepair, or makes the output empty otherwise. The latter
behaviour is designed to make reviewing of the changes in the context
of the whole changeset easier.
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GITDIFFCORE(7)GITDIFFCORE(7)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-* commands.
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
like this:
.ft C
README
Makefile
Documentation
*.h
*.c
t
.ft
SEE ALSOgit-diff(1), git-diff-files(1), git-diff-index(1), git-diff-tree(1),
git-format-patch(1), git-log(1), gitglossary(7), The Git User’s
Manual: user-manual.html
GIT
Part of the git(1) suite.
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