GITDIFFCORE(7)			  Git Manual			GITDIFFCORE(7)



NAME
       gitdiffcore - 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:

	   :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
       consideration.

       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:

       o   diffcore-break

       o   diffcore-rename

       o   diffcore-merge-broken

       o   diffcore-pickaxe

       o   diffcore-order

       o   diffcore-rotate

       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 DETECTING 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
       file file0:

	   :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 that when rename detection is on but both copy and break detection
       are off, rename detection adds a preliminary step that first checks if
       files are moved across directories while keeping their filename the
       same. If there is a file added to a directory whose contents is
       sufficiently similar to a file with the same name that got deleted from
       a different directory, it will mark them as renames and exclude them
       from the later quadratic step (the one that pairwise compares all
       unmatched files to find the "best" matches, determined by the highest
       content similarity). So, for example, if a deleted docs/ext.txt and an
       added docs/config/ext.txt are similar enough, they will be marked as a
       rename and prevent an added docs/ext.md that may be even more similar
       to the deleted docs/ext.txt from being considered as the rename
       destination in the later step. For this reason, the preliminary "match
       same filename" step uses a bit higher threshold to mark a file pair as
       a rename and stop considering other candidates for better matches. At
       most, one comparison is done per file in this preliminary pass; so if
       there are several remaining ext.txt files throughout the directory
       hierarchy after exact rename detection, this preliminary step may be
       skipped for those files.

       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
       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:

       o   -B50/60 (give 50% "break score" to diffcore-break, use 60% for
	   diffcore-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
       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
       expensive. To speed things up binary files without textconv filters
       will be ignored.

       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-*
       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:

	   README
	   Makefile
	   Documentation
	   *.h
	   *.c
	   t


DIFFCORE-ROTATE: FOR CHANGING AT WHICH PATH OUTPUT STARTS
       This transformation takes one pathname, and rotates the set of
       filepairs so that the filepair for the given pathname comes first,
       optionally discarding the paths that come before it. This is used to
       implement the --skip-to and the --rotate-to options. It is an error
       when the specified pathname is not in the set of filepairs, but it is
       not useful to error out when used with "git log" family of commands,
       because it is unreasonable to expect that a given path would be
       modified by each and every commit shown by the "git log" command. For
       this reason, when used with "git log", the filepair that sorts the same
       as, or the first one that sorts after, the given pathname is where the
       output starts.

       Use of this transformation combined with diffcore-order will produce
       unexpected results, as the input to this transformation is likely not
       sorted when diffcore-order is in effect.

SEE ALSO
       git-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[1]

GIT
       Part of the git(1) suite

NOTES
	1. The Git User's Manual
	   file:///usr/share/doc/git-2.38.4/user-manual.html



Git 2.38.4			  05/16/2024			GITDIFFCORE(7)