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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

       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. 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.

       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

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
	   git-htmldocs/user-manual.html

Git 2.13.2			  06/24/2017			GITDIFFCORE(7)

NAME | SYNOPSIS | DESCRIPTION | THE CHAIN OF OPERATION | DIFFCORE-BREAK: FOR SPLITTING UP COMPLETE REWRITES | DIFFCORE-RENAME: FOR DETECTING RENAMES AND COPIES | DIFFCORE-MERGE-BROKEN: FOR PUTTING COMPLETE REWRITES BACK TOGETHER | DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING | DIFFCORE-ORDER: FOR SORTING THE OUTPUT BASED ON FILENAMES | SEE ALSO | GIT | NOTES

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