Documentation/gitdiffcore.txt - jrn/git - Git at Google

 gitdiffcore(7)
 ==============

 NAME
 ----
 gitdiffcore - Tweaking diff output

 SYNOPSIS
 --------
 [verse]
 '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-{asterisk}' 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 directory;

  - '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:

 - diffcore-break
 - diffcore-rename
 - diffcore-merge-broken
 - diffcore-pickaxe
 - diffcore-order
 - diffcore-rotate

 These are applied in sequence.  The set of filepairs 'git diff-{asterisk}'
 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-{asterisk}' 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-{asterisk}' 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-{asterisk}' 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-{asterisk}' 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-{asterisk}' 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:

 * -B50/60 (give 50% "break score" to diffcore-break, use 60%
   for diffcore-merge-broken).

 * -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-{asterisk}' 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
 --------
 linkgit:git-diff[1],
 linkgit:git-diff-files[1],
 linkgit:git-diff-index[1],
 linkgit:git-diff-tree[1],
 linkgit:git-format-patch[1],
 linkgit:git-log[1],
 linkgit:gitglossary[7],
 link:user-manual.html[The Git User's Manual]

 GIT
 ---
 Part of the linkgit:git[1] suite
	gitdiffcore(7)
	==============

	NAME
	----
	gitdiffcore - Tweaking diff output

	SYNOPSIS
	--------
	[verse]
	'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-{asterisk}' 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 directory;

	- '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:

	- diffcore-break
	- diffcore-rename
	- diffcore-merge-broken
	- diffcore-pickaxe
	- diffcore-order
	- diffcore-rotate

	These are applied in sequence. The set of filepairs 'git diff-{asterisk}'
	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-{asterisk}' 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-{asterisk}' 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-{asterisk}' 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-{asterisk}' 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-{asterisk}' 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:

	* -B50/60 (give 50% "break score" to diffcore-break, use 60%
	for diffcore-merge-broken).

	* -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-{asterisk}' 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
	--------
	linkgit:git-diff[1],
	linkgit:git-diff-files[1],
	linkgit:git-diff-index[1],
	linkgit:git-diff-tree[1],
	linkgit:git-format-patch[1],
	linkgit:git-log[1],
	linkgit:gitglossary[7],
	link:user-manual.html[The Git User's Manual]

	GIT
	---
	Part of the linkgit:git[1] suite