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

 Tweaking diff output
 ====================
 June 2005


 Introduction
 ------------

 The diff commands git-diff-index, git-diff-files, git-diff-tree, and
 git-diff-stages can be told to manipulate differences they find in
 unconventional ways before showing diff(1) output.  The manipulation
 is collectively called "diffcore transformation".  This short note
 describes what they are and how to use them to produce diff outputs
 that are 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 directory;

  - git-diff-tree compares contents of two "tree" objects;

  - git-diff-stages compares contents of blobs at two stages in an
    unmerged index file.

 In all of these cases, the commands themselves compare
 corresponding paths in the two sets of files.  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 6 such transformations:

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

 These are applied in sequence.  The set of filepairs git-diff-\*
 commands find are used as the input to diffcore-pathspec, and
 the output from diffcore-pathspec 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-pathspec: For Ignoring Files Outside Our Consideration
 ---------------------------------------------------------------

 The first transformation in the chain is diffcore-pathspec, and
 is controlled by giving the pathname parameters to the
 git-diff-* commands on the command line.  The pathspec is used
 to limit the world diff operates in.  It removes the filepairs
 outside the specified set 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.

 Implementation note.  For performance reasons, git-diff-tree
 uses the pathname parameters on the command line to cull set of
 filepairs it feeds the diffcore mechanism itself, and does not
 use diffcore-pathspec, but the end result is the same.


 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 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 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 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:

 * -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 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 "original" side has the specified string and
 whose "result" side does not.  Such a filepair represents "the
 string appeared in this changeset".  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 `\--pickaxe-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.


 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
 ------------------------------------------------
	Tweaking diff output
	====================
	June 2005


	Introduction
	------------

	The diff commands git-diff-index, git-diff-files, git-diff-tree, and
	git-diff-stages can be told to manipulate differences they find in
	unconventional ways before showing diff(1) output. The manipulation
	is collectively called "diffcore transformation". This short note
	describes what they are and how to use them to produce diff outputs
	that are 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 directory;

	- git-diff-tree compares contents of two "tree" objects;

	- git-diff-stages compares contents of blobs at two stages in an
	unmerged index file.

	In all of these cases, the commands themselves compare
	corresponding paths in the two sets of files. 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 6 such transformations:

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

	These are applied in sequence. The set of filepairs git-diff-\*
	commands find are used as the input to diffcore-pathspec, and
	the output from diffcore-pathspec 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-pathspec: For Ignoring Files Outside Our Consideration
	---------------------------------------------------------------

	The first transformation in the chain is diffcore-pathspec, and
	is controlled by giving the pathname parameters to the
	git-diff-* commands on the command line. The pathspec is used
	to limit the world diff operates in. It removes the filepairs
	outside the specified set 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.

	Implementation note. For performance reasons, git-diff-tree
	uses the pathname parameters on the command line to cull set of
	filepairs it feeds the diffcore mechanism itself, and does not
	use diffcore-pathspec, but the end result is the same.


	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 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 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 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:

	* -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 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 "original" side has the specified string and
	whose "result" side does not. Such a filepair represents "the
	string appeared in this changeset". 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 `\--pickaxe-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.


	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
	------------------------------------------------