--- /dev/null
+Tweaking diff output
+====================
+June 2005
+
+
+Introduction
+------------
+
+The diff commands git-diff-cache, git-diff-files, and
+git-diff-tree 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-cache 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 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... N 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 7 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
+-----------------
+
+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.
+
+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
+--------------
+
+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... N 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
+---------------
+
+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... N 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 contents of unchanged files are considered as
+candidates of the source files in rename/copy operation, in
+addition to the deleted files. 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... 0123456... N 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... 0123456... 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
+similarity score different from the default 50% by giving a
+number after "-M" or "-C" option (e.g. "-M8" to tell it to use
+8/10 = 80%).
+
+Note. When the "-C" option is used, git-diff-cache and
+git-diff-file commands feed not just modified filepairs but
+unmodified ones to diffcore mechanism as well. This lets the
+copy detector consider unmodified files as copy source
+candidates at the expense of making it slower. Currently
+git-diff-tree does not feed unmodified filepairs even when the
+"-C" option is used, so it can detect copies only if the file
+that was copied happened to have been modified in the same
+changeset.
+
+
+diffcore-merge-broken
+---------------------
+
+This transformation is used to merge filepairs broken by
+diffcore-break, and were 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 defautls to
+ 50%).
+
+
+diffcore-pickaxe
+----------------
+
+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 touches 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
+--------------
+
+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 line 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, typical orderfile for the core GIT probably
+should look like this:
+
+ README
+ Makefile
+ Documentation
+ *.h
+ *.c
+ t
+
projects / git.git / commitdiff