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+Tweaking diff output
+====================
+June 2005
+
+
+Introduction
+------------
+
+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(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;
+
+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
+------------------------------------------------
+