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       gitdiffcore - Tweaking diff output


       git diff *


       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 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

       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

       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.


       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%).


       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.


       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

       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 +.


       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.


       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-*

       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:



       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


       Part of the git(1) suite.


        1. The Git User's Manual