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GITDIFFCORE(7)                                   Git Manual                                   GITDIFFCORE(7)



NAME
       gitdiffcore - Tweaking diff output

SYNOPSIS
       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-* 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

       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.

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 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 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 "result" side and whose
       "origin" side have different number of specified string. 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


SEE ALSO
       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 Manual[1]

GIT
       Part of the git(1) suite.

NOTES
        1. The Git User's Manual
           git-htmldocs/user-manual.html



Git 1.8.3                                        05/24/2013                                   GITDIFFCORE(7)

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