# gitdiffcore

## 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
    
           •   diffcore-rotate
    
           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 DETECTING 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 that when rename detection is on but both copy and break
           detection are off, rename detection adds a preliminary step that
           first checks if files are moved across directories while keeping
           their filename the same. If there is a file added to a directory
           whose contents are sufficiently similar to a file with the same
           name that got deleted from a different directory, it will mark
           them as renames and exclude them from the later quadratic step
           (the one that pairwise compares all unmatched files to find the
           "best" matches, determined by the highest content similarity). So,
           for example, if a deleted docs/ext.txt and an added
           docs/config/ext.txt are similar enough, they will be marked as a
           rename and prevent an added docs/ext.md that may be even more
           similar to the deleted docs/ext.txt from being considered as the
           rename destination in the later step. For this reason, the
           preliminary "match same filename" step uses a bit higher threshold
           to mark a file pair as a rename and stop considering other
           candidates for better matches. At most, one comparison is done per
           file in this preliminary pass; so if there are several remaining
           ext.txt files throughout the directory hierarchy after exact
           rename detection, this preliminary step may be skipped for those
           files.
    
           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**
           **%%%SH%%%**
           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 a 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 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 the old version prefixed with _-_ ,
           followed by the entire contents of the new version prefixed with
           _+_.
    
    **DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING**
           **%%%SH%%%**
           This transformation limits the set of filepairs to those that
           change specified strings between the preimage and the postimage in
           a certain way. -S<block-of-text> and -G<regular-expression>
           options are used to specify different ways these strings are
           sought.
    
           "-S<block-of-text>" detects filepairs whose preimage and postimage
           have different number of occurrences of the specified block of
           text. By definition, it will not detect in-file moves. Also, when
           a changeset moves a file wholesale without affecting the
           interesting string, diffcore-rename kicks in as usual, and **-S**
           omits the filepair (since the number of occurrences of that string
           didn’t change in that rename-detected filepair). When used with
           **--pickaxe-regex** , treat the <block-of-text> as an extended POSIX
           regular expression to match, instead of a literal string.
    
           "-G<regular-expression>" (mnemonic: grep) detects filepairs whose
           textual diff has an added or a deleted line that matches the given
           regular expression. This means that it will detect in-file (or
           what rename-detection considers the same file) moves, which is
           noise. The implementation runs diff twice and greps, and this can
           be quite expensive. To speed things up, binary files without
           textconv filters will be ignored.
    
           When **-S** or **-G** are used without **--pickaxe-all** , only filepairs that
           match their respective criterion are kept in the output. When
           **--pickaxe-all** is used, if even one filepair matches their
           respective criterion in a changeset, the entire changeset is kept.
           This behavior is designed to make reviewing 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
    


## DIFFCORE-ROTATE: FOR CHANGING AT WHICH PATH OUTPUT STARTS



           This transformation takes one pathname, and rotates the set of
           filepairs so that the filepair for the given pathname comes first,
           optionally discarding the paths that come before it. This is used
           to implement the **--skip-to** and the **--rotate-to** options. It is an
           error when the specified pathname is not in the set of filepairs,
           but it is not useful to error out when used with "git log" family
           of commands, because it is unreasonable to expect that a given
           path would be modified by each and every commit shown by the "git
           log" command. For this reason, when used with "git log", the
           filepair that sorts the same as, or the first one that sorts
           after, the given pathname is where the output starts.
    
           Use of this transformation combined with diffcore-order will
           produce unexpected results, as the input to this transformation is
           likely not sorted when diffcore-order is in effect.
    


## GIT



           Part of the git(1) suite
    


## NOTES



            1. The Git User’s Manual
               file:///home/mtk/share/doc/git-doc/user-manual.html
    


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