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 is 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

       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 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.

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
           file:///usr/share/doc/git/html/user-manual.html