GITDIFFCORE(7)            (08/17/2021)             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:

          +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 directory;

          +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 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.

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              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

          +o   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

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          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

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          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

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          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 diffcore-merge-broken).

          +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 +.

     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 didncqt 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.

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          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
          usercqs (or projectcqs) 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-

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          diff-tree(1), git-format-patch(1), git-log(1),
          gitglossary(7), m[blue]The Git Usercqs Manualm[][1]

     GIT
          Part of the git(1) suite

     NOTES
           1. The Git Usercqs Manual
              file:///usr/share/doc/git/html/user-manual.html

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