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    1 gitdiffcore(7)
    2 ==============
    3 
    4 NAME
    5 ----
    6 gitdiffcore - Tweaking diff output
    7 
    8 SYNOPSIS
    9 --------
   10 [verse]
   11 'git diff' *
   12 
   13 DESCRIPTION
   14 -----------
   15 
   16 The diff commands 'git diff-index', 'git diff-files', and 'git diff-tree'
   17 can be told to manipulate differences they find in
   18 unconventional ways before showing 'diff' output.  The manipulation
   19 is collectively called "diffcore transformation".  This short note
   20 describes what they are and how to use them to produce 'diff' output
   21 that is easier to understand than the conventional kind.
   22 
   23 
   24 The chain of operation
   25 ----------------------
   26 
   27 The 'git diff-{asterisk}' family works by first comparing two sets of
   28 files:
   29 
   30  - 'git diff-index' compares contents of a "tree" object and the
   31    working directory (when `--cached` flag is not used) or a
   32    "tree" object and the index file (when `--cached` flag is
   33    used);
   34 
   35  - 'git diff-files' compares contents of the index file and the
   36    working directory;
   37 
   38  - 'git diff-tree' compares contents of two "tree" objects;
   39 
   40 In all of these cases, the commands themselves first optionally limit
   41 the two sets of files by any pathspecs given on their command-lines,
   42 and compare corresponding paths in the two resulting sets of files.
   43 
   44 The pathspecs are used to limit the world diff operates in.  They remove
   45 the filepairs outside the specified sets of pathnames.  E.g. If the
   46 input set of filepairs included:
   47 
   48 ------------------------------------------------
   49 :100644 100644 bcd1234... 0123456... M junkfile
   50 ------------------------------------------------
   51 
   52 but the command invocation was `git diff-files myfile`, then the
   53 junkfile entry would be removed from the list because only "myfile"
   54 is under consideration.
   55 
   56 The result of comparison is passed from these commands to what is
   57 internally called "diffcore", in a format similar to what is output
   58 when the -p option is not used.  E.g.
   59 
   60 ------------------------------------------------
   61 in-place edit  :100644 100644 bcd1234... 0123456... M file0
   62 create         :000000 100644 0000000... 1234567... A file4
   63 delete         :100644 000000 1234567... 0000000... D file5
   64 unmerged       :000000 000000 0000000... 0000000... U file6
   65 ------------------------------------------------
   66 
   67 The diffcore mechanism is fed a list of such comparison results
   68 (each of which is called "filepair", although at this point each
   69 of them talks about a single file), and transforms such a list
   70 into another list.  There are currently 5 such transformations:
   71 
   72 - diffcore-break
   73 - diffcore-rename
   74 - diffcore-merge-broken
   75 - diffcore-pickaxe
   76 - diffcore-order
   77 
   78 These are applied in sequence.  The set of filepairs 'git diff-{asterisk}'
   79 commands find are used as the input to diffcore-break, and
   80 the output from diffcore-break is used as the input to the
   81 next transformation.  The final result is then passed to the
   82 output routine and generates either diff-raw format (see Output
   83 format sections of the manual for 'git diff-{asterisk}' commands) or
   84 diff-patch format.
   85 
   86 
   87 diffcore-break: For Splitting Up Complete Rewrites
   88 --------------------------------------------------
   89 
   90 The second transformation in the chain is diffcore-break, and is
   91 controlled by the -B option to the 'git diff-{asterisk}' commands.  This is
   92 used to detect a filepair that represents "complete rewrite" and
   93 break such filepair into two filepairs that represent delete and
   94 create.  E.g.  If the input contained this filepair:
   95 
   96 ------------------------------------------------
   97 :100644 100644 bcd1234... 0123456... M file0
   98 ------------------------------------------------
   99 
  100 and if it detects that the file "file0" is completely rewritten,
  101 it changes it to:
  102 
  103 ------------------------------------------------
  104 :100644 000000 bcd1234... 0000000... D file0
  105 :000000 100644 0000000... 0123456... A file0
  106 ------------------------------------------------
  107 
  108 For the purpose of breaking a filepair, diffcore-break examines
  109 the extent of changes between the contents of the files before
  110 and after modification (i.e. the contents that have "bcd1234..."
  111 and "0123456..." as their SHA-1 content ID, in the above
  112 example).  The amount of deletion of original contents and
  113 insertion of new material are added together, and if it exceeds
  114 the "break score", the filepair is broken into two.  The break
  115 score defaults to 50% of the size of the smaller of the original
  116 and the result (i.e. if the edit shrinks the file, the size of
  117 the result is used; if the edit lengthens the file, the size of
  118 the original is used), and can be customized by giving a number
  119 after "-B" option (e.g. "-B75" to tell it to use 75%).
  120 
  121 
  122 diffcore-rename: For Detecting Renames and Copies
  123 -------------------------------------------------
  124 
  125 This transformation is used to detect renames and copies, and is
  126 controlled by the -M option (to detect renames) and the -C option
  127 (to detect copies as well) to the 'git diff-{asterisk}' commands.  If the
  128 input contained these filepairs:
  129 
  130 ------------------------------------------------
  131 :100644 000000 0123456... 0000000... D fileX
  132 :000000 100644 0000000... 0123456... A file0
  133 ------------------------------------------------
  134 
  135 and the contents of the deleted file fileX is similar enough to
  136 the contents of the created file file0, then rename detection
  137 merges these filepairs and creates:
  138 
  139 ------------------------------------------------
  140 :100644 100644 0123456... 0123456... R100 fileX file0
  141 ------------------------------------------------
  142 
  143 When the "-C" option is used, the original contents of modified files,
  144 and deleted files (and also unmodified files, if the
  145 "--find-copies-harder" option is used) are considered as candidates
  146 of the source files in rename/copy operation.  If the input were like
  147 these filepairs, that talk about a modified file fileY and a newly
  148 created file file0:
  149 
  150 ------------------------------------------------
  151 :100644 100644 0123456... 1234567... M fileY
  152 :000000 100644 0000000... bcd3456... A file0
  153 ------------------------------------------------
  154 
  155 the original contents of fileY and the resulting contents of
  156 file0 are compared, and if they are similar enough, they are
  157 changed to:
  158 
  159 ------------------------------------------------
  160 :100644 100644 0123456... 1234567... M fileY
  161 :100644 100644 0123456... bcd3456... C100 fileY file0
  162 ------------------------------------------------
  163 
  164 In both rename and copy detection, the same "extent of changes"
  165 algorithm used in diffcore-break is used to determine if two
  166 files are "similar enough", and can be customized to use
  167 a similarity score different from the default of 50% by giving a
  168 number after the "-M" or "-C" option (e.g. "-M8" to tell it to use
  169 8/10 = 80%).
  170 
  171 Note.  When the "-C" option is used with `--find-copies-harder`
  172 option, 'git diff-{asterisk}' commands feed unmodified filepairs to
  173 diffcore mechanism as well as modified ones.  This lets the copy
  174 detector consider unmodified files as copy source candidates at
  175 the expense of making it slower.  Without `--find-copies-harder`,
  176 'git diff-{asterisk}' commands can detect copies only if the file that was
  177 copied happened to have been modified in the same changeset.
  178 
  179 
  180 diffcore-merge-broken: For Putting Complete Rewrites Back Together
  181 ------------------------------------------------------------------
  182 
  183 This transformation is used to merge filepairs broken by
  184 diffcore-break, and not transformed into rename/copy by
  185 diffcore-rename, back into a single modification.  This always
  186 runs when diffcore-break is used.
  187 
  188 For the purpose of merging broken filepairs back, it uses a
  189 different "extent of changes" computation from the ones used by
  190 diffcore-break and diffcore-rename.  It counts only the deletion
  191 from the original, and does not count insertion.  If you removed
  192 only 10 lines from a 100-line document, even if you added 910
  193 new lines to make a new 1000-line document, you did not do a
  194 complete rewrite.  diffcore-break breaks such a case in order to
  195 help diffcore-rename to consider such filepairs as candidate of
  196 rename/copy detection, but if filepairs broken that way were not
  197 matched with other filepairs to create rename/copy, then this
  198 transformation merges them back into the original
  199 "modification".
  200 
  201 The "extent of changes" parameter can be tweaked from the
  202 default 80% (that is, unless more than 80% of the original
  203 material is deleted, the broken pairs are merged back into a
  204 single modification) by giving a second number to -B option,
  205 like these:
  206 
  207 * -B50/60 (give 50% "break score" to diffcore-break, use 60%
  208   for diffcore-merge-broken).
  209 
  210 * -B/60 (the same as above, since diffcore-break defaults to 50%).
  211 
  212 Note that earlier implementation left a broken pair as a separate
  213 creation and deletion patches.  This was an unnecessary hack and
  214 the latest implementation always merges all the broken pairs
  215 back into modifications, but the resulting patch output is
  216 formatted differently for easier review in case of such
  217 a complete rewrite by showing the entire contents of old version
  218 prefixed with '-', followed by the entire contents of new
  219 version prefixed with '+'.
  220 
  221 
  222 diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
  223 ---------------------------------------------------------------------
  224 
  225 This transformation limits the set of filepairs to those that change
  226 specified strings between the preimage and the postimage in a certain
  227 way.  -S<block of text> and -G<regular expression> options are used to
  228 specify different ways these strings are sought.
  229 
  230 "-S<block of text>" detects filepairs whose preimage and postimage
  231 have different number of occurrences of the specified block of text.
  232 By definition, it will not detect in-file moves.  Also, when a
  233 changeset moves a file wholesale without affecting the interesting
  234 string, diffcore-rename kicks in as usual, and `-S` omits the filepair
  235 (since the number of occurrences of that string didn't change in that
  236 rename-detected filepair).  When used with `--pickaxe-regex`, treat
  237 the <block of text> as an extended POSIX regular expression to match,
  238 instead of a literal string.
  239 
  240 "-G<regular expression>" (mnemonic: grep) detects filepairs whose
  241 textual diff has an added or a deleted line that matches the given
  242 regular expression.  This means that it will detect in-file (or what
  243 rename-detection considers the same file) moves, which is noise.  The
  244 implementation runs diff twice and greps, and this can be quite
  245 expensive.
  246 
  247 When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
  248 that match their respective criterion are kept in the output.  When
  249 `--pickaxe-all` is used, if even one filepair matches their respective
  250 criterion in a changeset, the entire changeset is kept.  This behavior
  251 is designed to make reviewing changes in the context of the whole
  252 changeset easier.
  253 
  254 diffcore-order: For Sorting the Output Based on Filenames
  255 ---------------------------------------------------------
  256 
  257 This is used to reorder the filepairs according to the user's
  258 (or project's) taste, and is controlled by the -O option to the
  259 'git diff-{asterisk}' commands.
  260 
  261 This takes a text file each of whose lines is a shell glob
  262 pattern.  Filepairs that match a glob pattern on an earlier line
  263 in the file are output before ones that match a later line, and
  264 filepairs that do not match any glob pattern are output last.
  265 
  266 As an example, a typical orderfile for the core Git probably
  267 would look like this:
  268 
  269 ------------------------------------------------
  270 README
  271 Makefile
  272 Documentation
  273 *.h
  274 *.c
  275 t
  276 ------------------------------------------------
  277 
  278 SEE ALSO
  279 --------
  280 linkgit:git-diff[1],
  281 linkgit:git-diff-files[1],
  282 linkgit:git-diff-index[1],
  283 linkgit:git-diff-tree[1],
  284 linkgit:git-format-patch[1],
  285 linkgit:git-log[1],
  286 linkgit:gitglossary[7],
  287 link:user-manual.html[The Git User's Manual]
  288 
  289 GIT
  290 ---
  291 Part of the linkgit:git[1] suite