Documentation/gitdiffcore.txt

   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 that when rename detection is on but both copy and break
 172 detection are off, rename detection adds a preliminary step that first
 173 checks if files are moved across directories while keeping their
 174 filename the same.  If there is a file added to a directory whose
 175 contents is sufficiently similar to a file with the same name that got
 176 deleted from a different directory, it will mark them as renames and
 177 exclude them from the later quadratic step (the one that pairwise
 178 compares all unmatched files to find the "best" matches, determined by
 179 the highest content similarity).  So, for example, if a deleted
 180 docs/ext.txt and an added docs/config/ext.txt are similar enough, they
 181 will be marked as a rename and prevent an added docs/ext.md that may
 182 be even more similar to the deleted docs/ext.txt from being considered
 183 as the rename destination in the later step.  For this reason, the
 184 preliminary "match same filename" step uses a bit higher threshold to
 185 mark a file pair as a rename and stop considering other candidates for
 186 better matches.  At most, one comparison is done per file in this
 187 preliminary pass; so if there are several remaining ext.txt files
 188 throughout the directory hierarchy after exact rename detection, this
 189 preliminary step may be skipped for those files.
 190
 191 Note.  When the "-C" option is used with `--find-copies-harder`
 192 option, 'git diff-{asterisk}' commands feed unmodified filepairs to
 193 diffcore mechanism as well as modified ones.  This lets the copy
 194 detector consider unmodified files as copy source candidates at
 195 the expense of making it slower.  Without `--find-copies-harder`,
 196 'git diff-{asterisk}' commands can detect copies only if the file that was
 197 copied happened to have been modified in the same changeset.
 198
 199
 200 diffcore-merge-broken: For Putting Complete Rewrites Back Together
 201 ------------------------------------------------------------------
 202
 203 This transformation is used to merge filepairs broken by
 204 diffcore-break, and not transformed into rename/copy by
 205 diffcore-rename, back into a single modification.  This always
 206 runs when diffcore-break is used.
 207
 208 For the purpose of merging broken filepairs back, it uses a
 209 different "extent of changes" computation from the ones used by
 210 diffcore-break and diffcore-rename.  It counts only the deletion
 211 from the original, and does not count insertion.  If you removed
 212 only 10 lines from a 100-line document, even if you added 910
 213 new lines to make a new 1000-line document, you did not do a
 214 complete rewrite.  diffcore-break breaks such a case in order to
 215 help diffcore-rename to consider such filepairs as candidate of
 216 rename/copy detection, but if filepairs broken that way were not
 217 matched with other filepairs to create rename/copy, then this
 218 transformation merges them back into the original
 219 "modification".
 220
 221 The "extent of changes" parameter can be tweaked from the
 222 default 80% (that is, unless more than 80% of the original
 223 material is deleted, the broken pairs are merged back into a
 224 single modification) by giving a second number to -B option,
 225 like these:
 226
 227 * -B50/60 (give 50% "break score" to diffcore-break, use 60%
 228   for diffcore-merge-broken).
 229
 230 * -B/60 (the same as above, since diffcore-break defaults to 50%).
 231
 232 Note that earlier implementation left a broken pair as a separate
 233 creation and deletion patches.  This was an unnecessary hack and
 234 the latest implementation always merges all the broken pairs
 235 back into modifications, but the resulting patch output is
 236 formatted differently for easier review in case of such
 237 a complete rewrite by showing the entire contents of old version
 238 prefixed with '-', followed by the entire contents of new
 239 version prefixed with '+'.
 240
 241
 242 diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
 243 ---------------------------------------------------------------------
 244
 245 This transformation limits the set of filepairs to those that change
 246 specified strings between the preimage and the postimage in a certain
 247 way.  -S<block of text> and -G<regular expression> options are used to
 248 specify different ways these strings are sought.
 249
 250 "-S<block of text>" detects filepairs whose preimage and postimage
 251 have different number of occurrences of the specified block of text.
 252 By definition, it will not detect in-file moves.  Also, when a
 253 changeset moves a file wholesale without affecting the interesting
 254 string, diffcore-rename kicks in as usual, and `-S` omits the filepair
 255 (since the number of occurrences of that string didn't change in that
 256 rename-detected filepair).  When used with `--pickaxe-regex`, treat
 257 the <block of text> as an extended POSIX regular expression to match,
 258 instead of a literal string.
 259
 260 "-G<regular expression>" (mnemonic: grep) detects filepairs whose
 261 textual diff has an added or a deleted line that matches the given
 262 regular expression.  This means that it will detect in-file (or what
 263 rename-detection considers the same file) moves, which is noise.  The
 264 implementation runs diff twice and greps, and this can be quite
 265 expensive.  To speed things up binary files without textconv filters
 266 will be ignored.
 267
 268 When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
 269 that match their respective criterion are kept in the output.  When
 270 `--pickaxe-all` is used, if even one filepair matches their respective
 271 criterion in a changeset, the entire changeset is kept.  This behavior
 272 is designed to make reviewing changes in the context of the whole
 273 changeset easier.
 274
 275 diffcore-order: For Sorting the Output Based on Filenames
 276 ---------------------------------------------------------
 277
 278 This is used to reorder the filepairs according to the user's
 279 (or project's) taste, and is controlled by the -O option to the
 280 'git diff-{asterisk}' commands.
 281
 282 This takes a text file each of whose lines is a shell glob
 283 pattern.  Filepairs that match a glob pattern on an earlier line
 284 in the file are output before ones that match a later line, and
 285 filepairs that do not match any glob pattern are output last.
 286
 287 As an example, a typical orderfile for the core Git probably
 288 would look like this:
 289
 290 ------------------------------------------------
 291 README
 292 Makefile
 293 Documentation
 294 *.h
 295 *.c
 296 t
 297 ------------------------------------------------
 298
 299 SEE ALSO
 300 --------
 301 linkgit:git-diff[1],
 302 linkgit:git-diff-files[1],
 303 linkgit:git-diff-index[1],
 304 linkgit:git-diff-tree[1],
 305 linkgit:git-format-patch[1],
 306 linkgit:git-log[1],
 307 linkgit:gitglossary[7],
 308 link:user-manual.html[The Git User's Manual]
 309
 310 GIT
 311 ---
 312 Part of the linkgit:git[1] suite