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4a1332d0 JH |
1 | Tweaking diff output |
2 | ==================== | |
3 | June 2005 | |
4 | ||
5 | ||
6 | Introduction | |
7 | ------------ | |
8 | ||
59df2a11 CS |
9 | The diff commands git-diff-index, git-diff-files, git-diff-tree, and |
10 | git-diff-stages can be told to manipulate differences they find in | |
11 | unconventional ways before showing diff(1) output. The manipulation | |
12 | is collectively called "diffcore transformation". This short note | |
13 | describes what they are and how to use them to produce diff outputs | |
14 | that are easier to understand than the conventional kind. | |
4a1332d0 JH |
15 | |
16 | ||
17 | The chain of operation | |
18 | ---------------------- | |
19 | ||
20 | The git-diff-* family works by first comparing two sets of | |
21 | files: | |
22 | ||
215a7ad1 | 23 | - git-diff-index compares contents of a "tree" object and the |
e1ccf53a YS |
24 | working directory (when '\--cached' flag is not used) or a |
25 | "tree" object and the index file (when '\--cached' flag is | |
4a1332d0 JH |
26 | used); |
27 | ||
28 | - git-diff-files compares contents of the index file and the | |
29 | working directory; | |
30 | ||
59df2a11 CS |
31 | - git-diff-tree compares contents of two "tree" objects; |
32 | ||
33 | - git-diff-stages compares contents of blobs at two stages in an | |
34 | unmerged index file. | |
4a1332d0 JH |
35 | |
36 | In all of these cases, the commands themselves compare | |
37 | corresponding paths in the two sets of files. The result of | |
38 | comparison is passed from these commands to what is internally | |
39 | called "diffcore", in a format similar to what is output when | |
40 | the -p option is not used. E.g. | |
41 | ||
8db9307c JH |
42 | ------------------------------------------------ |
43 | in-place edit :100644 100644 bcd1234... 0123456... M file0 | |
44 | create :000000 100644 0000000... 1234567... A file4 | |
45 | delete :100644 000000 1234567... 0000000... D file5 | |
46 | unmerged :000000 000000 0000000... 0000000... U file6 | |
47 | ------------------------------------------------ | |
4a1332d0 JH |
48 | |
49 | The diffcore mechanism is fed a list of such comparison results | |
50 | (each of which is called "filepair", although at this point each | |
51 | of them talks about a single file), and transforms such a list | |
28f8faff | 52 | into another list. There are currently 6 such transformations: |
4a1332d0 | 53 | |
8db9307c JH |
54 | - diffcore-pathspec |
55 | - diffcore-break | |
56 | - diffcore-rename | |
57 | - diffcore-merge-broken | |
58 | - diffcore-pickaxe | |
59 | - diffcore-order | |
4a1332d0 | 60 | |
8db9307c | 61 | These are applied in sequence. The set of filepairs git-diff-\* |
4a1332d0 JH |
62 | commands find are used as the input to diffcore-pathspec, and |
63 | the output from diffcore-pathspec is used as the input to the | |
64 | next transformation. The final result is then passed to the | |
65 | output routine and generates either diff-raw format (see Output | |
8db9307c | 66 | format sections of the manual for git-diff-\* commands) or |
4a1332d0 JH |
67 | diff-patch format. |
68 | ||
69 | ||
59df2a11 | 70 | diffcore-pathspec: For Ignoring Files Outside Our Consideration |
a67c1d08 | 71 | --------------------------------------------------------------- |
4a1332d0 JH |
72 | |
73 | The first transformation in the chain is diffcore-pathspec, and | |
74 | is controlled by giving the pathname parameters to the | |
75 | git-diff-* commands on the command line. The pathspec is used | |
76 | to limit the world diff operates in. It removes the filepairs | |
59df2a11 CS |
77 | outside the specified set of pathnames. E.g. If the input set |
78 | of filepairs included: | |
79 | ||
80 | ------------------------------------------------ | |
81 | :100644 100644 bcd1234... 0123456... M junkfile | |
82 | ------------------------------------------------ | |
83 | ||
84 | but the command invocation was "git-diff-files myfile", then the | |
85 | junkfile entry would be removed from the list because only "myfile" | |
86 | is under consideration. | |
4a1332d0 JH |
87 | |
88 | Implementation note. For performance reasons, git-diff-tree | |
89 | uses the pathname parameters on the command line to cull set of | |
90 | filepairs it feeds the diffcore mechanism itself, and does not | |
91 | use diffcore-pathspec, but the end result is the same. | |
92 | ||
93 | ||
59df2a11 | 94 | diffcore-break: For Splitting Up "Complete Rewrites" |
a67c1d08 | 95 | ---------------------------------------------------- |
4a1332d0 JH |
96 | |
97 | The second transformation in the chain is diffcore-break, and is | |
98 | controlled by the -B option to the git-diff-* commands. This is | |
99 | used to detect a filepair that represents "complete rewrite" and | |
100 | break such filepair into two filepairs that represent delete and | |
101 | create. E.g. If the input contained this filepair: | |
102 | ||
8db9307c JH |
103 | ------------------------------------------------ |
104 | :100644 100644 bcd1234... 0123456... M file0 | |
105 | ------------------------------------------------ | |
4a1332d0 JH |
106 | |
107 | and if it detects that the file "file0" is completely rewritten, | |
108 | it changes it to: | |
109 | ||
8db9307c JH |
110 | ------------------------------------------------ |
111 | :100644 000000 bcd1234... 0000000... D file0 | |
112 | :000000 100644 0000000... 0123456... A file0 | |
113 | ------------------------------------------------ | |
4a1332d0 JH |
114 | |
115 | For the purpose of breaking a filepair, diffcore-break examines | |
116 | the extent of changes between the contents of the files before | |
117 | and after modification (i.e. the contents that have "bcd1234..." | |
118 | and "0123456..." as their SHA1 content ID, in the above | |
119 | example). The amount of deletion of original contents and | |
120 | insertion of new material are added together, and if it exceeds | |
121 | the "break score", the filepair is broken into two. The break | |
122 | score defaults to 50% of the size of the smaller of the original | |
123 | and the result (i.e. if the edit shrinks the file, the size of | |
124 | the result is used; if the edit lengthens the file, the size of | |
125 | the original is used), and can be customized by giving a number | |
126 | after "-B" option (e.g. "-B75" to tell it to use 75%). | |
127 | ||
128 | ||
59df2a11 | 129 | diffcore-rename: For Detection Renames and Copies |
a67c1d08 | 130 | ------------------------------------------------- |
4a1332d0 JH |
131 | |
132 | This transformation is used to detect renames and copies, and is | |
133 | controlled by the -M option (to detect renames) and the -C option | |
134 | (to detect copies as well) to the git-diff-* commands. If the | |
135 | input contained these filepairs: | |
136 | ||
8db9307c JH |
137 | ------------------------------------------------ |
138 | :100644 000000 0123456... 0000000... D fileX | |
139 | :000000 100644 0000000... 0123456... A file0 | |
140 | ------------------------------------------------ | |
4a1332d0 JH |
141 | |
142 | and the contents of the deleted file fileX is similar enough to | |
143 | the contents of the created file file0, then rename detection | |
144 | merges these filepairs and creates: | |
145 | ||
8db9307c JH |
146 | ------------------------------------------------ |
147 | :100644 100644 0123456... 0123456... R100 fileX file0 | |
148 | ------------------------------------------------ | |
4a1332d0 | 149 | |
59df2a11 CS |
150 | When the "-C" option is used, the original contents of modified files, |
151 | and deleted files (and also unmodified files, if the | |
152 | "\--find-copies-harder" option is used) are considered as candidates | |
153 | of the source files in rename/copy operation. If the input were like | |
154 | these filepairs, that talk about a modified file fileY and a newly | |
4a1332d0 JH |
155 | created file file0: |
156 | ||
8db9307c JH |
157 | ------------------------------------------------ |
158 | :100644 100644 0123456... 1234567... M fileY | |
59df2a11 | 159 | :000000 100644 0000000... bcd3456... A file0 |
8db9307c | 160 | ------------------------------------------------ |
4a1332d0 JH |
161 | |
162 | the original contents of fileY and the resulting contents of | |
163 | file0 are compared, and if they are similar enough, they are | |
164 | changed to: | |
165 | ||
8db9307c JH |
166 | ------------------------------------------------ |
167 | :100644 100644 0123456... 1234567... M fileY | |
59df2a11 | 168 | :100644 100644 0123456... bcd3456... C100 fileY file0 |
8db9307c | 169 | ------------------------------------------------ |
4a1332d0 JH |
170 | |
171 | In both rename and copy detection, the same "extent of changes" | |
172 | algorithm used in diffcore-break is used to determine if two | |
173 | files are "similar enough", and can be customized to use | |
59df2a11 CS |
174 | a similarity score different from the default of 50% by giving a |
175 | number after the "-M" or "-C" option (e.g. "-M8" to tell it to use | |
4a1332d0 JH |
176 | 8/10 = 80%). |
177 | ||
e1ccf53a | 178 | Note. When the "-C" option is used with `\--find-copies-harder` |
8db9307c | 179 | option, git-diff-\* commands feed unmodified filepairs to |
232b75ab JH |
180 | diffcore mechanism as well as modified ones. This lets the copy |
181 | detector consider unmodified files as copy source candidates at | |
e1ccf53a | 182 | the expense of making it slower. Without `\--find-copies-harder`, |
8db9307c | 183 | git-diff-\* commands can detect copies only if the file that was |
232b75ab | 184 | copied happened to have been modified in the same changeset. |
4a1332d0 JH |
185 | |
186 | ||
59df2a11 | 187 | diffcore-merge-broken: For Putting "Complete Rewrites" Back Together |
a67c1d08 | 188 | -------------------------------------------------------------------- |
4a1332d0 JH |
189 | |
190 | This transformation is used to merge filepairs broken by | |
f73ae1fc | 191 | diffcore-break, and not transformed into rename/copy by |
4a1332d0 JH |
192 | diffcore-rename, back into a single modification. This always |
193 | runs when diffcore-break is used. | |
194 | ||
195 | For the purpose of merging broken filepairs back, it uses a | |
196 | different "extent of changes" computation from the ones used by | |
197 | diffcore-break and diffcore-rename. It counts only the deletion | |
198 | from the original, and does not count insertion. If you removed | |
199 | only 10 lines from a 100-line document, even if you added 910 | |
200 | new lines to make a new 1000-line document, you did not do a | |
201 | complete rewrite. diffcore-break breaks such a case in order to | |
202 | help diffcore-rename to consider such filepairs as candidate of | |
203 | rename/copy detection, but if filepairs broken that way were not | |
204 | matched with other filepairs to create rename/copy, then this | |
205 | transformation merges them back into the original | |
206 | "modification". | |
207 | ||
208 | The "extent of changes" parameter can be tweaked from the | |
209 | default 80% (that is, unless more than 80% of the original | |
210 | material is deleted, the broken pairs are merged back into a | |
211 | single modification) by giving a second number to -B option, | |
212 | like these: | |
213 | ||
8db9307c JH |
214 | * -B50/60 (give 50% "break score" to diffcore-break, use 60% |
215 | for diffcore-merge-broken). | |
216 | ||
217 | * -B/60 (the same as above, since diffcore-break defaults to 50%). | |
4a1332d0 | 218 | |
366175ef | 219 | Note that earlier implementation left a broken pair as a separate |
f73ae1fc | 220 | creation and deletion patches. This was an unnecessary hack and |
366175ef JH |
221 | the latest implementation always merges all the broken pairs |
222 | back into modifications, but the resulting patch output is | |
f73ae1fc | 223 | formatted differently for easier review in case of such |
366175ef JH |
224 | a complete rewrite by showing the entire contents of old version |
225 | prefixed with '-', followed by the entire contents of new | |
226 | version prefixed with '+'. | |
227 | ||
4a1332d0 | 228 | |
59df2a11 | 229 | diffcore-pickaxe: For Detecting Addition/Deletion of Specified String |
a67c1d08 | 230 | --------------------------------------------------------------------- |
4a1332d0 JH |
231 | |
232 | This transformation is used to find filepairs that represent | |
233 | changes that touch a specified string, and is controlled by the | |
e1ccf53a | 234 | -S option and the `\--pickaxe-all` option to the git-diff-* |
4a1332d0 JH |
235 | commands. |
236 | ||
237 | When diffcore-pickaxe is in use, it checks if there are | |
238 | filepairs whose "original" side has the specified string and | |
239 | whose "result" side does not. Such a filepair represents "the | |
240 | string appeared in this changeset". It also checks for the | |
241 | opposite case that loses the specified string. | |
242 | ||
e1ccf53a | 243 | When `\--pickaxe-all` is not in effect, diffcore-pickaxe leaves |
59df2a11 | 244 | only such filepairs that touch the specified string in its |
e1ccf53a | 245 | output. When `\--pickaxe-all` is used, diffcore-pickaxe leaves all |
4a1332d0 JH |
246 | filepairs intact if there is such a filepair, or makes the |
247 | output empty otherwise. The latter behaviour is designed to | |
248 | make reviewing of the changes in the context of the whole | |
249 | changeset easier. | |
250 | ||
251 | ||
59df2a11 | 252 | diffcore-order: For Sorting the Output Based on Filenames |
a67c1d08 | 253 | --------------------------------------------------------- |
4a1332d0 JH |
254 | |
255 | This is used to reorder the filepairs according to the user's | |
256 | (or project's) taste, and is controlled by the -O option to the | |
257 | git-diff-* commands. | |
258 | ||
59df2a11 | 259 | This takes a text file each of whose lines is a shell glob |
4a1332d0 JH |
260 | pattern. Filepairs that match a glob pattern on an earlier line |
261 | in the file are output before ones that match a later line, and | |
262 | filepairs that do not match any glob pattern are output last. | |
263 | ||
59df2a11 | 264 | As an example, a typical orderfile for the core git probably |
8db9307c | 265 | would look like this: |
4a1332d0 | 266 | |
8db9307c | 267 | ------------------------------------------------ |
df8baa42 JF |
268 | README |
269 | Makefile | |
270 | Documentation | |
271 | *.h | |
272 | *.c | |
273 | t | |
8db9307c | 274 | ------------------------------------------------ |
4a1332d0 | 275 |