mirror of
https://github.com/git/git.git
synced 2024-12-22 16:23:38 +08:00
276 lines
11 KiB
Plaintext
276 lines
11 KiB
Plaintext
Tweaking diff output
|
|
====================
|
|
June 2005
|
|
|
|
|
|
Introduction
|
|
------------
|
|
|
|
The diff commands git-diff-index, git-diff-files, git-diff-tree, and
|
|
git-diff-stages can be told to manipulate differences they find in
|
|
unconventional ways before showing diff(1) output. The manipulation
|
|
is collectively called "diffcore transformation". This short note
|
|
describes what they are and how to use them to produce diff outputs
|
|
that are 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;
|
|
|
|
- git-diff-stages compares contents of blobs at two stages in an
|
|
unmerged index file.
|
|
|
|
In all of these cases, the commands themselves compare
|
|
corresponding paths in the two sets of files. 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 6 such transformations:
|
|
|
|
- diffcore-pathspec
|
|
- diffcore-break
|
|
- diffcore-rename
|
|
- diffcore-merge-broken
|
|
- diffcore-pickaxe
|
|
- diffcore-order
|
|
|
|
These are applied in sequence. The set of filepairs git-diff-\*
|
|
commands find are used as the input to diffcore-pathspec, and
|
|
the output from diffcore-pathspec 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-pathspec: For Ignoring Files Outside Our Consideration
|
|
---------------------------------------------------------------
|
|
|
|
The first transformation in the chain is diffcore-pathspec, and
|
|
is controlled by giving the pathname parameters to the
|
|
git-diff-* commands on the command line. The pathspec is used
|
|
to limit the world diff operates in. It removes the filepairs
|
|
outside the specified set 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.
|
|
|
|
Implementation note. For performance reasons, git-diff-tree
|
|
uses the pathname parameters on the command line to cull set of
|
|
filepairs it feeds the diffcore mechanism itself, and does not
|
|
use diffcore-pathspec, but the end result is the same.
|
|
|
|
|
|
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 SHA1 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 Detection 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. 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 is used to find filepairs that represent
|
|
changes that touch a specified string, and is controlled by the
|
|
-S option and the `\--pickaxe-all` option to the git-diff-*
|
|
commands.
|
|
|
|
When diffcore-pickaxe is in use, it checks if there are
|
|
filepairs whose "original" side has the specified string and
|
|
whose "result" side does not. Such a filepair represents "the
|
|
string appeared in this changeset". It also checks for the
|
|
opposite case that loses the specified string.
|
|
|
|
When `\--pickaxe-all` is not in effect, diffcore-pickaxe leaves
|
|
only such filepairs that touch the specified string in its
|
|
output. When `\--pickaxe-all` is used, diffcore-pickaxe leaves all
|
|
filepairs intact if there is such a filepair, or makes the
|
|
output empty otherwise. The latter behaviour is designed to
|
|
make reviewing of the 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
|
|
------------------------------------------------
|
|
|