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dd4048d1c7
Rename detection rework continues. * en/ort-perf-batch-8: diffcore-rename: compute dir_rename_guess from dir_rename_counts diffcore-rename: limit dir_rename_counts computation to relevant dirs diffcore-rename: compute dir_rename_counts in stages diffcore-rename: extend cleanup_dir_rename_info() diffcore-rename: move dir_rename_counts into dir_rename_info struct diffcore-rename: add function for clearing dir_rename_count Move computation of dir_rename_count from merge-ort to diffcore-rename diffcore-rename: add a mapping of destination names to their indices diffcore-rename: provide basic implementation of idx_possible_rename() diffcore-rename: use directory rename guided basename comparisons
333 lines
14 KiB
Text
333 lines
14 KiB
Text
gitdiffcore(7)
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==============
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NAME
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----
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gitdiffcore - Tweaking diff output
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SYNOPSIS
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--------
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[verse]
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'git diff' *
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DESCRIPTION
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-----------
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The diff commands 'git diff-index', 'git diff-files', and 'git diff-tree'
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can be told to manipulate differences they find in
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unconventional ways before showing 'diff' output. The manipulation
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is collectively called "diffcore transformation". This short note
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describes what they are and how to use them to produce 'diff' output
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that is easier to understand than the conventional kind.
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The chain of operation
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----------------------
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The 'git diff-{asterisk}' family works by first comparing two sets of
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files:
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- 'git diff-index' compares contents of a "tree" object and the
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working directory (when `--cached` flag is not used) or a
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"tree" object and the index file (when `--cached` flag is
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used);
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- 'git diff-files' compares contents of the index file and the
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working directory;
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- 'git diff-tree' compares contents of two "tree" objects;
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In all of these cases, the commands themselves first optionally limit
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the two sets of files by any pathspecs given on their command-lines,
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and compare corresponding paths in the two resulting sets of files.
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The pathspecs are used to limit the world diff operates in. They remove
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the filepairs outside the specified sets of pathnames. E.g. If the
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input set of filepairs included:
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------------------------------------------------
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:100644 100644 bcd1234... 0123456... M junkfile
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------------------------------------------------
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but the command invocation was `git diff-files myfile`, then the
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junkfile entry would be removed from the list because only "myfile"
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is under consideration.
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The result of comparison is passed from these commands to what is
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internally called "diffcore", in a format similar to what is output
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when the -p option is not used. E.g.
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------------------------------------------------
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in-place edit :100644 100644 bcd1234... 0123456... M file0
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create :000000 100644 0000000... 1234567... A file4
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delete :100644 000000 1234567... 0000000... D file5
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unmerged :000000 000000 0000000... 0000000... U file6
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------------------------------------------------
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The diffcore mechanism is fed a list of such comparison results
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(each of which is called "filepair", although at this point each
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of them talks about a single file), and transforms such a list
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into another list. There are currently 5 such transformations:
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- diffcore-break
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- diffcore-rename
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- diffcore-merge-broken
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- diffcore-pickaxe
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- diffcore-order
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- diffcore-rotate
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These are applied in sequence. The set of filepairs 'git diff-{asterisk}'
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commands find are used as the input to diffcore-break, and
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the output from diffcore-break is used as the input to the
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next transformation. The final result is then passed to the
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output routine and generates either diff-raw format (see Output
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format sections of the manual for 'git diff-{asterisk}' commands) or
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diff-patch format.
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diffcore-break: For Splitting Up Complete Rewrites
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--------------------------------------------------
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The second transformation in the chain is diffcore-break, and is
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controlled by the -B option to the 'git diff-{asterisk}' commands. This is
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used to detect a filepair that represents "complete rewrite" and
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break such filepair into two filepairs that represent delete and
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create. E.g. If the input contained this filepair:
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------------------------------------------------
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:100644 100644 bcd1234... 0123456... M file0
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------------------------------------------------
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and if it detects that the file "file0" is completely rewritten,
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it changes it to:
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------------------------------------------------
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:100644 000000 bcd1234... 0000000... D file0
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:000000 100644 0000000... 0123456... A file0
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------------------------------------------------
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For the purpose of breaking a filepair, diffcore-break examines
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the extent of changes between the contents of the files before
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and after modification (i.e. the contents that have "bcd1234..."
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and "0123456..." as their SHA-1 content ID, in the above
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example). The amount of deletion of original contents and
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insertion of new material are added together, and if it exceeds
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the "break score", the filepair is broken into two. The break
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score defaults to 50% of the size of the smaller of the original
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and the result (i.e. if the edit shrinks the file, the size of
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the result is used; if the edit lengthens the file, the size of
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the original is used), and can be customized by giving a number
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after "-B" option (e.g. "-B75" to tell it to use 75%).
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diffcore-rename: For Detecting Renames and Copies
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-------------------------------------------------
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This transformation is used to detect renames and copies, and is
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controlled by the -M option (to detect renames) and the -C option
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(to detect copies as well) to the 'git diff-{asterisk}' commands. If the
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input contained these filepairs:
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------------------------------------------------
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:100644 000000 0123456... 0000000... D fileX
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:000000 100644 0000000... 0123456... A file0
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------------------------------------------------
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and the contents of the deleted file fileX is similar enough to
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the contents of the created file file0, then rename detection
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merges these filepairs and creates:
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------------------------------------------------
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:100644 100644 0123456... 0123456... R100 fileX file0
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------------------------------------------------
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When the "-C" option is used, the original contents of modified files,
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and deleted files (and also unmodified files, if the
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"--find-copies-harder" option is used) are considered as candidates
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of the source files in rename/copy operation. If the input were like
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these filepairs, that talk about a modified file fileY and a newly
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created file file0:
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------------------------------------------------
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:100644 100644 0123456... 1234567... M fileY
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:000000 100644 0000000... bcd3456... A file0
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------------------------------------------------
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the original contents of fileY and the resulting contents of
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file0 are compared, and if they are similar enough, they are
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changed to:
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------------------------------------------------
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:100644 100644 0123456... 1234567... M fileY
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:100644 100644 0123456... bcd3456... C100 fileY file0
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------------------------------------------------
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In both rename and copy detection, the same "extent of changes"
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algorithm used in diffcore-break is used to determine if two
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files are "similar enough", and can be customized to use
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a similarity score different from the default of 50% by giving a
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number after the "-M" or "-C" option (e.g. "-M8" to tell it to use
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8/10 = 80%).
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Note that when rename detection is on but both copy and break
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detection are off, rename detection adds a preliminary step that first
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checks if files are moved across directories while keeping their
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filename the same. If there is a file added to a directory whose
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contents is sufficiently similar to a file with the same name that got
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deleted from a different directory, it will mark them as renames and
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exclude them from the later quadratic step (the one that pairwise
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compares all unmatched files to find the "best" matches, determined by
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the highest content similarity). So, for example, if a deleted
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docs/ext.txt and an added docs/config/ext.txt are similar enough, they
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will be marked as a rename and prevent an added docs/ext.md that may
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be even more similar to the deleted docs/ext.txt from being considered
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as the rename destination in the later step. For this reason, the
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preliminary "match same filename" step uses a bit higher threshold to
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mark a file pair as a rename and stop considering other candidates for
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better matches. At most, one comparison is done per file in this
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preliminary pass; so if there are several remaining ext.txt files
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throughout the directory hierarchy after exact rename detection, this
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preliminary step may be skipped for those files.
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Note. When the "-C" option is used with `--find-copies-harder`
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option, 'git diff-{asterisk}' commands feed unmodified filepairs to
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diffcore mechanism as well as modified ones. This lets the copy
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detector consider unmodified files as copy source candidates at
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the expense of making it slower. Without `--find-copies-harder`,
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'git diff-{asterisk}' commands can detect copies only if the file that was
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copied happened to have been modified in the same changeset.
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diffcore-merge-broken: For Putting Complete Rewrites Back Together
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------------------------------------------------------------------
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This transformation is used to merge filepairs broken by
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diffcore-break, and not transformed into rename/copy by
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diffcore-rename, back into a single modification. This always
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runs when diffcore-break is used.
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For the purpose of merging broken filepairs back, it uses a
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different "extent of changes" computation from the ones used by
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diffcore-break and diffcore-rename. It counts only the deletion
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from the original, and does not count insertion. If you removed
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only 10 lines from a 100-line document, even if you added 910
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new lines to make a new 1000-line document, you did not do a
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complete rewrite. diffcore-break breaks such a case in order to
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help diffcore-rename to consider such filepairs as candidate of
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rename/copy detection, but if filepairs broken that way were not
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matched with other filepairs to create rename/copy, then this
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transformation merges them back into the original
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"modification".
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The "extent of changes" parameter can be tweaked from the
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default 80% (that is, unless more than 80% of the original
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material is deleted, the broken pairs are merged back into a
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single modification) by giving a second number to -B option,
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like these:
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* -B50/60 (give 50% "break score" to diffcore-break, use 60%
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for diffcore-merge-broken).
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* -B/60 (the same as above, since diffcore-break defaults to 50%).
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Note that earlier implementation left a broken pair as a separate
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creation and deletion patches. This was an unnecessary hack and
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the latest implementation always merges all the broken pairs
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back into modifications, but the resulting patch output is
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formatted differently for easier review in case of such
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a complete rewrite by showing the entire contents of old version
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prefixed with '-', followed by the entire contents of new
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version prefixed with '+'.
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diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
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---------------------------------------------------------------------
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This transformation limits the set of filepairs to those that change
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specified strings between the preimage and the postimage in a certain
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way. -S<block of text> and -G<regular expression> options are used to
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specify different ways these strings are sought.
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"-S<block of text>" detects filepairs whose preimage and postimage
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have different number of occurrences of the specified block of text.
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By definition, it will not detect in-file moves. Also, when a
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changeset moves a file wholesale without affecting the interesting
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string, diffcore-rename kicks in as usual, and `-S` omits the filepair
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(since the number of occurrences of that string didn't change in that
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rename-detected filepair). When used with `--pickaxe-regex`, treat
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the <block of text> as an extended POSIX regular expression to match,
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instead of a literal string.
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"-G<regular expression>" (mnemonic: grep) detects filepairs whose
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textual diff has an added or a deleted line that matches the given
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regular expression. This means that it will detect in-file (or what
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rename-detection considers the same file) moves, which is noise. The
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implementation runs diff twice and greps, and this can be quite
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expensive. To speed things up binary files without textconv filters
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will be ignored.
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When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
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that match their respective criterion are kept in the output. When
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`--pickaxe-all` is used, if even one filepair matches their respective
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criterion in a changeset, the entire changeset is kept. This behavior
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is designed to make reviewing changes in the context of the whole
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changeset easier.
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diffcore-order: For Sorting the Output Based on Filenames
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---------------------------------------------------------
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This is used to reorder the filepairs according to the user's
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(or project's) taste, and is controlled by the -O option to the
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'git diff-{asterisk}' commands.
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This takes a text file each of whose lines is a shell glob
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pattern. Filepairs that match a glob pattern on an earlier line
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in the file are output before ones that match a later line, and
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filepairs that do not match any glob pattern are output last.
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As an example, a typical orderfile for the core Git probably
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would look like this:
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------------------------------------------------
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README
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Makefile
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Documentation
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*.h
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*.c
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t
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------------------------------------------------
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diffcore-rotate: For Changing At Which Path Output Starts
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---------------------------------------------------------
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This transformation takes one pathname, and rotates the set of
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filepairs so that the filepair for the given pathname comes first,
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optionally discarding the paths that come before it. This is used
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to implement the `--skip-to` and the `--rotate-to` options. It is
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an error when the specified pathname is not in the set of filepairs,
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but it is not useful to error out when used with "git log" family of
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commands, because it is unreasonable to expect that a given path
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would be modified by each and every commit shown by the "git log"
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command. For this reason, when used with "git log", the filepair
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that sorts the same as, or the first one that sorts after, the given
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pathname is where the output starts.
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Use of this transformation combined with diffcore-order will produce
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unexpected results, as the input to this transformation is likely
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not sorted when diffcore-order is in effect.
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SEE ALSO
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--------
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linkgit:git-diff[1],
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linkgit:git-diff-files[1],
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linkgit:git-diff-index[1],
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linkgit:git-diff-tree[1],
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linkgit:git-format-patch[1],
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linkgit:git-log[1],
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linkgit:gitglossary[7],
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link:user-manual.html[The Git User's Manual]
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GIT
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---
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Part of the linkgit:git[1] suite
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