Splitting Up Complete Rewrites
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The second transformation in the chain is diffcore-break, and is
controlled by the -B option to the 'git diff-{asterisk}' 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:
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:100644 100644 bcd1234... 0123456... M file0
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and if it detects that the file "file0" is completely rewritten,
it changes it to:
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:100644 000000 bcd1234... 0000000... D file0
:000000 100644 0000000... 0123456... A file0
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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 SHA-1 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 Detecting Renames and Copies
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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-{asterisk}' commands. If the
input contained these filepairs:
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:100644 000000 0123456... 0000000... D fileX
:000000 100644 0000000... 0123456... A file0
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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:
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:100644 100644 0123456... 0123456... R100 fileX file0
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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:
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:100644 100644 0123456... 1234567... M fileY
:000000 100644 0000000... bcd3456... A file0
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the original contents of fileY and the resulting contents of
file0 are compared, and if they are similar enough, they are
changed to:
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:100644 100644 0123456... 1234567... M fileY
:100644 100644 0123456... bcd3456... C100 fileY file0
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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 that when rename detection is on but both copy and break
detection are off, rename detection adds a preliminary step that first
checks if files are moved across directories while keeping their
filename the same. If there is a file added to a directory whose
contents are sufficiently similar to a file with the same name that got
deleted from a different directory, it will mark