This commit integrates the fuzzy fingerprint heuristic into
guess_line_blames().
We actually make two passes. The first pass uses the fuzzy algorithm to
find a match within the current diff chunk. If that fails, the second
pass searches the entire parent file for the best match.
For an example of scanning the entire parent for a match, consider:
commit-a 30) #include <sys/header_a.h>
commit-b 31) #include <header_b.h>
commit-c 32) #include <header_c.h>
Then commit X alphabetizes them:
commit-X 30) #include <header_b.h>
commit-X 31) #include <header_c.h>
commit-X 32) #include <sys/header_a.h>
If we just check the parent's chunk (i.e. the first pass), we'd get:
commit-b 30) #include <header_b.h>
commit-c 31) #include <header_c.h>
commit-X 32) #include <sys/header_a.h>
That's because commit X actually consists of two chunks: one chunk is
removing sys/header_a.h, then some context, and the second chunk is
adding sys/header_a.h.
If we scan the entire parent file, we get:
commit-b 30) #include <header_b.h>
commit-c 31) #include <header_c.h>
commit-a 32) #include <sys/header_a.h>
Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This algorithm will replace the heuristic used to identify lines from
ignored commits with one that finds likely candidate lines in the
parent's version of the file. The actual replacement occurs in an
upcoming commit.
The old heuristic simply assigned lines in the target to the same line
number (plus offset) in the parent. The new function uses a
fingerprinting algorithm to detect similarity between lines.
The new heuristic is designed to accurately match changes made
mechanically by formatting tools such as clang-format and clang-tidy.
These tools make changes such as breaking up lines to fit within a
character limit or changing identifiers to fit with a naming convention.
The heuristic is not intended to match more extensive refactoring
changes and may give misleading results in such cases.
In most cases formatting tools preserve line ordering, so the heuristic
is optimised for such cases. (Some types of changes do reorder lines
e.g. sorting keep the line content identical, the git blame -M option
can already be used to address this). The reason that it is advantageous
to rely on ordering is due to source code repeating the same character
sequences often e.g. declaring an identifier on one line and using that
identifier on several subsequent lines. This means that lines can look
very similar to each other which presents a problem when doing fuzzy
matching. Relying on ordering gives us extra clues to point towards the
true match.
The heuristic operates on a single diff chunk change at a time. It
creates a “fingerprint” for each line on each side of the change.
Fingerprints are described in detail in the comment for `struct
fingerprint`, but essentially are a multiset of the character pairs in a
line. The heuristic first identifies the line in the target entry whose
fingerprint is most clearly matched to a line fingerprint in the parent
entry. Where fingerprints match identically, the position of the lines
is used as a tie-break. The heuristic locks in the best match, and
subtracts the fingerprint of the line in the target entry from the
fingerprint of the line in the parent entry to prevent other lines being
matched on the same parts of that line. It then repeats the process
recursively on the section of the chunk before the match, and then the
section of the chunk after the match.
Here's an example of the difference the fingerprinting makes. Consider
a file with two commits:
commit-a 1) void func_1(void *x, void *y);
commit-b 2) void func_2(void *x, void *y);
After a commit 'X', we have:
commit-X 1) void func_1(void *x,
commit-X 2) void *y);
commit-X 3) void func_2(void *x,
commit-X 4) void *y);
When we blame-ignored with the old algorithm, we get:
commit-a 1) void func_1(void *x,
commit-b 2) void *y);
commit-X 3) void func_2(void *x,
commit-X 4) void *y);
Where commit-b is blamed for 2 instead of 3. With the fingerprint
algorithm, we get:
commit-a 1) void func_1(void *x,
commit-a 2) void *y);
commit-b 3) void func_2(void *x,
commit-b 4) void *y);
Note line 2 could be matched with either commit-a or commit-b as it is
equally similar to both lines, but is matched with commit-a because its
position as a fraction of the new line range is more similar to commit-a
as a fraction of the old line range. Line 4 is also equally similar to
both lines, but as it appears after line 3 which will be matched first
it cannot be matched with an earlier line.
For many more examples, see t/t8014-blame-ignore-fuzzy.sh which contains
example parent and target files and the line numbers in the parent that
must be matched.
Signed-off-by: Michael Platings <michael@platin.gs>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
