The document discusses various diff algorithms in Git, specifically Myers, Minimal, Patience, and Histogram, and their differences in output such as the number and position of changed lines. A mapping study reveals that Histogram is more effective in describing code changes compared to Myers. The authors encourage using the Histogram algorithm for analyzing code changes to achieve better accuracy in identifying modified lines.

1. How different are different
diff algorithms in Git?
Yusuf S. Nugroho Hideaki Hata Kenichi Matsumoto

2. diff is essential in SE research field
2
Empirical Software
Engineering Research git diff [<options>] <commit> <commit> [--] [<path>…]

3. Git offers 4 diff algorithms
3
• Myers
• Minimal (improved Myers)
• Patience (try to give contextual diff)
• Histogram (enhanced Patience, normally faster)
--diff-algorithm={algorithm name}
Documentation
default algorithm
Histogram was introduced in git 1.7.7 in 2011

4. Different algorithms produce different diff outputs
4
Differences:
• Number of changed lines
• Position of changed lines
9 added lines
2 deleted lines
4 deleted lines
9 added lines

5. 2 sequential analyses
• Systematic Mapping Study
• How previous studies used git diff?
• Comparisons Study
• differences of diff outputs between
Myers and Histogram
5

6. Results of Mapping Study
6
52
0
Default Other diff
0
20
40
60
Type of diff algorithm
24
13 12
2 1
0
10
20
30
Patches Metrics SZZ Merges Authorship
Top 3 purposes of using diff
TSE
3 Journals
EMSE TOSEM
8 Proceedings FSE ICSE OOPSLA PLDI
ASE ICSME ISSTA MSR

7. Comparing diff outputs in 3 applications
7
Manual Comparison: Patches
Myers Histogram
... some code ...
- a deleted line
+ an added line
+ an added line
- a deleted line
... some code ...
... some code ...
- a deleted line
- a deleted line
+ an added line
+ an added line
... some code ...

8. Different diff algorithms can report different location of
identified changed lines of code
4.17%
4.62%
7.79%
9.68%
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
12.00%
Metrics SZZ Algorithm
Frequency of different location of identified changed lines of code
#Files
#Commits
8

9. Histogram is better for describing code changes
9
13.4%
14.9%
71.6%
62.6%
16.9%
20.6%
0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0%
Histogram
Myers
Same
in-Code diff in-Non-Code diff
code changes
non-code
changes

10. Use histogram diff algorithm when analyzing code changes
10
• Different diff algorithms can produce different amount and
location of changed lines.
• Histogram detects the changed lines more appropriately from
source code.

11. • Nugroho, Y.S., Hata, H. & Matsumoto, K., ”How Different are
Different diff Algorithms in Git? Use --histogram for Code
Changes", Empirical Software Engineering 25, 79-823 (2020).
Available at: https://doi.org/10.1007/s10664-019-09772-z
11
Publication

12. Application on actual tools
• Git extension -- (Feature request)
https://github.com/gitextensions/gitextensions/
issues/6991
12
• Pydriller
https://pydriller.readthedocs.io/en/latest/
configuration.html#git-diff-algorithms