What does it mean when Git says 'rewrite' or 'rename' in a commit message?

Question

Running a git commit leads to the following output:

[manu@host] git: git commit -a -m "StreamIt instrumentation"
[master 263410f] StreamIt instrumentation
62 files changed, 6117 insertions(+), 5748 deletions(-)
rewrite Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/cluster/ClusterBackend.java (91%)
rewrite Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/cluster/ClusterCodeGenerator.java (95%)
rewrite Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/cluster/code/FlatIRToCluster.java (94%)
rewrite Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/common/ToCCommon.java (92%)
rewrite Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/flatgraph/ScheduledStaticStreamGraph.java (93%)
rename Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/sir/lowering/fission/{StatelessDuplicate.java => HorizontalFission.java} (98%)
rewrite Code/ALCHEMY/streamit-src-2.1.1/src/at/dms/kjc/sir/lowering/partition/dynamicprog/DynamicProgPartitioner.java (93%)

• What does git mean when it reports a file as 'rewrite'?
• What is the meaning of the associated percentage?
• Why is a percentage only associated with these lines among the 62 changed in this commit?

Answer 1

Git uses heuristics to determine if a change was a renaming or copying of a file, and also if it is a "rewriting" of the file. Roughly speaking, if the diff between the old and new version is bigger than the new version itself, it's a "rewrite".

This is tuned for git's original use case of changing source files, most often making localised changes: since it is based on a line-by-line diff, things like reindenting a source file can trigger it. Also, since it is determined on-the-fly, diff options like "-b" and "-w" can change the evaluation of whether a change is a rewrite (or a copy, or a rename).

The percentage is git's "dissimilarity index" (as opposed to the percentage "similarity index" for a rename or copy). Probably something like the percentage of lines in the file that have changed.