John Keeping <j...@keeping.me.uk> writes:

> When the source file is empty, the calculation of the merge score
> results in a division by zero.  Since the merge score is initialized to
> zero, it makes sense to just leave it as it is if the source size is
> zero.  This means that we still use the extent of damage metric to
> decide whether to break the filepair.

Well spotted.  An empty blob to another blob that is larger than 400
bytes will trigger div0, and it makes sense to leave merge-score to
0 (i.e. do not show as whole-delete-then-whole-add after rename
detection is done and the broken filepair is merged back).

Actually, if src->size is 0, we probably shouldn't break the filepair
in the first place.  That is, if your preimage and postimage looked
like these:


     == preimage ==             == postimage ==

     F (empty file)             F (a large file)
                                E (a new empty file)

do we want to see F renamed to E and then a new file created as F
while running "git diff -B -M"?  I doubt it.

So in that sense, this might be a better solution.  I dunno.

 diffcore-break.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/diffcore-break.c b/diffcore-break.c
index 44f8678..eabafd5 100644
--- a/diffcore-break.c
+++ b/diffcore-break.c
@@ -67,6 +67,8 @@ static int should_break(struct diff_filespec *src,
        max_size = ((src->size > dst->size) ? src->size : dst->size);
        if (max_size < MINIMUM_BREAK_SIZE)
                return 0; /* we do not break too small filepair */
+       if (src->size == 0)
+               return 0; /* we do not let empty files get renamed */
 
        if (diffcore_count_changes(src, dst,
                                   &src->cnt_data, &dst->cnt_data,
--
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