[PATCH 06/14] run_diff_files(): de-mystify the size of combine_diff_path struct

We allocate a combine_diff_path struct with space for 5 parents. Why 5?

The history is not particularly enlightening. The allocation comes from b4b1550315 (Don't instantiate structures with FAMs., 2006-06-18), which just switched to xmalloc from a stack struct with 5 elements. That struct changed to 5 from 4 in 2454c962fb (combine-diff: show mode changes as well., 2006-02-06), when we also moved from storing raw sha1 bytes to the combine_diff_parent struct. But no explanation is given. That 4 comes from the earliest code in ea726d02e9 (diff-files: -c and --cc options., 2006-01-28).

One might guess it is for the 4 stages we can store in the index. But this code path only ever diffs the current state against stages 2 and 3. So we only need two slots.

And it's easy to see this is still the case. We fill the parent slots by subtracting 2 from the ce_stage() values, ignoring values below 2. And since ce_stage() is only 2 bits, there are 4 values, and thus we need 2 slots.

Let's use the correct value (saving a tiny bit of memory) and add a comment explaining what's going on (saving a tiny bit of programmer brain power).

Arguably we could use:

  1 + (STAGEMASK >> STAGESHIFT) - 2

which lets the compiler enforce that we will not go out-of-bounds if we see an unexpected value from ce_stage(). But that is more confusing to explain, and the constant "2" is baked into other parts of the function. It is a fundamental constant, not something where somebody might bump a macro and forget to update this code.

Signed-off-by: Jeff King <peff@peff.net>
---
 diff-lib.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)