On Sat, Aug 04, 2012 at 03:49:12PM -0700, Junio C Hamano wrote:
> Jeff King <p...@peff.net> writes:
> > There are some calculations that git makes repeatedly, even
> > though the results are invariant for a certain input (e.g.,
> > the patch-id of a certain commit). We can make a space/time
> > tradeoff by caching these on disk between runs.
> > Even though these may be immutable for a certain commit, we
> > don't want to directly store the results in the commit
> > objects themselves, for a few reasons:
> > 1. They are not necessarily used by all algorithms, so
> > bloating the commit object might slow down other
> > algorithms.
> > 2. Because they can be calculated from the existing
> > commits, they are redundant with the existing
> > information. Thus they are an implementation detail of
> > our current algorithms, and should not be cast in stone
> > by including them in the commit sha1.
> > 3. They may only be immutable under a certain set of
> > conditions (e.g., which grafts or replace refs we are
> > using). Keeping the storage external means we can
> > invalidate and regenerate the cache whenever those
> > conditions change.
> 4. The algorithm used to compute such values could improve over
> time. The same advantage argument as 3 applies to this case.
Yeah, agreed. That commit message is a year old, and was written for an
earlier iteration of the patch which was used for caching commit
generations. There's not really a better algorithm there, but your
comment certainly applies to rename similarities.
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