Re: [PATCH 1/2] diffcore-rename: support rename cache

[Jeff King <peff@peff.net>]

On Sat, Nov 08, 2008 at 10:24:10AM +0100, Yann Dirson wrote:

> Well, it could be that an out-of-pathspec pair would have a better
> score than an in-pathspec one.  Maybe cache recording should be turned
> off when doing pathspec limitation ?

One thing I notice is that the cache works at the level of "here is the
best rename for this commit." Maybe it could go down a level and say
"here is the inexact rename score between these blobs". Then you would
still find the best score between two blobs each time, but save the
really computationally intensive part (which is comparing the actual
_content_ of the blobs).

That should work in the face of path limiting or any other option,
because it is caching something immutable: this is the similarity score
between two pieces of content. And then you get arbitrary tree-to-tree
speedups for free, since such a cache would be valid for every commit.

The downsides are:

 - your cache is potentially bigger, since you are caching the score of
   every pair you look at, instead of just "good" pairs (OTOH, you are
   not doing a per-commit cache, which helps reduce the size)

 - you can still "lie" about a score to pre-seed imported SVN renames,
   but such lying will actually apply to all commits.

-Peff