On Thu, Apr 16, 2020 at 09:26:49AM -0400, Derrick Stolee wrote: > On 4/15/2020 8:52 PM, Taylor Blau wrote: > > On Thu, Apr 16, 2020 at 12:18:33AM +0200, Jakub Narębski wrote: > >> On Wed, 15 Apr 2020 at 20:37, Derrick Stolee <stolee@xxxxxxxxx> wrote: > >> [...] > >>> -->8-- > >>> From 89beb9598daabb19e3c896bbceeb0fc1b9ccc6ca Mon Sep 17 00:00:00 2001 > >>> From: Derrick Stolee <dstolee@xxxxxxxxxxxxx> > >>> Date: Wed, 15 Apr 2020 18:04:25 +0000 > >>> Subject: [PATCH] bloom: compute all Bloom hashes from lowercase > >>> > >>> The changed-path Bloom filters currently hash path strings using > >>> the exact string for the path. This makes it difficult* to use the > >>> filters when restricting to case-insensitive pathspecs. > >>> > >>> * I say "difficult" because it is possible to generate all 2^n > >>> options for the case of a path and test them all, but this is > >>> a bad idea and should not be done. "Impossible" is an appropriate > >>> alternative. > >>> > >>> THIS IS A BREAKING CHANGE. Commit-graph files with changed-path > >>> Bloom filters computed by a previous commit will not be compatible > >>> with the filters computed in this commit, nor will we get correct > >>> results when testing across these incompatible versions. Normally, > >>> this would be a completely unacceptable change, but the filters > >>> have not been released and hence are still possible to update > >>> before release. > >>> > >>> TODO: If we decide to move in this direction, then the following > >>> steps should be done (and some of them should be done anyway): > >>> > >>> * We need to document the Bloom filter format to specify exactly > >>> how we compute the filter data. The details should be careful > >>> enough that someone can reproduce the exact file format without > >>> looking at the C code. > >>> > >>> * That document would include the tolower() transformation that is > >>> being done here. > >> > >> Why not modify the BDAT chunk to include version of > >> case folding transformation or other collation algorithm > >> (other transformation).that is done prior to computing > >> the Bloom filter key? Though that might be unnecessary > >> flexibility... > > > > If this ends up being something that we want to do, I agree with > > Stolee's reasoning that this should be a breaking change. If we were, > > say, several months into having Bloom filters in a release and decided > > at that point to make the change, then: sure, supporting both by writing > > a bit in the BDAT chunk makes sense. > > > > But, we're many months away from that state yet, and so I don't think > > the cost of rebuilding what few commit-graphs exist with bloom filters > > in them today to support both ordinary and lower-cased paths in the > > filter. > > > > Anyway, I'm still not sold on this idea in general (nor do I understand > > it that others are), so I'll respond in more detail in another part of > > the thread... > > I agree that this is not a good direction to go. I created the patch > because I was curious how difficult it would be, and it is good to have > a record of the possible direction. However, it complicates the file > format and will have unpredictable effects on the entropy (or on the > performance of history for case-colliding paths). > > It is good that we have the capability to extend the filter data in > the future if we really need to. > > I'll make a TODO item for myself to try writing that detailed Bloom > filter format documentation as a follow-up. In the meantime, I'll try > to close this out by responding to the feedback we have so far. Sounds good, and thanks for investigating. > Thanks, > -Stolee Thanks, Taylor
