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... > For example the value of 0x00 in such field of BDAT > chunk header would mean no transformation, while > the value of 0x01 would mean per-character tolower() > or Unicode equivalent of it. > > Best, > -- > Jakub Narębski Thanks, Taylor
