Re: [PATCH v2 05/12] bloom: de-duplicate directory entries

> From: Derrick Stolee <dstolee@microsoft.com>
> 
> When computing a changed-path Bloom filter, we need to take the
> files that changed from the diff computation and extract the parent
> directories. That way, a directory pathspec such as "Documentation"
> could match commits that change "Documentation/git.txt".
> 
> However, the current code does a poor job of this process. The paths
> are added to a hashmap, but we do not check if an entry already
> exists with that path. This can create many duplicate entries and
> cause the filter to have a much larger length than it should. This
> means that the filter is more sparse than intended, which helps the
> false positive rate, but wastes a lot of space.
> 
> Properly use hashmap_get() before hashmap_add(). Also be sure to
> include a comparison function so these can be matched correctly.
> 
> This has an effect on a test in t0095-bloom.sh. This makes sense,
> there are ten changes inside "smallDir" so the total number of
> paths in the filter should be 11. This would result in 11 * 10 bits
> required, and with 8 bits per byte, this results in 14 bytes.

> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  bloom.c          | 35 ++++++++++++++++++++++++++---------
>  t/t0095-bloom.sh |  4 ++--
>  2 files changed, 28 insertions(+), 11 deletions(-)
> 
> diff --git a/bloom.c b/bloom.c
> index 96792782719..196cda0a1bd 100644
> --- a/bloom.c
> +++ b/bloom.c
> @@ -158,6 +158,19 @@ void init_bloom_filters(void)
>  	init_bloom_filter_slab(&bloom_filters);
>  }
>  
> +static int pathmap_cmp(const void *hashmap_cmp_fn_data,
> +		       const struct hashmap_entry *eptr,
> +		       const struct hashmap_entry *entry_or_key,
> +		       const void *keydata)
> +{
> +	const struct pathmap_hash_entry *e1, *e2;
> +
> +	e1 = container_of(eptr, const struct pathmap_hash_entry, entry);
> +	e2 = container_of(entry_or_key, const struct pathmap_hash_entry, entry);
> +
> +	return strcmp(e1->path, e2->path);
> +}
> +
>  struct bloom_filter *get_bloom_filter(struct repository *r,
>  				      struct commit *c,
>  				      int compute_if_not_present)
> @@ -206,25 +219,29 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
>  		struct hashmap pathmap;
>  		struct pathmap_hash_entry *e;
>  		struct hashmap_iter iter;
> -		hashmap_init(&pathmap, NULL, NULL, 0);
> +		hashmap_init(&pathmap, pathmap_cmp, NULL, 0);
>  
>  		for (i = 0; i < diff_queued_diff.nr; i++) {
>  			const char *path = diff_queued_diff.queue[i]->two->path;
>  
>  			/*
> -			* Add each leading directory of the changed file, i.e. for
> -			* 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so
> -			* the Bloom filter could be used to speed up commands like
> -			* 'git log dir/subdir', too.
> -			*
> -			* Note that directories are added without the trailing '/'.
> -			*/
> +			 * Add each leading directory of the changed file, i.e. for
> +			 * 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so
> +			 * the Bloom filter could be used to speed up commands like
> +			 * 'git log dir/subdir', too.
> +			 *
> +			 * Note that directories are added without the trailing '/'.
> +			 */
>  			do {
>  				char *last_slash = strrchr(path, '/');
>  
>  				FLEX_ALLOC_STR(e, path, path);

>  				hashmap_entry_init(&e->entry, strhash(path));
> -				hashmap_add(&pathmap, &e->entry);
> +
> +				if (!hashmap_get(&pathmap, &e->entry, NULL))
> +					hashmap_add(&pathmap, &e->entry);
> +				else
> +					free(e);
>  
>  				if (!last_slash)
>  					last_slash = (char*)path;
> diff --git a/t/t0095-bloom.sh b/t/t0095-bloom.sh
> index 8f9eef116dc..6defeb544f1 100755
> --- a/t/t0095-bloom.sh
> +++ b/t/t0095-bloom.sh
> @@ -89,8 +89,8 @@ test_expect_success 'get bloom filter for commit with 10 changes' '
>  	git add smallDir &&
>  	git commit -m "commit with 10 changes" &&
>  	cat >expect <<-\EOF &&
> -	Filter_Length:25
> -	Filter_Data:82|a0|65|47|0c|92|90|c0|a1|40|02|a0|e2|40|e0|04|0a|9a|66|cf|80|19|85|42|23|
> +	Filter_Length:14
> +	Filter_Data:02|b3|c4|a0|34|e7|fe|eb|cb|47|fe|a0|e8|72|
>  	EOF
>  	test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&
>  	test_cmp expect actual
> -- 
> gitgitgadget
>