- The Bloom filter of the commit carrying the paths that were changed between

the commit and its first parent, if requested.

Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]

* The ith entry, BIDX[i], stores the number of 8-byte word blocks in all

Bloom filters from commit 0 to commit i (inclusive) in lexicographic

order. The Bloom filter for the i-th commit spans from BIDX[i-1] to

BIDX[i] (plus header length), where BIDX[-1] is 0.

* The BIDX chunk is ignored if the BDAT chunk is not present.

Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]

* It starts with header consisting of three unsigned 32-bit integers:

- Version of the hash algorithm being used. We currently only support

value 1 which corresponds to the 32-bit version of the murmur3 hash

implemented exactly as described in

https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double

hashing technique using seed values 0x293ae76f and 0x7e646e2 as

described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters

in Probabilistic Verification"

- The number of times a path is hashed and hence the number of bit positions

that cumulatively determine whether a file is present in the commit.

- The minimum number of bits 'b' per entry in the Bloom filter. If the filter

contains 'n' entries, then the filter size is the minimum number of 64-bit

words that contain n*b bits.

* The rest of the chunk is the concatenation of all the computed Bloom

filters for the commits in lexicographic order.

* Note: Commits with no changes or more than 512 changes have Bloom filters

of length zero.

* The BDAT chunk is present if and only if BIDX is present.

TEST_BUILTINS_OBJS += test-bloom.o

LIB_OBJS += bloom.o

#include "git-compat-util.h"

#include "bloom.h"

#include "diff.h"

#include "diffcore.h"

#include "revision.h"

#include "hashmap.h"

#include "commit-graph.h"

#include "commit.h"

define_commit_slab(bloom_filter_slab, struct bloom_filter);

struct bloom_filter_slab bloom_filters;

struct pathmap_hash_entry {

struct hashmap_entry entry;

const char path[FLEX_ARRAY];

};

static uint32_t rotate_left(uint32_t value, int32_t count)

{

uint32_t mask = 8 * sizeof(uint32_t) - 1;

count &= mask;

return ((value << count) | (value >> ((-count) & mask)));

}

static inline unsigned char get_bitmask(uint32_t pos)

{

return ((unsigned char)1) << (pos & (BITS_PER_WORD - 1));

}

static int load_bloom_filter_from_graph(struct commit_graph *g,

struct bloom_filter *filter,

struct commit *c)

{

uint32_t lex_pos, start_index, end_index;

while (c->graph_pos < g->num_commits_in_base)

g = g->base_graph;

/* The commit graph commit 'c' lives in doesn't carry bloom filters. */

if (!g->chunk_bloom_indexes)

return 0;

lex_pos = c->graph_pos - g->num_commits_in_base;

end_index = get_be32(g->chunk_bloom_indexes + 4 * lex_pos);

if (lex_pos > 0)

start_index = get_be32(g->chunk_bloom_indexes + 4 * (lex_pos - 1));

else

start_index = 0;

filter->len = end_index - start_index;

filter->data = (unsigned char *)(g->chunk_bloom_data +

sizeof(unsigned char) * start_index +

BLOOMDATA_CHUNK_HEADER_SIZE);

return 1;

}

uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len)

{

const uint32_t c1 = 0xcc9e2d51;

const uint32_t c2 = 0x1b873593;

const uint32_t r1 = 15;

const uint32_t r2 = 13;

const uint32_t m = 5;

const uint32_t n = 0xe6546b64;

int i;

uint32_t k1 = 0;

const char *tail;

int len4 = len / sizeof(uint32_t);

uint32_t k;

for (i = 0; i < len4; i++) {

uint32_t byte1 = (uint32_t)data[4*i];

uint32_t byte2 = ((uint32_t)data[4*i + 1]) << 8;

uint32_t byte3 = ((uint32_t)data[4*i + 2]) << 16;

uint32_t byte4 = ((uint32_t)data[4*i + 3]) << 24;

k = byte1 | byte2 | byte3 | byte4;

k *= c1;

k = rotate_left(k, r1);

k *= c2;

seed ^= k;

seed = rotate_left(seed, r2) * m + n;

}

tail = (data + len4 * sizeof(uint32_t));

switch (len & (sizeof(uint32_t) - 1)) {

case 3:

k1 ^= ((uint32_t)tail[2]) << 16;

/*-fallthrough*/

case 2:

k1 ^= ((uint32_t)tail[1]) << 8;

/*-fallthrough*/

case 1:

k1 ^= ((uint32_t)tail[0]) << 0;

k1 *= c1;

k1 = rotate_left(k1, r1);

k1 *= c2;

seed ^= k1;

break;

}

seed ^= (uint32_t)len;

seed ^= (seed >> 16);

seed *= 0x85ebca6b;

seed ^= (seed >> 13);

seed *= 0xc2b2ae35;

seed ^= (seed >> 16);

return seed;

}

void fill_bloom_key(const char *data,

size_t len,

struct bloom_key *key,

const struct bloom_filter_settings *settings)

{

int i;

const uint32_t seed0 = 0x293ae76f;

const uint32_t seed1 = 0x7e646e2c;

const uint32_t hash0 = murmur3_seeded(seed0, data, len);

const uint32_t hash1 = murmur3_seeded(seed1, data, len);

key->hashes = (uint32_t *)xcalloc(settings->num_hashes, sizeof(uint32_t));

for (i = 0; i < settings->num_hashes; i++)

key->hashes[i] = hash0 + i * hash1;

}

void add_key_to_filter(const struct bloom_key *key,

struct bloom_filter *filter,

const struct bloom_filter_settings *settings)

{

int i;

uint64_t mod = filter->len * BITS_PER_WORD;

for (i = 0; i < settings->num_hashes; i++) {

uint64_t hash_mod = key->hashes[i] % mod;

uint64_t block_pos = hash_mod / BITS_PER_WORD;

filter->data[block_pos] |= get_bitmask(hash_mod);

}

}

void init_bloom_filters(void)

{

init_bloom_filter_slab(&bloom_filters);

}

struct bloom_filter *get_bloom_filter(struct repository *r,

struct commit *c,

int compute_if_not_present)

{

struct bloom_filter *filter;

struct bloom_filter_settings settings = DEFAULT_BLOOM_FILTER_SETTINGS;

int i;

struct diff_options diffopt;

int max_changes = 512;

if (bloom_filters.slab_size == 0)

return NULL;

filter = bloom_filter_slab_at(&bloom_filters, c);

if (!filter->data) {

load_commit_graph_info(r, c);

if (c->graph_pos != COMMIT_NOT_FROM_GRAPH &&

r->objects->commit_graph->chunk_bloom_indexes) {

if (load_bloom_filter_from_graph(r->objects->commit_graph, filter, c))

return filter;

else

return NULL;

}

}

if (filter->data || !compute_if_not_present)

return filter;

repo_diff_setup(r, &diffopt);

diffopt.flags.recursive = 1;

diffopt.detect_rename = 0;

diffopt.max_changes = max_changes;

diff_setup_done(&diffopt);

if (c->parents)

diff_tree_oid(&c->parents->item->object.oid, &c->object.oid, "", &diffopt);

else

diff_tree_oid(NULL, &c->object.oid, "", &diffopt);

diffcore_std(&diffopt);

if (diff_queued_diff.nr <= max_changes) {

struct hashmap pathmap;

struct pathmap_hash_entry *e;

struct hashmap_iter iter;

hashmap_init(&pathmap, NULL, NULL, 0);

for (i = 0; i < diff_queued_diff.nr; i++) {

const char *path = diff_queued_diff.queue[i]->two->path;

/*

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 (!last_slash)

last_slash = (char*)path;

*last_slash = '\0';

} while (*path);

diff_free_filepair(diff_queued_diff.queue[i]);

}

filter->len = (hashmap_get_size(&pathmap) * settings.bits_per_entry + BITS_PER_WORD - 1) / BITS_PER_WORD;

filter->data = xcalloc(filter->len, sizeof(unsigned char));

hashmap_for_each_entry(&pathmap, &iter, e, entry) {

struct bloom_key key;

fill_bloom_key(e->path, strlen(e->path), &key, &settings);

add_key_to_filter(&key, filter, &settings);

}

hashmap_free_entries(&pathmap, struct pathmap_hash_entry, entry);

} else {

for (i = 0; i < diff_queued_diff.nr; i++)

diff_free_filepair(diff_queued_diff.queue[i]);

filter->data = NULL;

filter->len = 0;

}

free(diff_queued_diff.queue);

DIFF_QUEUE_CLEAR(&diff_queued_diff);

return filter;

}

int bloom_filter_contains(const struct bloom_filter *filter,

const struct bloom_key *key,

const struct bloom_filter_settings *settings)

{

int i;

uint64_t mod = filter->len * BITS_PER_WORD;

if (!mod)

return -1;

for (i = 0; i < settings->num_hashes; i++) {

uint64_t hash_mod = key->hashes[i] % mod;

uint64_t block_pos = hash_mod / BITS_PER_WORD;

if (!(filter->data[block_pos] & get_bitmask(hash_mod)))

return 0;

}

return 1;

}