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-rw-r--r--src/bloom.cpp13
1 files changed, 11 insertions, 2 deletions
diff --git a/src/bloom.cpp b/src/bloom.cpp
index f07b5b6066..f8e28edded 100644
--- a/src/bloom.cpp
+++ b/src/bloom.cpp
@@ -245,6 +245,14 @@ static inline uint32_t RollingBloomHash(unsigned int nHashNum, uint32_t nTweak,
return MurmurHash3(nHashNum * 0xFBA4C795 + nTweak, vDataToHash);
}
+
+// A replacement for x % n. This assumes that x and n are 32bit integers, and x is a uniformly random distributed 32bit value
+// which should be the case for a good hash.
+// See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
+static inline uint32_t FastMod(uint32_t x, size_t n) {
+ return ((uint64_t)x * (uint64_t)n) >> 32;
+}
+
void CRollingBloomFilter::insert(const std::vector<unsigned char>& vKey)
{
if (nEntriesThisGeneration == nEntriesPerGeneration) {
@@ -268,7 +276,8 @@ void CRollingBloomFilter::insert(const std::vector<unsigned char>& vKey)
for (int n = 0; n < nHashFuncs; n++) {
uint32_t h = RollingBloomHash(n, nTweak, vKey);
int bit = h & 0x3F;
- uint32_t pos = (h >> 6) % data.size();
+ /* FastMod works with the upper bits of h, so it is safe to ignore that the lower bits of h are already used for bit. */
+ uint32_t pos = FastMod(h, data.size());
/* The lowest bit of pos is ignored, and set to zero for the first bit, and to one for the second. */
data[pos & ~1] = (data[pos & ~1] & ~(((uint64_t)1) << bit)) | ((uint64_t)(nGeneration & 1)) << bit;
data[pos | 1] = (data[pos | 1] & ~(((uint64_t)1) << bit)) | ((uint64_t)(nGeneration >> 1)) << bit;
@@ -286,7 +295,7 @@ bool CRollingBloomFilter::contains(const std::vector<unsigned char>& vKey) const
for (int n = 0; n < nHashFuncs; n++) {
uint32_t h = RollingBloomHash(n, nTweak, vKey);
int bit = h & 0x3F;
- uint32_t pos = (h >> 6) % data.size();
+ uint32_t pos = FastMod(h, data.size());
/* If the relevant bit is not set in either data[pos & ~1] or data[pos | 1], the filter does not contain vKey */
if (!(((data[pos & ~1] | data[pos | 1]) >> bit) & 1)) {
return false;