Add a nTweak to bloom filters to tweak the seed.

This commit is contained in:
Matt Corallo 2012-11-02 18:33:50 -04:00
parent 4c8fc1a588
commit b1f99bed6f
3 changed files with 57 additions and 24 deletions

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@ -15,7 +15,7 @@ using namespace std;
static const unsigned char bit_mask[8] = {0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80}; static const unsigned char bit_mask[8] = {0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80};
CBloomFilter::CBloomFilter(unsigned int nElements, double nFPRate) : CBloomFilter::CBloomFilter(unsigned int nElements, double nFPRate, unsigned int nTweakIn) :
// The ideal size for a bloom filter with a given number of elements and false positive rate is: // The ideal size for a bloom filter with a given number of elements and false positive rate is:
// - nElements * log(fp rate) / ln(2)^2 // - nElements * log(fp rate) / ln(2)^2
// We ignore filter parameters which will create a bloom filter larger than the protocol limits // We ignore filter parameters which will create a bloom filter larger than the protocol limits
@ -23,14 +23,15 @@ vData(min((unsigned int)(-1 / LN2SQUARED * nElements * log(nFPRate)), MAX_BLOOM
// The ideal number of hash functions is filter size * ln(2) / number of elements // The ideal number of hash functions is filter size * ln(2) / number of elements
// Again, we ignore filter parameters which will create a bloom filter with more hash functions than the protocol limits // Again, we ignore filter parameters which will create a bloom filter with more hash functions than the protocol limits
// See http://en.wikipedia.org/wiki/Bloom_filter for an explanation of these formulas // See http://en.wikipedia.org/wiki/Bloom_filter for an explanation of these formulas
nHashFuncs(min((unsigned int)(vData.size() * 8 / nElements * LN2), MAX_HASH_FUNCS)) nHashFuncs(min((unsigned int)(vData.size() * 8 / nElements * LN2), MAX_HASH_FUNCS)),
nTweak(nTweakIn)
{ {
} }
inline unsigned int CBloomFilter::Hash(unsigned int nHashNum, const std::vector<unsigned char>& vDataToHash) const inline unsigned int CBloomFilter::Hash(unsigned int nHashNum, const std::vector<unsigned char>& vDataToHash) const
{ {
// 0xFBA4C795 chosen as it guarantees a reasonable bit difference between nHashNum values. // 0xFBA4C795 chosen as it guarantees a reasonable bit difference between nHashNum values.
return MurmurHash3(nHashNum * 0xFBA4C795, vDataToHash) % (vData.size() * 8); return MurmurHash3(nHashNum * 0xFBA4C795 + nTweak, vDataToHash) % (vData.size() * 8);
} }
void CBloomFilter::insert(const vector<unsigned char>& vKey) void CBloomFilter::insert(const vector<unsigned char>& vKey)

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@ -33,6 +33,7 @@ class CBloomFilter
private: private:
std::vector<unsigned char> vData; std::vector<unsigned char> vData;
unsigned int nHashFuncs; unsigned int nHashFuncs;
unsigned int nTweak;
unsigned int Hash(unsigned int nHashNum, const std::vector<unsigned char>& vDataToHash) const; unsigned int Hash(unsigned int nHashNum, const std::vector<unsigned char>& vDataToHash) const;
@ -41,7 +42,9 @@ public:
// Note that if the given parameters will result in a filter outside the bounds of the protocol limits, // Note that if the given parameters will result in a filter outside the bounds of the protocol limits,
// the filter created will be as close to the given parameters as possible within the protocol limits. // the filter created will be as close to the given parameters as possible within the protocol limits.
// This will apply if nFPRate is very low or nElements is unreasonably high. // This will apply if nFPRate is very low or nElements is unreasonably high.
CBloomFilter(unsigned int nElements, double nFPRate); // nTweak is a constant which is added to the seed value passed to the hash function
// It should generally always be a random value (and is largely only exposed for unit testing)
CBloomFilter(unsigned int nElements, double nFPRate, unsigned int nTweak);
// Using a filter initialized with this results in undefined behavior // Using a filter initialized with this results in undefined behavior
// Should only be used for deserialization // Should only be used for deserialization
CBloomFilter() {} CBloomFilter() {}
@ -50,6 +53,7 @@ public:
( (
READWRITE(vData); READWRITE(vData);
READWRITE(nHashFuncs); READWRITE(nHashFuncs);
READWRITE(nTweak);
) )
void insert(const std::vector<unsigned char>& vKey); void insert(const std::vector<unsigned char>& vKey);

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