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Add a nTweak to bloom filters to tweak the seed.

tags/v0.15.1
Matt Corallo 8 years ago
parent
commit
b1f99bed6f
3 changed files with 57 additions and 24 deletions
  1. 4
    3
      src/bloom.cpp
  2. 5
    1
      src/bloom.h
  3. 48
    20
      src/test/bloom_tests.cpp

+ 4
- 3
src/bloom.cpp View File

@@ -15,7 +15,7 @@ using namespace std;

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:
// - nElements * log(fp rate) / ln(2)^2
// 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
// 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
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
{
// 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)

+ 5
- 1
src/bloom.h View File

@@ -33,6 +33,7 @@ class CBloomFilter
private:
std::vector<unsigned char> vData;
unsigned int nHashFuncs;
unsigned int nTweak;

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,
// 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.
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
// Should only be used for deserialization
CBloomFilter() {}
@@ -50,6 +53,7 @@ public:
(
READWRITE(vData);
READWRITE(nHashFuncs);
READWRITE(nTweak);
)

void insert(const std::vector<unsigned char>& vKey);

+ 48
- 20
src/test/bloom_tests.cpp
File diff suppressed because it is too large
View File


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