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More conservative hash functions.

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This commit is contained in:
Davi de Castro Reis 2012-06-09 02:27:39 -03:00
parent c06ad3e25e
commit b3e2ef709d
4 changed files with 9 additions and 43 deletions
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2
cxxmph/mph_index.h
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@ -252,7 +252,7 @@ struct FlexibleMPHIndex<false, false, Key, HashFcn>
: public SimpleMPHIndex<Key, HashFcn> {
FlexibleMPHIndex() : SimpleMPHIndex<Key, HashFcn>(false) {}
uint32_t index(const Key& key) const {
return MPHIndex::index<HashFcn>(key); }
return MPHIndex::perfect_hash<HashFcn>(key); }
uint32_t size() const { return MPHIndex::perfect_hash_size(); }
};
// From a trade-off perspective this case does not make much sense.

46
cxxmph/seeded_hash.h
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@ -10,11 +10,6 @@
#include "MurmurHash3.h"
#include "stringpiece.h"
// From murmur, only used naively to extend 32 bits functions to 128 bits.
uint32_t fmix ( uint32_t h );
// Used for a quick and dirty hash function for integers. Probably a bad idea.
uint64_t fmix ( uint64_t h );
namespace cxxmph {
struct h128 {
@ -33,15 +28,16 @@ template <class HashFcn>
struct seeded_hash_function {
template <class Key>
uint32_t operator()(const Key& k, uint32_t seed) const {
return HashFcn()(k) ^ seed;
uint32_t h;
uint32_t h0 = HashFcn()(k);
MurmurHash3_x86_32(reinterpret_cast<const void*>(&h0), 4, seed, &h);
return h;
}
template <class Key>
h128 hash128(const Key& k, uint32_t seed) const {
h128 h;
for (int i = 0; i < 4; ++i) {
h.uint32[i] = HashFcn()(k) ^ seed;
seed = fmix(seed);
}
uint32_t h0 = HashFcn()(k);
MurmurHash3_x64_128(reinterpret_cast<const void*>(&h0), 4, seed, &h);
return h;
}
};
@ -78,20 +74,6 @@ struct Murmur3StringPiece {
}
};
struct Murmur3Fmix64bitsType {
template <class Key>
uint32_t operator()(const Key& k) const {
return fmix(*reinterpret_cast<const uint64_t*>(&k));
}
template <class Key>
h128 hash128(const Key& k) const {
h128 h;
h.set64(fmix(k), 0);
h.set64(fmix(h.get64(0)), 1);
return h;
}
};
template <>
struct seeded_hash_function<Murmur3> {
template <class Key>
@ -126,22 +108,6 @@ struct seeded_hash_function<Murmur3StringPiece> {
}
};
template <>
struct seeded_hash_function<Murmur3Fmix64bitsType> {
template <class Key>
uint32_t operator()(const Key& k, uint32_t seed) const {
return fmix(k + seed);
}
template <class Key>
h128 hash128(const Key& k, uint32_t seed) const {
h128 h;
h.set64(fmix(k ^ seed), 0);
h.set64(fmix(h.get64(0)), 1);
return h;
}
};
template <class HashFcn> struct seeded_hash
{ typedef seeded_hash_function<HashFcn> hash_function; };
// Use Murmur3 instead for all types defined in std::hash, plus

2
cxxmph/seeded_hash_test.cc
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@ -21,7 +21,7 @@ int main(int argc, char** argv) {
exit(-1);
}
unordered_map<uint64_t, int, Murmur3Fmix64bitsType> g;
unordered_map<uint64_t, int> g;
for (int i = 0; i < 1000; ++i) g[i] = i;
for (int i = 0; i < 1000; ++i) if (g[i] != i) exit(-1);