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Added nice optimization to avoid mod 3.

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This commit is contained in:
Davi Reis 2012-03-16 02:54:16 -03:00
parent 2bfe38d2da
commit 11d54ea837
2 changed files with 18 additions and 6 deletions
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6
cxxmph/mph_bits.h
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@ -2,6 +2,8 @@
#define __CXXMPH_MPH_BITS_H__
#include <stdint.h> // for uint32_t and friends
#include <array>
#include <cassert>
#include <climits>
#include <cmath>
@ -9,6 +11,7 @@
#include <cstring>
#include <limits>
#include <vector>
#include <utility>
namespace cxxmph {
@ -62,6 +65,9 @@ static uint32_t nextpoweroftwo(uint32_t k) {
return k+1;
}
// Interesting bit tricks that might end up here:
// http://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord
} // namespace cxxmph
#endif

18
cxxmph/mph_index.h
View File

@ -45,7 +45,8 @@ class MPHIndex {
public:
MPHIndex(double c = 1.23, uint8_t b = 7) :
c_(c), b_(b), m_(0), n_(0), k_(0), r_(1),
g_(NULL), g_size_(0), ranktable_(NULL), ranktable_size_(0),
g_(NULL), g_size_(0),
ranktable_(NULL), ranktable_size_(0),
deserialized_(false) { }
~MPHIndex();
@ -112,6 +113,7 @@ class MPHIndex {
// c++ vector to make mmap based backing easier.
const uint8_t* g_;
uint32_t g_size_;
uint8_t threebit_mod3[10]; // speed up mod3 calculation for 3bit ints
// The table used for the rank step of the minimal perfect hash function
const uint32_t* ranktable_;
uint32_t ranktable_size_;
@ -135,12 +137,13 @@ bool MPHIndex::Reset(
m_ = size;
r_ = static_cast<uint32_t>(ceil((c_*m_)/3));
if ((r_ % 2) == 0) r_ += 1;
nest_displacement_[0] = 0;
nest_displacement_[1] = r_;
nest_displacement_[2] = (r_ << 1);
// This can be used to speed mods, but increases occupation too much.
// Needs to try http://gmplib.org/manual/Integer-Exponentiation.html instead
// r_ = nextpoweroftwo(r_);
nest_displacement_[0] = 0;
nest_displacement_[1] = r_;
nest_displacement_[2] = (r_ << 1);
for (int i = 0; i < sizeof(threebit_mod3); ++i) threebit_mod3[i] = i % 3;
n_ = 3*r_;
k_ = 1U << b_;
@ -215,15 +218,18 @@ uint32_t MPHIndex::perfect_hash(const Key& key) const {
uint32_t h[4];
if (!g_size_) return 0;
SeededHashFcn().hash64(key, hash_seed_[0], h);
// for (int i = 0; i < 3; ++i) h[i] = SeededHashFcn()(key, hash_seed_[i]);
h[0] = (h[0] % r_) + nest_displacement_[0];
h[1] = (h[1] % r_) + nest_displacement_[1];
h[2] = (h[2] % r_) + nest_displacement_[2];
// h[0] = (h[0] & (r_-1)) + nest_displacement_[0];
// h[1] = (h[1] & (r_-1)) + nest_displacement_[1];
// h[2] = (h[2] & (r_-1)) + nest_displacement_[2];
// cerr << "g_.size() " << g_size_ << " h0 >> 2 " << (h[0] >> 2) << endl;
assert((h[0] >> 2) <g_size_);
assert((h[1] >> 2) <g_size_);
assert((h[2] >> 2) <g_size_);
uint8_t nest = (get_2bit_value(g_, h[0]) + get_2bit_value(g_, h[1]) + get_2bit_value(g_, h[2])) % 3;
uint8_t nest = threebit_mod3[
get_2bit_value(g_, h[0]) + get_2bit_value(g_, h[1]) + get_2bit_value(g_, h[2])];
uint32_t vertex = h[nest];
return vertex;
}