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turbonss/cxxmph/mph_index.h

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#ifndef __CXXMPH_MPH_INDEX_H__
#define __CXXMPH_MPH_INDEX_H__
// Minimal perfect hash abstraction implementing the BDZ algorithm
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//
// This is a data structure that given a set of known keys S, will create a
// mapping from S to [0..|S|). The class is informed about S through the Reset
// method and the mapping is queried by calling index(key).
//
// This is a pretty uncommon data structure, and if you application has a real
// use case for it, chances are that it is a real win. If all you are doing is
// a straightforward implementation of an in-memory associative mapping data
// structure (e.g., mph_map.h), then it will probably be slower, since that the
// evaluation of index() is typically slower than the total cost of running a
// traditional hash function over a key and doing 2-3 conflict resolutions on
// 100byte-ish strings.
//
// Notes:
//
// Most users can use the SimpleMPHIndex wrapper instead of the MPHIndex which
// have confusing template parameters.
// This class only implements a minimal perfect hash function, it does not
// implement an associative mapping data structure.
#include <stdint.h>
#include <cassert>
#include <cmath>
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#include <unordered_map> // for std::hash
#include <vector>
#include <iostream>
using std::cerr;
using std::endl;
#include "seeded_hash.h"
#include "trigraph.h"
namespace cxxmph {
class MPHIndex {
public:
MPHIndex(double c = 1.23, uint8_t b = 7) :
c_(c), b_(b), m_(0), n_(0), k_(0), r_(0),
g_(NULL), g_size_(0), ranktable_(NULL), ranktable_size_(0),
deserialized_(false) { }
~MPHIndex();
template <class SeededHashFcn, class ForwardIterator>
bool Reset(ForwardIterator begin, ForwardIterator end);
template <class SeededHashFcn, class Key> // must agree with Reset
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// Get a unique identifier for k, in the range [0;size()). If x wasn't part
// of the input in the last Reset call, returns a random value.
uint32_t index(const Key& x) const;
uint32_t size() const { return m_; }
void clear();
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// Advanced users functions. Please avoid unless you know what you are doing.
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uint32_t perfect_hash_size() const { return n_; }
template <class SeededHashFcn, class Key> // must agree with Reset
uint32_t perfect_hash(const Key& x) const;
template <class SeededHashFcn, class Key> // must agree with Reset
uint32_t minimal_perfect_hash(const Key& x) const;
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// Serialization for mmap usage - not tested well, ping me if you care.
// Serialized tables are not guaranteed to work across versions or different
// endianness (although they could easily be made to be).
uint32_t serialize_bytes_needed() const;
void serialize(char *memory) const;
bool deserialize(const char* serialized_memory);
private:
template <class SeededHashFcn, class ForwardIterator>
bool Mapping(ForwardIterator begin, ForwardIterator end,
std::vector<TriGraph::Edge>* edges,
std::vector<uint32_t>* queue);
bool GenerateQueue(TriGraph* graph, std::vector<uint32_t>* queue);
void Assigning(const std::vector<TriGraph::Edge>& edges,
const std::vector<uint32_t>& queue);
void Ranking();
uint32_t Rank(uint32_t vertex) const;
// Algorithm parameters
double c_; // Number of bits per key (? is it right)
uint8_t b_; // Number of bits of the kth index in the ranktable
// Values used during generation
uint32_t m_; // edges count
uint32_t n_; // vertex count
uint32_t k_; // kth index in ranktable, $k = log_2(n=3r)\varepsilon$
// Values used during search
// Partition vertex count, derived from c parameter.
uint32_t r_;
// The array containing the minimal perfect hash function graph. Do not use
// c++ vector to make mmap based backing easier.
const uint8_t* g_;
uint32_t g_size_;
// The table used for the rank step of the minimal perfect hash function
const uint32_t* ranktable_;
uint32_t ranktable_size_;
// The selected hash seed triplet for finding the edges in the minimal
// perfect hash function graph.
uint32_t hash_seed_[3];
bool deserialized_;
static const uint8_t valuemask[];
static void set_2bit_value(uint8_t *d, uint32_t i, uint8_t v) {
d[(i >> 2)] &= ((v << ((i & 3) << 1)) | valuemask[i & 3]);
}
static uint32_t get_2bit_value(const uint8_t* d, uint32_t i) {
return (d[(i >> 2)] >> (((i & 3) << 1)) & 3);
}
};
// Template method needs to go in the header file.
template <class SeededHashFcn, class ForwardIterator>
bool MPHIndex::Reset(ForwardIterator begin, ForwardIterator end) {
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if (end == begin) {
clear();
return true;
}
m_ = end - begin;
r_ = static_cast<uint32_t>(ceil((c_*m_)/3));
if ((r_ % 2) == 0) r_ += 1;
n_ = 3*r_;
k_ = 1U << b_;
// cerr << "m " << m_ << " n " << n_ << " r " << r_ << endl;
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int iterations = 1000;
std::vector<TriGraph::Edge> edges;
std::vector<uint32_t> queue;
while (1) {
// cerr << "Iterations missing: " << iterations << endl;
for (int i = 0; i < 3; ++i) hash_seed_[i] = random();
if (Mapping<SeededHashFcn>(begin, end, &edges, &queue)) break;
else --iterations;
if (iterations == 0) break;
}
if (iterations == 0) return false;
Assigning(edges, queue);
std::vector<TriGraph::Edge>().swap(edges);
Ranking();
deserialized_ = false;
return true;
}
template <class SeededHashFcn, class ForwardIterator>
bool MPHIndex::Mapping(
ForwardIterator begin, ForwardIterator end,
std::vector<TriGraph::Edge>* edges, std::vector<uint32_t>* queue) {
TriGraph graph(n_, m_);
for (ForwardIterator it = begin; it != end; ++it) {
uint32_t h[3];
for (int i = 0; i < 3; ++i) h[i] = SeededHashFcn()(*it, hash_seed_[i]);
uint32_t v0 = h[0] % r_;
uint32_t v1 = h[1] % r_ + r_;
uint32_t v2 = h[2] % r_ + (r_ << 1);
// cerr << "Key: " << *it << " edge " << it - begin << " (" << v0 << "," << v1 << "," << v2 << ")" << endl;
graph.AddEdge(TriGraph::Edge(v0, v1, v2));
}
if (GenerateQueue(&graph, queue)) {
graph.ExtractEdgesAndClear(edges);
return true;
}
return false;
}
template <class SeededHashFcn, class Key>
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uint32_t MPHIndex::perfect_hash(const Key& key) const {
uint32_t h[3];
for (int i = 0; i < 3; ++i) h[i] = SeededHashFcn()(key, hash_seed_[i]);
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assert(r_);
h[0] = h[0] % r_;
h[1] = h[1] % r_ + r_;
h[2] = h[2] % r_ + (r_ << 1);
assert(g_size_);
// 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_);
uint32_t vertex = h[(get_2bit_value(g_, h[0]) + get_2bit_value(g_, h[1]) + get_2bit_value(g_, h[2])) % 3];
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return vertex;
}
template <class SeededHashFcn, class Key>
uint32_t MPHIndex::minimal_perfect_hash(const Key& key) const {
return Rank(perfect_hash<SeededHashFcn, Key>(key));
}
template <class SeededHashFcn, class Key>
uint32_t MPHIndex::index(const Key& key) const {
return minimal_perfect_hash<SeededHashFcn, Key>(key);
}
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// Simple wrapper around MPHIndex to simplify calling code. Please refer to the
// MPHIndex class for documentation.
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template <class Key, class HashFcn = typename seeded_hash<std::hash<Key> >::hash_function>
class SimpleMPHIndex : public MPHIndex {
public:
template <class ForwardIterator>
bool Reset(ForwardIterator begin, ForwardIterator end) {
return MPHIndex::Reset<HashFcn>(begin, end);
}
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uint32_t index(const Key& key) const { return MPHIndex::index<HashFcn>(key); }
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uint32_t perfect_hash(const Key& key) const { return MPHIndex::perfect_hash<HashFcn>(key); }
uint32_t minimal_perfect_hash(const Key& key) const { return MPHIndex::minimal_perfect_hash<HashFcn>(key); }
};
} // namespace cxxmph
#endif // __CXXMPH_MPH_INDEX_H__