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If hashing is too slow this might mean your hash table implementation is very bad.
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// g++ -O3 -o uniquevalues uniquevalues.cpp -std=c++11 -Wall -Wextra -DNDEBUG | |
// results for N = 10000000: | |
// distinct_count_stdhash(values,N) : 1340.096 cycles per operation (best) 1340.096 cycles per operation (avg) | |
// distinct_count_goodhash(values,N) : 137.917 cycles per operation (best) 137.917 cycles per operation (avg) | |
// distinct_count_sort(values,N) : 258.619 cycles per operation (best) 258.619 cycles per operation (avg) | |
#include <iostream> | |
#include <algorithm> | |
#include <unordered_set> | |
#include <vector> | |
#include <stdint.h> | |
#include <stdio.h> | |
#include <assert.h> | |
#include "benchmark.h" | |
size_t hash_value(uint64_t x) | |
{ | |
x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9ull; | |
x = (x ^ (x >> 27)) * 0x94d049bb133111ebull; | |
x = x ^ (x >> 31); | |
return x; | |
} | |
struct IntSet | |
{ | |
std::vector<uint64_t> data; | |
size_t size; | |
IntSet(size_t capacity = 0): data(capacity), size(0) | |
{ | |
assert((capacity & (capacity - 1)) == 0); | |
} | |
void insert(uint64_t key) | |
{ | |
size_t m = data.size() - 1; | |
size_t h = hash_value(key) & m; | |
size_t i = 0; | |
while (data[h] != key) | |
{ | |
if (data[h] == 0) | |
{ | |
data[h] = key; | |
size++; | |
break; | |
} | |
i = i + 1; | |
h = (h + i) & m; | |
} | |
} | |
static size_t optimalCapacity(size_t count) | |
{ | |
size_t capacity = 1; | |
while (count >= capacity / 2) | |
capacity *= 2; | |
return capacity; | |
} | |
}; | |
size_t distinct_count_goodhash(const uint64_t * values, size_t howmany) { | |
bool zero = false; | |
IntSet hash(IntSet::optimalCapacity(howmany)); | |
for (size_t i = 0; i < howmany; ++i) | |
if (values[i] == 0) | |
zero = true; | |
else | |
hash.insert(values[i]); | |
return zero + hash.size; | |
} | |
size_t distinct_count_stdhash(const uint64_t * values, size_t howmany) { | |
std::unordered_set<uint64_t> hash(values, values + howmany); | |
return hash.size(); | |
} | |
size_t distinct_count_sort(const uint64_t * values, size_t howmany) { | |
std::vector<uint64_t> array(values, values + howmany); | |
std::sort(array.begin(), array.end()); | |
return std::unique(array.begin(), array.end()) - array.begin(); | |
} | |
void demo(size_t N) { | |
std::cout << "N = " << N <<std::endl; | |
uint64_t * values = new uint64_t[N]; | |
for(size_t i = 0; i < N; ++i) { | |
values[i] = ((uint16_t)rand()) | ((uint32_t)rand() << 16) | ((uint64_t)rand()<<32) |((uint64_t)rand()<<48); | |
} | |
values[N/2] = values[0]; | |
values[N-1] = values[1]; | |
size_t expected = distinct_count_stdhash(values,N); | |
std::cout <<" expected = "<<expected << std::endl; | |
const int repeat = 1; | |
BEST_TIME(distinct_count_stdhash(values,N), expected, , repeat, N, true); | |
BEST_TIME(distinct_count_goodhash(values,N), expected, , repeat, N, true); | |
BEST_TIME(distinct_count_sort(values,N), expected, , repeat, N, true); | |
delete[] values; | |
} | |
int main() { | |
for(size_t N = 10; N < 100000000; N*=10) demo(N); | |
} |
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