Created
October 1, 2011 21:46
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Inverse Sieve of Eratosthenes in C
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#include <stdio.h> | |
#include <stdlib.h> | |
typedef unsigned long long int t_item; | |
#define BITS sizeof(t_item) | |
void init(t_item *bitset, int maximum) { | |
int p = maximum / BITS; | |
int i; | |
for (i = 0; i <= p; i++) { | |
bitset[i] = 0; | |
} | |
} | |
t_item does_not_contain(t_item *bitset, int i) { | |
return !(bitset[i / BITS] & (1LL << (i % BITS))); | |
} | |
void add(t_item *bitset, int i) { | |
bitset[i / BITS] |= (1LL << (i % BITS)); | |
} | |
void primes(t_item *non_primes_bitset, int maximum) { | |
int m, km; | |
init(non_primes_bitset, maximum); | |
add(non_primes_bitset, 1); | |
for (km = 4; km <= maximum; km += 2) { | |
add(non_primes_bitset, km); | |
} | |
for (m = 3; m <= maximum; m += 2) { | |
if (does_not_contain(non_primes_bitset, m)) { | |
for (km = 2 * m; km <= maximum; km += m) { | |
add(non_primes_bitset, km); | |
} | |
} | |
} | |
} | |
int main(void) { | |
int maximum = 100; | |
t_item *non_primes_bitset = (t_item *)malloc(maximum + BITS); | |
primes(non_primes_bitset, maximum); | |
int n; | |
for (n = 1; n <= maximum; n++) { | |
if (does_not_contain(non_primes_bitset, n)) { | |
printf("%d\n", n); | |
} | |
} | |
free(non_primes_bitset); | |
return 0; | |
} |
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