位数组实现用筛法(Sieve of Eratosthnes)计算素数

//
//  main.c
//  bitarray
//
//  Created by Cheney Shen on 11-4-16.
//  Copyright 2011年 __MyCompanyName__. All rights reserved.
//

#include <stdio.h>
#include <limits.h>
#include <string.h>
#include <time.h>

#define BITMASK(b)  (1 << ((b) % CHAR_BIT))
#define BITSLOT(b)  ((b) / CHAR_BIT)
#define BITSET(a, b)    ((a)[BITSLOT(b)] |= BITMASK(b))
#define BITCLEAR(a, b)  ((a)[BITSLOT(b)] &= ~BITMASK(b))
#define BITTEST(a, b)   ((a)[BITSLOT(b)] & BITMASK(b))
#define BITNSLOTS(nb)    ((nb + CHAR_BIT - 1) / CHAR_BIT)

#define MAX 100000

int main (int argc, const char * argv[])
{
    char bitarray[MAX];
    int i, j;
    time_t  start, end;
    memset(bitarray, 0, BITNSLOTS(MAX));
    
    time(&start);
    printf("It's %s\n", ctime(&start));
    for (i = 2; i < MAX; i++) {
        if(!BITTEST(bitarray, i)) {
            printf("%d\t", i);
            for (j = i<<1; j < MAX; j += i)
                BITSET(bitarray, j);
        }
    }
    time(&end);
    printf("\n");
    printf("Now it's is %s", ctime(&end));
    return 0;
}