区间素数筛法

Posted 2020-08-04 Cumulonimbus

     b以内的合数

 1 //对区间[a,b)内的整数执行筛法,需要保证a>1。is_prime[i-a]表示整数i是素数。
 2 void segment_sieve(LL a, LL b)
 3 {
 4     for (LL i = 0; i*i < b; i++) is_prime_small[i] = true;
 5     for (LL i = 0; i < b - a; i++) is_prime[i] = true;
 6 
 7     for (LL i = 2; i*i < b; i++){
 8         if (is_prime_small[i]){
 9             for (LL j = 2 * i; j*j < b; j += i) is_prime_small[j] = false;
10             for (LL j = max(2LL, (a + i - 1) / i)*i; j < b; j += i) is_prime[j - a] = false;
11         }
12     }
13 }

例题 POJ 3978