primes generation

//Primes. Sieve of Eratosthenes
//nasarouf@cs.ubc.ca
int p[MAX],pc,f[MAX+1];
void gen() {
    pc = 0; p[pc++] = 2;
    for (int i = 3; i <= MAX; i += 2) if (!f[i]) {
        p[pc++] = i;
        if (i <= MAX / i) for (int j = i*i; j <= MAX; j += 2 * i) f[j] = 1;
    }
}