Optimize spfrhwotimize

Post small pieces of code and optimize each others anus

int is_prime(int num){
    if(num < 1)
        return 0;
    if(num < 3)
        return 1;
    if((num % 2) == 0)
        return 0;
    int i = 3;
    while(num <= i * i){
        if((num % i) == 0)
            return 0;
        i += 2;
    }
    return 1;
}

Time test:

int main(){
    for(int i = 0; i < 10000000; ++i)
        is_prime(i);
    return 0;
}

[code]gcc -O3 prime.c -std=c99
time ./a.out
real    0m0.046s
user    0m0.040s
sys     0m0.000s
[code]

int is_prime(int num){
  return PRIME[num];
}

where PRIME[] is a bit array representing all prime numbers less than 10000000

If you're going to be checking that many numbers there's no reason to do the same math over and over again.