Re: A simple sieve in Phobos?

[Chris Williams]

On Wednesday, 19 March 2014 at 18:40:49 UTC, Chris Williams wrote:
enum uint MAX_N = 10_000_000U;
void calcPrimes() pure nothrow {
    uint[][uint] markers;

    for (uint L = 2; L < MAX_N; L++) {
        uint[]* pList = (L in markers);
        if (pList is null) {
            markers[L + L] = [L];
        }
        else {
            uint[] list = *pList;
            size_t len = list.length - 1;
            for (uint L2 = 0; L2 < len; L2++) {
                uint val = list[L2];
                markers[ L + val ] ~= val;
            }

            // reuse the current list for the last item to save 
allocations
            list[0] = list[len];
            list.length = 1;
            markers[ L + list[len] ] = list;
        }
    }
}

void main() {
    calcPrimes;
}

Well bummer, my quick and easy optimization broke the result for 
some reason. Here's a version that actually produces the correct 
answers, while I try and debug:

enum uint MAX_N = 10_000_000U;

void calcPrimes() {
    uint[][uint] markers;

    for (uint L = 2; L < MAX_N; L++) {
        uint[]* pList = (L in markers);
        if (pList is null) {
            markers[L + L] = [L];
        }
        else {
            uint[] list = *pList;
            for (uint L2 = 0; L2 < list.length; L2++) {
                uint val = list[L2];
                markers[ L + val ] ~= val;
            }
        }
    }
}

void main() {
    calcPrimes;
}