project-euler/a075.cpp

#include <iostream>
#include <cmath>
#include <chrono>
#define lmax 1500000

using namespace std;
typedef unsigned char byte;

inline int isqrt(int n) {
    return (int)sqrt((float)n);
}

template<class T>
inline T gcd(T a, T b) {
    while(b) {
        auto t = a % b;
        a = b;
        b = t;
    }
    return a;
}

int main() {
    byte arr[lmax+1];
    auto start = chrono::system_clock::now();

    for (int i = 0; i <= lmax; i++)
        arr[i] = 0;

    for (int length = 12; length <= lmax; length += 2)
    for (int m = isqrt(length)/2+1; m <= isqrt(length/2); m++) {
        if (length % (2*m) == 0) 
            {
                int n = length / (2*m) - m;
                if (n>0 && n < m && n%2!= m%2 && gcd(m,n) == 1)
                arr[length]++;
                }
    }

    auto middle = std::chrono::system_clock::now();

    std::chrono::duration<double> elapsed_seconds = middle-start;
    cout << "generating primitives took: " << elapsed_seconds.count() << endl;


    for (int length = lmax; length >= 0; length--) {
        int sqrtlen = isqrt(length);
        for (int prev_length = 2; prev_length <= sqrtlen; prev_length++)
            if (length % prev_length == 0) {
                arr[length] += arr[prev_length];
                arr[length] += arr[length/prev_length];
                if (arr[length] > 1)
                    break;
            }
    }
    auto end = std::chrono::system_clock::now();

    elapsed_seconds = end-middle;
    cout << "adding multiples took: " << elapsed_seconds.count() << endl;


    int singles = 0;

    for (int i = 0; i <= lmax; i++)
        if (arr[i] == 1)
            singles++;

    for (int i = 0; i <= 60; i++)
        if (arr[i] == 1)
            cout << i << ", ";
    cout << endl;

    cout << singles;
    return 0;
}