projecteuler/problem044.cpp at master · js42721/projecteuler · GitHub

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#include<cmath>
#include<iostream>
#include<limits>

boolisPentagonal(longlong n)
{
returnfmod(sqrt(24 * n + 1), 6) == 5;
}

longlongminimalDifference()
{
longlong minD = std::numeric_limits<longlong>::max();
longlong current = 5;
longlong inc = 7;
// The outer loop generates pentagonal numbers until the distance to the
// next number exceeds the recorded minimal difference. After that point,
// all differences will be greater than the recorded minimal difference,
// meaning it is the true minimal difference.
while (inc < minD) {
longlong p = current - (inc - 3);
longlong dec = inc - 6;
do {
longlong d = current - p;
if (d >= minD) {
break;
 }
if (isPentagonal(d) && isPentagonal(current + p)) {
 minD = d;
break;
 }
 p -= dec;
 dec -= 3;
 } while (p >= 1);
 current += inc;
 inc += 3;
 }
return minD;
}

intmain()
{
 std::cout << "Answer: " << minimalDifference() << '\n';
return0;
}