int main(void)
{
assert(p(4) + p(7) == 92);
assert(isPentagon(p(4) + p(7)) == true);
assert(isPentagon(p(7) - p(4)) == false);
std::cout << problem44() << std::endl;
}
int problem44(void)
{
int min = 99999999;
for (int j = 1; j < 10000; ++j) {
for (int k = j + 1; k < 10000; ++k) {
const int diff = p(k) - p(j);
if (min < diff)
break;
if (isPentagon(diff) && isPentagon(p(j) + p(k)))
min = diff;
}
}
return min;
}
int main(){
// check that the pentagon function isn't broken
assert(P(5)==35);
long i,j,k;
int s=5000;
long D=100;
long pentagons[s+1];
// Generate s pentagon numbers:
for(i=1; i<=s; i++){
// another check on the algorithm
assert(P(i)>P(i-1));
pentagons[i]=P(i);
printf("P[%ld]=%ld\n",i,P(i));
}
long testsum,testdif, Pj, Pk;
// So BOTH the sum and difference of Pj,Pk must be pentagonal
// starting with the difference (easier to find).
// for general purposes, Pj > Pk
for(k=1; k<=s; k++){
for(j=2; j<=s; j++){
Pj = pentagons[j];
Pk = pentagons[k];
testdif = Pj - Pk;
testsum = Pj + Pk;
if(!(isPentagon(testdif,s,pentagons))){
//printf("For Pk=%ld, Pj=%d, dif is pentag'l",Pk,Pj);
if(!(isPentagon(testdif,s,pentagons))){
//printf(" and so is sum\n");
//check to see if this is a new D (minimised value)
if(labs(Pk-Pj)<=D) {
D = labs(Pk-Pj);
printf("new low score: %ld\n",D);
}
}else printf(".\n");
}
}//end of checking values for Pj
}//end of checking values for Pk
printf("lowest D=%ld\n",D);
return 0;
}
