On Wednesday, 30 October 2013 at 14:17:22 UTC, qznc wrote:
Why do you want to find the exact prime first? Just check
against sqrt(num) in the loop.
auto upper = cast(ulong)sqrt(cast(double)num)) + 1;
foreach(ulong prime; primes) {
if (prime > upper) return true;
if (num % prime == 0) return false;
}
assert (false); // should be unreachable?
Because having a branch inside the foreach causes a branch
prediction error when you've found a prime number. Simply
iterating up to the sqrt and then terminating the loop has no
branch prediction error. Try it for yourself.
