Fix the calculation of "is prime" for numbers after 1321.

[SeanTAllen]

No description provided.

[EpicEric]

Should this have regression tests?

One possibility to cover a wide range of values is to test for U128 Mersenne primes, for example:

class MersennePrimeTest is UnitTest
  fun name(): String => "math/MersennePrimeTest"

  fun apply(h: TestHelper) =>
    let mersenne_prime_exponents: Array[U8] = [
      2; 3; 5; 7
      13; 17; 19; 31
      61; 89; 107; 127
    ]
    var p: U8 = 128
    var x = U128.max_value()  // (2 ^ p) - 1
    while p >= 2 do
      if mersenne_prime_exponents.contains(p) then
        h.assert_true(IsPrime[U128](x))
      else
        h.assert_false(IsPrime[U128](x))
      end
      p = p - 1
      x = x >> 1
    end

[rhagenson]

Eric, I am always for more tests covering a wider range. I am not familiar with Mersenne primes though.

I am going to adding tests to this PR before it is merged and reduce the redundancy in the implementation.

[rhagenson]

I am working on my additions here.

I am currently looking at generalizing Mersenne prime production.

[SeanTAllen]

@rhagenson if the additional tests aren't ready in the next couple days, let's merge this so it goes out in the release and go from there.

[rhagenson]

Sounds good. I will push what I have done today (short of any WIP stuff) so we can get it ready. The issue right now is that Mersenne primes are computationally intensive so I am trying to find a way to pare that down slightly to test a large portion of the range, but not all.

[rhagenson]

The only problematic type is U128 because the calculations done within IsPrime increment upwards by 6 while the test is checking for the primality of (2 ^ 127) - 1. This takes a long, long time. Until we can find a way to shortcut this, I am okay with not having a Mersenne prime test for U128.

[rhagenson]

The force push is me using git fixup for the first time (I just learned about it last week thanks to Sean) so if it broke something I will need help fixing it. I had neglected to remove some debugging stuff, including the use "debug" itself.

[rhagenson]

I have going to remove myself as a reviewer since obviously at this time I have made additions so there is a general implicit approval.

[rhagenson]

Could not quickly determine how to unassign myself so going to stamp it with my approval anyhow as I have to switch what I am focusing on right now.