One (last?) trick to reduce the number of iterations.
I changed this:
var residues: seq[int] = @[]
var pc = 1
while pc < modpg: (pc += 2; if gcd(modpg, pc) == 1: residues.add(pc))
let rescnt = residues.len
to this:
var residues: seq[int] = @[]
var pc = 1
var inc = 4
while pc < modpg: (pc += inc; inc = inc xor 0b110; if gcd(modpg, pc) ==
1: residues.add(pc))
let rescnt = residues.len
This reduces the number of times `gcd` is called from half the numbers (all the
odds) upto modpg (modpg/2) to a third (modpg/3).
This change produces the P3 PG residue sequence: `pcs = 6m +|- 1`
pc = 1, 5, 7, 11, 13, 17, 19...
+4 +2 +4 +2 +4 +2
So while it shaves off some iterations of `gcd` it's still not enough to get
P17 to compile.
`A REQUEST:` It would be really, really nice to use the boolean operators like:
`x ^= y; x &= y`; etc.
