Only produce Unknown names that are just long enough

[rhagenson]

Produce names that are just long enough to cover all randomness. The necessary randomness would be enough to cover separating every known individual from every other known individual by --max-distance number of unknown individuals.

Characters needed  =ceiling( log(Number of possible unknown) / log(Number of possible characters) )

Number of possible unknown = maxDist * nKnown * (nKnown - 1) * 1/2

Number of possible characters = 26 = len("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

[rhagenson]

Because it is cheap and significantly reduces the possibility of random name clashes I would likely also add 1 to this value so there is always Number of possible characters * Characters needed possibilities.

[rhagenson]

gonum provides combinatorics calculations: https://godoc.org/gonum.org/v1/gonum/stat/combin#Binomial

[rhagenson]

Calculating the number of necessary characters to represent all possible unknowns with: 500 known individuals, all separated by a distance of 9, and 26 possible characters needs only 5 characters. How many are needed to represent 1000 known individuals, all separated by a distance of 9, and 26 possible characters?...5. 5^{26} = 1.49e18

The answer to having enough randomness is to just use 5 characters. Hand-rolling a way to produce 5 random characters also proved slow so I wen back to using xid, but I only take the final 6 (changing) characters. Why 6, because 6^{26} = 1.70e20 and I might as well add an additional character as it cheap to do so.