Chris Stewart - 1 year ago 81
Scala Question

# Scalacheck number generator between 0 <= x < 2^64

I'm trying to right a good number generator that covers

uint64_t
in C. Here is what I have so far.

def uInt64s : Gen[BigInt] = Gen.choose(0,64).map(pow2(_) - 1)

It is a good start, but it only generates numbers
2^n - 1
. Is there a more effective way to generate random BigInts while preserving the number range
0 <= n < 2^64
?

Okay, maybe I am missing something here, but isn't it as simple as this?

def uInt64s : Gen[BigInt] = Gen.chooseNum(Long.MinValue,Long.MaxValue)
.map(x => BigInt(x) + BigInt(2).pow(63))

Longs already have the correct number of bits - just adding 2^63 so Long.MinValue becomes 0 and Long.MaxValue becomes 2^64 - 1. And doing the addition with BigInts of course.

I was curious about the distribution of generated values. Apparently the distribution of chooseNum is not uniform, since it prefers special values, but the edge cases for Longs are probably also interesting for UInt64s:

/** Generates numbers within the given inclusive range, with
*  extra weight on zero, +/- unity, both extremities, and any special
*  numbers provided. The special numbers must lie within the given range,
*  otherwise they won't be included. */
def chooseNum[T](minT: T, maxT: T, specials: T*)(
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