Uniformly distributed random numbers for @CocoawithLove's Xoroshiro implementation

CwlRandom+NWA.swift

//
//  CwlRandom+NWA.swift
//  Butterfly
//
//  Created by Bruno Scheele on 10/07/16.
//  Copyright © 2016 Noodlewerk Apps V.o.f. All rights reserved.
//

import Foundation

extension Xoroshiro {
    /// Calculates a uniform random number less than *upperBound* iff *upperBound* > 0. Else, returns 0.
    ///
    /// - parameter upperBound: The upper bound of the random number.
    /// - returns: A random number between 0 and *upperbound*.
    /// - note: Adapted from arc4random_uniform to avoid 'modulo bias'.
    public mutating func randomUniform(upperbound ub: UInt64) -> UInt64 {
        let minimum = UInt64.max % ub
        var result: UInt64!
        repeat {
            result = randomWord()
        } while result <= minimum
        return result % ub
    }
}