struct SplitMix64 : RandomNumberGenerator {
    public typealias State = UInt64
    public private(set) var state: State

    init(state: UInt64) {
        self.state = state
    }

    public mutating func next() -> UInt64 {
        state &+= 0x9E3779B97F4A7C15
        var z = state
        z = (z ^ (z >> 30)) &* 0xBF58476D1CE4E5B9
        z = (z ^ (z >> 27)) &* 0x94D049BB133111EB
        return z ^ (z >> 31)
    }

    public mutating func nextDouble() -> Double {
        Double(next() >> 11) * 0x1.0p-53
    }
}