import Security
import QuartzCore

//-----------------------------------------------------------------------------
// The static property .ulpOfOne is unnecessarily slow (as it seems to perform
// actual function calls to Double(1).ulp on every access). The test() func
// below is almost 5 times faster when setting usingMyUlpOfOne to true.
//-----------------------------------------------------------------------------
let usingMyUlpOfOne = false

extension Double {
    static let myUlpOfOne = Double.ulpOfOne
    init(unitRange i: UInt64) {
        let uoo = usingMyUlpOfOne ? Double.myUlpOfOne : Double.ulpOfOne
        self = Double(i >> Double.exponentBitCount) * (uoo / 2.0)
    }
}

func unsafeRandomBitPatternElements<T>(count: Int) -> [T]
    where T: ExpressibleByIntegerLiteral
{
    var a = [T](repeating: 0, count: count)
    let r = SecRandomCopyBytes(nil, count * MemoryLayout<T>.stride, &a)
    precondition(r == errSecSuccess)
    return a
}

func test() {
    for _ in 0 ..< 10 {
        var sum = Double(0)
        let a: [UInt64] = unsafeRandomBitPatternElements(count: 100_000_000)
        let t0 = CACurrentMediaTime()
        for e in a { sum += Double(unitRange: e) }
        let t1 = CACurrentMediaTime()
        print("time:", t1 - t0, "seconds, ( sum:", sum, ")")
    }
}

print("usingMyUlpOfOne == \(usingMyUlpOfOne):")
test()

//-----------------------------------------------------------------------------
// Example output when run on my MBP late 2013, dev snapshot 2018-01-30:
//-----------------------------------------------------------------------------
//
// usingMyUlpOfOne == false:
// time: 0.506870910001453 seconds, ( sum: 49998178.5861357 )
// time: 0.508704126987141 seconds, ( sum: 50000915.5541131 )
// time: 0.48335500099347 seconds, ( sum: 50002202.6118007 )
// time: 0.490296787000261 seconds, ( sum: 49997649.5106063 )
// time: 0.502311271004146 seconds, ( sum: 49998730.1718012 )
// time: 0.487179237010423 seconds, ( sum: 50001758.3728186 )
// time: 0.530522076995112 seconds, ( sum: 49998896.5738505 )
// time: 0.524156589992344 seconds, ( sum: 50002229.7349948 )
// time: 0.515008457994554 seconds, ( sum: 50001545.8855311 )
// time: 0.50125320898951 seconds, ( sum: 49999437.9996973 )
//
//-----------------------------------------------------------------------------
//
// usingMyUlpOfOne == true:
// time: 0.122320255992236 seconds, ( sum: 49998913.4932828 )
// time: 0.113033984991489 seconds, ( sum: 50000117.6091672 )
// time: 0.111723182984861 seconds, ( sum: 50002081.104568 )
// time: 0.111441375978757 seconds, ( sum: 49997609.1379723 )
// time: 0.111161193985026 seconds, ( sum: 49998541.0318086 )
// time: 0.121517223014962 seconds, ( sum: 49994113.287431 )
// time: 0.116348965995712 seconds, ( sum: 50003603.4877935 )
// time: 0.112562206981238 seconds, ( sum: 49998787.9100643 )
// time: 0.116928763978649 seconds, ( sum: 50003593.6316356 )
// time: 0.111437907005893 seconds, ( sum: 49999608.8038312 )
//
//-----------------------------------------------------------------------------