为什么Int.random()比arc4random_uniform()慢?

时间:2019-04-26 17:26:42

标签: swift random

我已经使用Int.random()方法和arc4random_uniform()进行了数字生成速度测试。
两项测试均在macOS控制台中运行,并将构建配置设置为release。 以下是我用于测试的代码。

public func randomGen1() {
    let n = 1_000_000
    let startTime = CFAbsoluteTimeGetCurrent()
    for i in 0..<n {
        _ = arc4random_uniform(10)
    }
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print(timeElapsed)
}
public func randomGen2() {
    let n = 1_000_000
    let startTime = CFAbsoluteTimeGetCurrent()
    for i in 0..<n {
        _ = Int.random(in: 0..<10)
    }
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print(timeElapsed)
}

我的时间是
0.029475092887878418(用于arc4random_uniform(10))
0.20298802852630615(用于Int.random(in:0 .. <10))

为什么Int.random()这么慢?
有没有优化的方法?
有没有更快的快速生成随机数的方法?

1 个答案:

答案 0 :(得分:2)

第二个问题的答案:

  

“有没有更快的快速生成随机数的方法?”

我已经previously used Xoshiro伪随机数生成器,它非常快。

这里是用于基准测试的代码:

  • randomGen1
import Foundation

public func randomGen1() {
    let n = 1_000_000
    var sum: UInt32 = 0
    let startTime = CFAbsoluteTimeGetCurrent()
    for _ in 0..<n {
        sum = sum &+ arc4random_uniform(10)
    }
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print(sum, timeElapsed)
}

do {
    randomGen1()
}
  • randomGen2
public func randomGen2() {
    let n = 1_000_000
    var sum: UInt32 = 0
    let startTime = CFAbsoluteTimeGetCurrent()
    for _ in 0..<n {
        sum = sum &+ UInt32.random(in: 0..<10)
    }
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print(sum, timeElapsed)
}


do {
    randomGen2()
}
struct Xoshiro: RandomNumberGenerator {
    public typealias StateType = (UInt32, UInt32, UInt32, UInt32)

    private var state: StateType

    public init(seed: StateType) {
        self.state = seed
    }

    public mutating func next() -> Int {
        let x = state.1 &* 5
        let result = ((x &<< 7) | (x &>> 25)) &* 9
        let t = state.1 &<< 9
        state.2 ^= state.0
        state.3 ^= state.1
        state.1 ^= state.2
        state.0 ^= state.3
        state.2 ^= t
        state.3 = (state.3 &<< 21) | (state.3 &>> 11)
        return Int(result)
    }
}

var x = Xoshiro(seed: (UInt32.random(in: 0..<10),  //Other upper limits could be used to increase randomness
    UInt32.random(in: 0..<10),
    UInt32.random(in: 0..<10),
    UInt32.random(in: 0..<10)))

public func randomGen3() {
    let n = 1_000_000
    var sum: UInt32 = 0
    let startTime = CFAbsoluteTimeGetCurrent()
    for _ in 0..<n {
        sum = sum &+ UInt32(abs(x.next()) % 10)
    }
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print(sum, timeElapsed)
}

do {
    randomGen3()
}

Xoshiro速度很快,但没有通过所有随机性测试。如果需要考虑安全性,则可以使用Wyhash

Daniel Lemirethis论文的作者)刚寄给我Swift implementation的Wyhash:

class WyhashGenerator {
    var seed : UInt64

    let multiplier1 : UInt64 = 0xa3b195354a39b70d
    let multiplier2 : UInt64 = 0x1b03738712fad5c9
    let increment : UInt64 = 0x60bee2bee120fc15

    init(userSeed : UInt64) {
        seed = userSeed;
    }

    func random() -> UInt64 {
        seed &+= increment
        let fullmult1 = seed.multipliedFullWidth(by: multiplier1)
        let m1 = fullmult1.high ^ fullmult1.low;
        let fullmult2 = m1.multipliedFullWidth(by: multiplier2)
        let m2 = fullmult2.high ^ fullmult2.low;
        return m2
    }
}

它可以像这样使用:

public func randomGen4() {
    let n = 1_000_000
    var sum: UInt64 = 0
    let startTime = CFAbsoluteTimeGetCurrent()
    let gen = WyhashGenerator(userSeed: 0)
    for _ in 0..<n {
        sum = sum &+ gen.random() % 10
    }
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print(sum, timeElapsed)
}

do {
    randomGen4()
}

这是基准测试结果,在终端中使用优化(-O)编译了代码:

arc4random_uniform()  : 0.034s
UInt32.random(in:)    : 0.243s
WyHash64              : 0.002s
Xoshiro               : 0.001s

您可以找到更多的随机数生成器here