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

转载 作者:搜寻专家 更新时间:2023-10-31 08:03:38 26 4
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我已经使用 Int.random() 方法和 arc4random_uniform() 进行数字生成速度测试。
这两个测试都在 macOS 控制台中运行,构建配置设置为发布。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)
}

我得到的时间是
0.029475092887878418(对于 arc4random_uniform(10))
0.20298802852630615(对于 Int.random(in: 0..<10))

为什么 Int.random() 这么慢?
有什么办法可以优化吗?
在 swift 中有没有更快的随机数生成方法?

最佳答案

更新

This区间内随机数生成器的实现已合并到标准库中,性能应该比以前更好:

// s = upperBound; r1, r2 = random numbers from generator
func bounded(s: UInt64, r1:UInt64, r2: UInt64) -> UInt64 {
// r1 would come from invoking generator's next()
var m = r1.multipliedFullWidth(by: s)
if m.low < s {
// let t = (0 &- s) % s // Lemire's original form
var t = 0 &- s // O'Neill's modulo optimization
if t >= s {
t &-= s
if t >= s {
t %= s
}
}
while m.low < t {
// r2 would come from invoking generator's next()
m = r2.multipliedFullWidth(by: s)
}
}
return m.high
}

有关详细信息,请参阅下面的答案。

回答

第二个问题的答案:

"Are there any faster methods for random number generation in swift?"

我已经 previously used Xoshiro非常快的伪随机数生成器。

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。如果担心安全问题,那么您可以使用 Wyhash .

Daniel Lemire (this 论文的作者)刚刚给我发了一份 Swift implementation怀哈希的:

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
}
}

可以这样使用:

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()
}

下面是基准测试结果,代码在终端中编译并进行了优化 (-O):

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

您可以找到更多随机数生成器 here .

关于swift - 为什么 Int.random() 比 arc4random_uniform() 慢?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/55872415/

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