Swift：64位整数的随机数？ [英] Swift: Random number for 64-bit integers?

问题描述

所以，在我目前的项目中，我需要使用64位整数，我需要在高达1000亿的范围内获取随机数。 arc4random（）/ arc4random_uniform（）仅适用于无符号32位整数。

So, with my current project, I need to work with 64-bit integers and I need to grab random numbers between ranges up to 100 billion. arc4random()/arc4random_uniform() only works with unsigned 32-bit integers.

我可能会稍微捏一下因为我的每次调用的最小/最大范围可能不会超过20亿，但我想在未来的情况下保护自己，以防我决定，我确实需要更广泛的范围。

I can probably fudge it a little because my min/max range for every call will likely not exceed 2 billion, but I'd like to futureproof myself in case I decide that, well, I do need a broader range.

有什么建议吗？

推荐答案

更新：截至 Swift 4.2 （目前处于测试阶段）有一个统一的随机Swift标准库中的API，参见

Update: As of Swift 4.2 (currently in beta) there is a unified random API in the Swift standard library, see

• SE 0202随机统一。

• SE 0202 Random Unification.

使用最近的Swift 4.2开发人员快照，您只需调用

With a recent Swift 4.2 developer snapshot you can simply call

UInt64.random(in: minValue ... maxValue)

获取给定范围内的随机数。

to get a random number in the given range.

（上一个答案f或斯威夫特< 4.2：）使用 arc4random_buf（）你可以创建任意大的随机数，
所以这将是一个可能的解决方案：

(Previous answer for Swift < 4.2:) With arc4random_buf() you can create "arbitrary large" random numbers, so this would be a possible solution:

// Swift 2:
func random64(upper_bound: UInt64) -> UInt64 {

    // Generate 64-bit random number:
    var rnd : UInt64 = 0
    arc4random_buf(&rnd, sizeofValue(rnd))

    return rnd % upper_bound
}

// Swift 3:
func random64(upper_bound: UInt64) -> UInt64 {

    // Generate 64-bit random number:
    var rnd : UInt64 = 0
    arc4random_buf(&rnd, MemoryLayout.size(ofValue: rnd))

    return rnd % upper_bound
}

此方法受到模数偏差当上限不是2的幂时出现问题（参见为什么人们说使用随机数生成器时存在模偏差？）。在这里，我从上面的帖子中翻译了答案
https://stackoverflow.com/a/10989061/1187415 到Swift：

This method suffers from the "modulo bias" problem when the upper bound is not a power of 2 (See Why do people say there is modulo bias when using a random number generator?). Here I have translated the answer https://stackoverflow.com/a/10989061/1187415 from above thread to Swift:

// Swift 2:
func random64(upper_bound: UInt64) -> UInt64 {

    // Generate 64-bit random value in a range that is
    // divisible by upper_bound:
    let range = UInt64.max - UInt64.max % upper_bound
    var rnd : UInt64 = 0
    repeat {
        arc4random_buf(&rnd, sizeofValue(rnd))
    } while rnd >= range

    return rnd % upper_bound
}

// Swift 3:
func random64(upper_bound: UInt64) -> UInt64 {

    // Generate 64-bit random value in a range that is
    // divisible by upper_bound:
    let range = UInt64.max - UInt64.max % upper_bound
    var rnd : UInt64 = 0
    repeat {
        arc4random_buf(&rnd, MemoryLayout.size(ofValue: rnd))
    } while rnd >= range

    return rnd % upper_bound
}

（乍一看，它看起来好像是循环可能不会终止，但可以显示
，平均需要少于2次迭代。）

(At first sight it looks as if the loop might not terminate, but it can be shown that on average less than 2 iterations are needed.)

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