swift2 - 如何从 CollectionType 中获取随机元素？

Question

这是我的示例代码

import Foundation // ar4random_uniform

extension CollectionType where Self.Index.Distance == Int {
    var randomElement: Self.Generator.Element? {
        if !isEmpty {
            let i = Int(arc4random_uniform(UInt32(count)))
            return self[startIndex.advancedBy(i)]
        } else {
            return nil
        }
    }
}

let arr = ["alfa", "beta","gama"]
let view = "abcdefghijklmnopqrstuvwxyz".characters
let dict:Dictionary<Int,Any> = [1:"a",2:true,3:1]
let range = Range<Int>(start: -99, end: 100)
let b = UnsafeMutablePointer<Int>.alloc(10)
for i in 0..<10 { (b+i).memory = 1000*i }
let buff = UnsafeMutableBufferPointer<Int>(start: b, count: 10)
for i in 0...10 {
    print(arr.randomElement!,
        view.randomElement!,
        dict.randomElement!,
        range.randomElement!,
        buff.randomElement!)
}

/*
gama z (1, "a") 62 2000
alfa i (1, "a") 4 9000
alfa f (1, "a") 14 6000
gama g (1, "a") 68 8000
beta a (2, true) 42 7000
alfa y (1, "a") -76 1000
alfa i (1, "a") 76 4000
gama f (2, true) 79 3000
beta s (2, true) 46 3000
beta l (1, "a") -17 9000
gama q (3, 1) 62 9000
*/

代码按预期工作。

问题是，Swift 的哪个内置类型符合 CollectionType 协议(protocol)，其中 Self.Index.Distance != Int 以及如何在这种情况下实现相同的行为？

Answer 1

typealias Distance : _SignedIntegerType = Int

因此每个 Index.Distance 都必须符合 _SignedIntegerType。每个 _SignedIntegerType 都 promise 与 IntMax(最宽的可用整数)之间的转换:

init(_: IntMax)
func toIntMax() -> IntMax

将这两个事实放在一起，我们可以计算出一个随机索引(最多 2³¹-1 个元素；对于大于此的集合，这将崩溃):

let randomDistance = Index.Distance(arc4random_uniform(UInt32(count.toIntMax())).toIntMax())
let randomIndex = startIndex.advancedBy(randomDistance)
return self[randomIndex]