如何在Swift中生成大范围的随机数？

Question

我正在寻找一种有效的方法来在Swift中生成具有任意范围（甚至可能是UInt.max或Int.max）的大数（包括浮点类型！）。

我见过的所有现有问题要么因大值（UInt.max）而崩溃，要么不支持范围。我知道您可以从中读取/dev/urandom随机字节，但这无助于将这些值限制为给定的时间间隔（而且我敢肯定，循环直到无效为止）。

Answer 1

这是的可能解决方案UInt，Int并且Double可以与所有这些类型一起使用。它被编写为扩展方法（现已针对Swift2进行了更新），但是对于全局函数也可以做到这一点。

请注意，仅arc4random_uniform()产生32位数字，因此如果Int/ UInt是64位整数（所有OS
X机器和所有较新的iOS设备都是这种情况），则不能使用该数字。

因为UInt我们使用的技术（这只是的快速翻译）。如果范围涵盖的整个范围，则将UInt单独处理。

extension UInt {
    static func random(minValue minValue : UInt, maxValue : UInt) -> UInt {
        precondition(minValue <= maxValue, "attempt to call random() with minValue > maxValue")

        if minValue == UInt.min && maxValue == UInt.max {
            // Random number in the full range of UInt:

            var rnd : UInt = 0
            arc4random_buf(&rnd, sizeofValue(rnd))
            return rnd
        } else {
            // Compute random number in the range 0 ... (maxValue-minValue),
            // using the technique from 
            // https://stackoverflow.com/a/26550169/1187415, https://stackoverflow.com/a/10989061/1187415
            // and avoiding the "modulo bias problem":

            let range = maxValue - minValue + 1
            let randLimit = UInt.max - UInt.max % range
            var rnd : UInt = 0
            repeat {
                arc4random_buf(&rnd, sizeofValue(rnd))
            } while rnd >= randLimit
            rnd = rnd % range

            // Transform `rnd` back to the range minValue ... maxValue:
            return minValue + rnd
        }
    }
}

例子：

let u1 = UInt.random(minValue: 1000, maxValue: 2000)
let u2 = UInt.random(minValue: UInt.min, maxValue: UInt.max)

使用溢出运算符和bitPattern:转换可以将有符号整数的情况减少为无符号情况：

extension Int {
    static func random(minValue minValue : Int, maxValue : Int) -> Int {
        precondition(minValue <= maxValue, "attempt to call random() with minValue > maxValue")

        // Compute unsigned random number in the range 0 ... (maxValue-minValue):
        let diff = UInt(bitPattern: maxValue &- minValue)
        let rnd = UInt.random(minValue: 0, maxValue: diff)

        // Transform `rnd` back to the range minValue ... maxValue:
        return minValue &+ Int(bitPattern: rnd)
    }
}

例子：

let i1 = Int.random(minValue: -1000, maxValue: 1000)
let i2 = Int.random(minValue: Int.min, maxValue: Int.max)

最后，一个简单的实现Double：

extension Double {
    static func random(minValue minValue : Double, maxValue : Double) -> Double {
        precondition(minValue <= maxValue, "attempt to call random() with minValue > maxValue")

        // Random floating point number in the range 0.0 ... 1.0:
        let rnd = Double(UInt.random(minValue: 0, maxValue: UInt.max))/Double(UInt.max)

        // Scale to range minValue ... maxValue:
        return minValue + rnd * (maxValue - minValue)
    }
}

例：

let d = Double.random(minValue: 10.5, maxValue: 123.5)

Swift 3更新：

extension UInt {
    static func random(minValue: UInt, maxValue: UInt) -> UInt {
        precondition(minValue <= maxValue, "attempt to call random() with minValue > maxValue")

        if minValue == UInt.min && maxValue == UInt.max {
            // Random number in the full range of UInt:

            var rnd: UInt = 0
            arc4random_buf(&rnd, MemoryLayout.size(ofValue: rnd))
            return rnd
        } else {
            // Compute random number in the range 0 ... (maxValue-minValue),
            // using the technique from 
            // https://stackoverflow.com/a/26550169/1187415, https://stackoverflow.com/a/10989061/1187415
            // and avoiding the "modulo bias problem":

            let range = maxValue - minValue + 1
            let randLimit = UInt.max - UInt.max % range
            var rnd: UInt = 0
            repeat {
                arc4random_buf(&rnd, MemoryLayout.size(ofValue: rnd))
            } while rnd >= randLimit
            rnd = rnd % range

            // Transform `rnd` back to the range minValue ... maxValue:
            return minValue + rnd
        }
    }
}

extension Int {
    static func random(minValue: Int, maxValue: Int) -> Int {
        precondition(minValue <= maxValue, "attempt to call random() with minValue > maxValue")

        // Compute unsigned random number in the range 0 ... (maxValue-minValue):
        let diff = UInt(bitPattern: maxValue &- minValue)
        let rnd = UInt.random(minValue: 0, maxValue: diff)

        // Transform `rnd` back to the range minValue ... maxValue:
        return minValue &+ Int(bitPattern: rnd)
    }
}

extension Double {
    static func random(minValue: Double, maxValue: Double) -> Double {
        precondition(minValue <= maxValue, "attempt to call random() with minValue > maxValue")

        // Random floating point number in the range 0.0 ... 1.0:
        let rnd = Double(UInt.random(minValue: 0, maxValue: UInt.max))/Double(UInt.max)

        // Scale to range minValue ... maxValue:
        return minValue + rnd * (maxValue - minValue)
    }
}