如何将UInt64转换为Int64？

Question

尝试在Swift中调用dispatch_time实在令人难以理解，原因如下：

 dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC), dispatch_get_main_queue(), {
            doSomething()
            })

结果为错误：“找不到接受提供的参数的’*’重载”。

NSEC_PER_SEC是UInt64，因此可以进行一些实验：

let x:UInt64 = 1000
let m:Int64 = 10 * x

结果与上述相同

let x:UInt64 = 1000
let m:Int64 = 10 * (Int64) x

“一行上的连续语句中的结果必须用’;’分隔”

let x:UInt64 = 1000
let m:Int64 = 10 * ((Int64) x)

结果为“预期的’，’分隔符”

let x:UInt64 = 1000
let m:Int64 = (Int64)10 * (Int64) x

“一行上的连续语句中的结果必须用’;’分隔”

等等

该死的Swift编译器，我放弃了。如何将UInt64强制转换为Int64，和/或如何快速使用dispatch_time？

Answer 1

将a强制转换UInt64为an Int64是不安全的，因为a UInt64可以具有大于的数字Int64.max，这将导致溢出。

以下是将转换UInt64为Int64，反之亦然的代码段：

// Extension for 64-bit integer signed <-> unsigned conversion

extension Int64 {
    var unsigned: UInt64 {
        let valuePointer = UnsafeMutablePointer<Int64>.allocate(capacity: 1)
        defer {
            valuePointer.deallocate(capacity: 1)
        }

        valuePointer.pointee = self

        return valuePointer.withMemoryRebound(to: UInt64.self, capacity: 1) { $0.pointee }
    }
}

extension UInt64 {
    var signed: Int64 {
        let valuePointer = UnsafeMutablePointer<UInt64>.allocate(capacity: 1)
        defer {
            valuePointer.deallocate(capacity: 1)
        }

        valuePointer.pointee = self

        return valuePointer.withMemoryRebound(to: Int64.self, capacity: 1) { $0.pointee }
    }
}

这只是将二进制数据解释UInt64为Int64，即Int64.max，由于64位整数的最高有效位处的符号位，其数字将大于负数。

如果只需要正整数，则获取绝对值。

编辑： 根据行为，您可以获取绝对值，或：

if currentValue < 0 {
    return Int64.max + currentValue + 1
} else {
    return currentValue
}

后一种选项类似于剥离符号位。例如：

// Using an 8-bit integer for simplicity

// currentValue
0b1111_1111 // If this is interpreted as Int8, this is -1.

// Strip sign bit
0b0111_1111 // As Int8, this is 127. To get this we can add Int8.max

// Int8.max + currentValue + 1
127 + (-1) + 1 = 127