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。
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
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