golang how can I convert uint64 to int64? [duplicate]

Question

anyone can help me? converting uint64 to int64 pls

//fmt.Println(int64(18446744073709551615)) 
//constant 18446744073709551615 overflows int64 

var x uint64 = 18446744073709551615
var y int64 = int64(x)
fmt.Println(y) //-1 

//just like(c)signed long long
//anyone can help me pls!
//How can I using like this?

// -9223372036854775808 +9223372036854775807

func BytesToInt(b []byte) int {
    bytesBuffer := bytes.NewBuffer(b)
    var tmp int32
    binary.Read(bytesBuffer, binary.BigEndian, &tmp)
    return int(tmp)
}

Answer 1

What you are asking (to store 18,446,744,073,709,551,615 as an int64 value) is impossible.

A unit64 stores positive integers and has 64 bits available to hold information. It can therefore store any positive integer between 0 and 18,446,744,073,709,551,615 (2^64-1).

An int64 uses one bit to hold the sign, leaving 63 bits to hold information about the number. It can store any value between -9,223,372,036,854,775,808 and +9,223,372,036,854,775,807 (-2^63 and 2^63-1).

Both types can hold 18,446,744,073,709,551,616 unique integers, it is just that the uint64 range starts at zero, where as the int64 values straddle zero.

To hold 18,446,744,073,709,551,615 as a signed integer would require 65 bits.

In your conversion, no information from the underlying bytes is lost. The difference in the integer values returned is due to how the the two types interpret and display the values.

uint64 will display a raw integer value, whereas int64 will use two's complement.

var x uint64 = 18446744073709551615
var y int64 = int64(x)

fmt.Printf("uint64: %v = %#[1]x, int64: %v = %#x\n", x, y, uint64(y))
// uint64: 18446744073709551615 = 0xffffffffffffffff
// int64:                    -1 = 0xffffffffffffffff


x -= 100
y -= 100
fmt.Printf("uint64: %v = %#[1]x, int64: %v = %#x\n", x, y, uint64(y))
// uint64: 18446744073709551515 = 0xffffffffffffff9b
// int64:                  -101 = 0xffffffffffffff9b 

https://play.golang.com/p/hlWqhnC9Dh