反向转换整数以反向生成IP

Question

I wanted to generate IP in reverse order.

Functionality which generate IP address is as follow:

val := 1<<(32-prefixLen) // let take prefixLen as 24
IP4(val) = IP4(256) = 0.0.1.0

I wanted the IP in reverse order i.e., 0.1.0.0, so convert the val in reverse order bytes and send it to IP4() function.

Functionality which I have tried resulted in integer overflow:

temp1:=byte(val*0xff<<24)    
temp2:=byte(val*0xff00<<16)   
temp3:=byte(val*0xff0000<<8)
temp4:=byte(val*0xff000000)  
valReverse := uint32(temp4)|uint32(temp3)|uint32(temp2)|uint32(temp1)
IP4(valReverse)=0.1.0.0 //expected

Answer 1

Reversing bytes

First, you have to use uint32 type instead of byte as a byte value only has 8 bits (bit positions above 8 don't exist).

Next, you have to use bitwise AND which is & and not the multiplication *.

Third, the shift values are incorrect. See this working example:

prefixLen := uint32(24)

var val uint32
val = 1 << (32 - prefixLen)
fmt.Printf("%08x
", val)

temp1 := uint32(val & 0xff << 24)
temp2 := uint32(val & 0xff00 << 8)
temp3 := uint32(val & 0xff0000 >> 8)
temp4 := uint32(val & 0xff000000 >> 24)
valReverse := uint32(temp4) | uint32(temp3) | uint32(temp2) | uint32(temp1)

fmt.Printf("%08x
", valReverse)

Output (try it on the Go Playground):

00000100
00010000

But personally I would just use the net.IP type which models an IP address with a byte slice, and using that you can simply reverse the bytes with a slice-reversing algorithm.

This is how it would look like:

ip := net.IPv4(0, 0, 1, 0).To4()
fmt.Println(ip)

// Reverse:
for i, j := 0, len(ip)-1; i < j; i, j = i+1, j-1 {
    ip[i], ip[j] = ip[j], ip[i]
}
fmt.Println(ip)

Output (try it on the Go Playground):

0.0.1.0
0.1.0.0

Note: if you have the IP as a value of uint32, you can still use the net.IP type, creating the net.IP value like this:

ip := net.IPv4(byte(val>>24), byte(val>>16), byte(val>>8), byte(val)).To4()

Reversing bits

If you want to reverse bits, you have to do that bit-by-bit. One possible solution is this:

prefixLen := uint32(24)

var val uint32
val = 1 << (32 - prefixLen)
fmt.Printf("%08x
", val)

var valReverse uint32
for i := 0; i < 32; i, val = i+1, val>>1 {
    valReverse = valReverse<<1 | val&0x01
}
fmt.Printf("%08x
", valReverse)

Output (try it on the Go Playground):

00000100
00800000

An alternative, maybe more complex way to reverse the bits:

var valReverse uint32
for mask1, mask2 := uint32(0x01), uint32(0x80000000); mask2 != 0; mask1, mask2 = mask1<<1, mask2>>1 {
    if val&mask1 != 0 {
        valReverse |= mask2
    }
}

Output is the same, try this variant on the Go Playground.