Formatting big integers as hexadecimal digits separated by colons

Question

I'm pretty sure this can be written as a couple of lines, but I'm unable to come up with a better solution.

package main

import (
    "fmt"
    "math/big"
)

func colonedSerial(i *big.Int) string {
    hex := fmt.Sprintf("%x", i)
    if len(hex)%2 == 1 {
        hex = "0" + hex
    }
    numberOfColons := len(hex)/2 - 1
    colonedHex := make([]byte, len(hex)+numberOfColons)
    for i, j := 0, 0; i < len(hex)-1; i, j = i+1, j+1 {
        colonedHex[j] = hex[i]
        if i%2 == 1 {
            j++
            colonedHex[j] = []byte(":")[0]
        }
    }
    // we skipped the last one to avoid the colon at the end
    colonedHex[len(colonedHex)-1] = hex[len(hex)-1]
    return string(colonedHex)
}


func main() {
    fmt.Println(colonedSerial(big.NewInt(1234)))
    // "04:d2"
    fmt.Println(colonedSerial(big.NewInt(123456)))
    // "01:e2:40"
    fmt.Println(colonedSerial(big.NewInt(1234567891011121314)))
    // "11:22:10:f4:b2:d2:30:a2"
}

Also the []byte(":")[0] is very ugly. Is there a better way to do that? (I did not wanted to write the ASCII value of colon (58) as it would be a magic number).

Go Playground version: https://play.golang.org/p/GTxajcr3NY

Answer 1

In Go, we usually expect reasonable performance and brevity. For example,

package main

import (
    "encoding/hex"
    "fmt"
    "math/big"
)

func formatSerial(serial *big.Int) string {
    b := serial.Bytes()
    buf := make([]byte, 0, 3*len(b))
    x := buf[1*len(b) : 3*len(b)]
    hex.Encode(x, b)
    for i := 0; i < len(x); i += 2 {
        buf = append(buf, x[i], x[i+1], ':')
    }
    return string(buf[:len(buf)-1])
}

func main() {
    fmt.Println(formatSerial(big.NewInt(1234)))
    // "04:d2"
    fmt.Println(formatSerial(big.NewInt(123456)))
    // "01:e2:40"
    fmt.Println(formatSerial(big.NewInt(1234567891011121314)))
    // "11:22:10:f4:b2:d2:30:a2"
}

Output:

04:d2
01:e2:40
11:22:10:f4:b2:d2:30:a2

Benchmark: serial := big.NewInt(1234567891011121314):

BenchmarkPeterSO         3000000       552 ns/op        72 B/op      3 allocs/op
BenchmarkKissgyorgy       500000      2545 ns/op       120 B/op      7 allocs/op
Benchmark200Success        30000     40675 ns/op     40675 B/op     35 allocs/op

Answer 2

Since the task mainly involves inserting a colon after every two hex digits, I would treat it as a search-and-replace operation, using a regular expression.

import (
    "fmt"
    "math/big"
    "regexp"
    "strings"
)

func colonedSerial(i *big.Int) string {
    re := regexp.MustCompile("..")

    hex := fmt.Sprintf("%x", i)
    if len(hex)%2 == 1 {
        hex = "0" + hex
    }
    return strings.TrimRight(re.ReplaceAllString(hex, "$0:"), ":")
}