Implementation of an URL shortener using base62 encoding in Go

base62.go

package main

import (
    "fmt"
    "strings"
)

const alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

// Integer power: compute a**b using binary powering algorithm
// See Donald Knuth, The Art of Computer Programming, Volume 2, Section 4.6.3
func Pow(a, b int) int {
        p := 1
        for b > 0 {
                if b&1 != 0 {
                        p *= a
                }
                b >>= 1
                a *= a
        }
        return p
}

func base62Encode(num int) []byte {

    code := make([]byte, 0, 128)

    for num > 0 {
        rem := num % len(alphabet)
        num = num / len(alphabet)
        
        code = append(code, alphabet[rem])
    }

    for i, j := 0, len(code)-1; i < j; i, j = i+1, j-1 {
        code[i], code[j] = code[j], code[i]
    }

    return code
}

func base62Decode(code string) int {

    num := 0
    i := 0
    for _, char := range code {
        power := (len(code) - (i + 1))
        num = num + strings.Index(alphabet, string(char)) * Pow(len(alphabet), power)
        i++

    }

    return num
}