A key part of shortening URLs is creating a reasonable mapping from an ID (could be database entity) to a short string that redirects to a correct new url.

urlshorty.go

package main

import (
	"errors"
	"fmt"
	"strings"
)

const (
	Alphabet string = "23456789abcdefghijkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ"
	Base     int    = len(Alphabet)
)

/*
	Accepts string and decodes it into an ID for lookup
*/
func Decode(url string) (base62Number int, err error) {
	if url == "" {
		return -1, errors.New("empty string not allowed")
	}

	var sum = 0

	for _, c := range url {
		if strings.Contains(Alphabet, string(c)) {
			sum = sum*Base + strings.Index(Alphabet, string(c))
		} else {
			return -1, fmt.Errorf("unable to find character %c in Alphabet", c)
		}
	}

	return sum, nil
}

/*
	Accepts an integer key and encodes it into a string using alphabet
*/
func Encode(key int) (url string, err error) {
	if key == 0 {
		return string(Alphabet[0]), nil
	}

	var output = ""

	// as long as we have more data, collect
	for key > 0 {
		output = string(Alphabet[key%Base]) + output
		key /= Base
	}

	return output, nil
}

func main() {
	/*
		Overall idea, user A with original url B inserts into DB and we get an ID
		Use ID and Encode to get a string and return short version of url
		User B with a GET request for something/short_url
		System looks up short_url, gets ID
		System retrieves original URL from DB
		return original URL
	*/

	for i := 0; i < 1000; i++ {
		var encoded, _ = Encode(i)
		fmt.Println(encoded)

		var decoded, _ = Decode(encoded)

		if i != decoded {
			fmt.Printf("string comparison failed, string %d is not a match for %d\n", i, decoded)
		}
	}

}