make a long url to short. It just allocate a ID for a URL, then convert the id to 62-base ID. Then format new id to string with kBase62CharMap.

shorturl.go

package shorturl

import (
    "math"
)

const kMinShortUrlLength = 6

var kBase62CharMap = []rune{
	'0', '1', '2', '3', '4', '5', '6',
	'7', '8', '9', 'A', 'B', 'C', 'D',
	'E', 'F', 'G', 'H', 'I', 'J', 'K',
	'L', 'M', 'N', 'O', 'P', 'Q', 'R',
	'S', 'T', 'U', 'V', 'W', 'X', 'Y',
	'Z', 'a', 'b', 'c', 'd', 'e', 'f',
	'g', 'h', 'i', 'j', 'k', 'l', 'm',
	'n', 'o', 'p', 'q', 'r', 's', 't',
	'u', 'v', 'w', 'x', 'y', 'z',
}

func convert_to_base62(id uint64) []byte {
	var quotient uint64 = id
	var remainder uint64
	var r []byte
	for quotient != 0 {
		quotient, remainder = quotient/62, quotient%62
		r = append(r, byte(remainder))
	}
	return r
}

func convert_from_base62(r string) uint64 {
	var id uint64
	var num_index uint64
	for _, v := range r {
		var i uint64
		if v >= '0' && v <= '9' {
			i = uint64(v - '0')
		} else if v >= 'A' && v <= 'Z' {
			i = uint64(v-'A') + 10
		} else if v >= 'a' && v <= 'z' {
			i = uint64(v-'a') + 36
		}
		id += i * uint64(math.Pow(float64(62), float64(num_index)))
		num_index++
	}
	return id
}

func generateShortUrl(id uint64) string {
	v := convert_to_base62(id)
	if i := kMinShortUrlLength - len(v); i > 0 {
		for j := 0; j < i; j++ {
			v = append(v, byte(0))
		}
	}
	var s []rune
	for _, r := range v {
		s = append(s, kBase62CharMap[r])
	}
	return string(s)
}

func Generate(url string, allocateId func(url string) (id uint64, err error)) string {
	id, err := allocateId(url)
	if err != nil {
		return ""
	}
	s := generateShortUrl(id)
	return s
}

func Query(short string, reserveQuery func(id uint64) (l string, err error)) string {
	id := convert_from_base62(short)
	s, err := reserveQuery(id)
	if err != nil {
		return ""
	}
	return s
}