main.go

package main

import (
	"fmt"
	"log"
	"net/http"
	"sync"

	"github.com/go-chi/chi/v5"
	"github.com/go-chi/chi/v5/middleware"
	"github.com/lithammer/shortuuid/v4"
)

type Mapper struct {
	Mapping map[string]string
	Lock    sync.Mutex
}

var urlMapper Mapper

func init() {
	// intialise mapper
	urlMapper = Mapper{
		Mapping: make(map[string]string),
	}
}

func main() {
	r := chi.NewRouter()

	r.Use(middleware.Logger)
	r.Get("/", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("server is running..."))
	})

	r.Post("/short-it", createShortUrlHandler)
	r.Get("/short/{key}", redirectHandler)

	http.ListenAndServe(":3000", r)

}

func createShortUrlHandler(w http.ResponseWriter, r *http.Request) {
	r.ParseForm()
	u := r.Form.Get("URL")
	if u == "" {
		w.WriteHeader(http.StatusBadRequest)
		w.Write([]byte("URL cannot be empty"))
		return
	}

	// generate key
	key := shortuuid.New()

	// insert
	insertMapping(key, u)

	log.Printf("shortened URL: %s\n", key)

	w.WriteHeader(http.StatusOK)
	w.Write([]byte(fmt.Sprintf("http://localhost:3000/short/%s", key)))
}

func insertMapping(key string, u string) {
	urlMapper.Lock.Lock()
	defer urlMapper.Lock.Unlock()

	urlMapper.Mapping[key] = u
}

func redirectHandler(w http.ResponseWriter, r *http.Request) {
	key := chi.URLParam(r, "key")
	if key == "" {
		w.WriteHeader(http.StatusBadRequest)
		w.Write([]byte("key cannot be empty"))
		return
	}

	// fetch mapping

	u := fetchMapping(key)
	if u == "" {
		w.WriteHeader(http.StatusBadRequest)
		w.Write([]byte("url cannot be empty"))
		return 
	}

	http.Redirect(w, r, u, http.StatusFound)
}

func fetchMapping(key string) string {
	urlMapper.Lock.Lock()
	defer urlMapper.Lock.Unlock()

	return urlMapper.Mapping[key]
}