package main

import (
	"crypto/rand"
	"fmt"
	"io/ioutil"
	"net"
	"net/http"
	"net/url"
	"os"
	"os/exec"
)

func main() {
	oa := OAuthFlow{
		ClientID:     os.Getenv("GH_OAUTH_CLIENT_ID"),
		ClientSecret: os.Getenv("GH_OAUTH_CLIENT_SECRET"),
	}
	token, err := oa.ObtainAccessToken()
	if err != nil {
		panic(err)
	}
	fmt.Printf("OAuth access token: %s\n", token)
}

func randomString(length int) (string, error) {
	b := make([]byte, length/2)
	_, err := rand.Read(b)
	if err != nil {
		return "", err
	}
	return fmt.Sprintf("%x", b), nil
}

type OAuthFlow struct {
	ClientID     string
	ClientSecret string
}

func (oa *OAuthFlow) ObtainAccessToken() (accessToken string, err error) {
	state, _ := randomString(20)

	code := ""
	listener, err := net.Listen("tcp", "localhost:0")
	if err != nil {
		return
	}
	port := listener.Addr().(*net.TCPAddr).Port

	q := url.Values{}
	q.Set("client_id", oa.ClientID)
	q.Set("redirect_uri", fmt.Sprintf("http://localhost:%d", port))
	q.Set("scope", "repo")
	q.Set("state", state)

	cmd := exec.Command("open", fmt.Sprintf("https://github.com/login/oauth/authorize?%s", q.Encode()))
	err = cmd.Run()
	if err != nil {
		return
	}

	http.Serve(listener, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		defer listener.Close()
		rq := r.URL.Query()
		if state != rq.Get("state") {
			fmt.Fprintf(w, "Error: state mismatch")
			return
		}
		code = rq.Get("code")
		w.Header().Add("content-type", "text/html")
		fmt.Fprintf(w, "<p>You have authenticated <strong>GitHub CLI</strong>. You may now close this page.</p>")
	}))

	resp, err := http.PostForm("https://github.com/login/oauth/access_token",
		url.Values{
			"client_id":     {oa.ClientID},
			"client_secret": {oa.ClientSecret},
			"code":          {code},
			"state":         {state},
		})
	if err != nil {
		return
	}

	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return
	}
	tokenValues, err := url.ParseQuery(string(body))
	if err != nil {
		return
	}
	accessToken = tokenValues.Get("access_token")
	return
}