package main

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"encoding/json"
	"errors"
	"net/http"
	"net/url"
	"os"
	"os/exec"
	"strings"
	"time"
)

// GitHub App user authorization: the browser is sent to GitHub, the callback exchanges the
// code for a user token held in memory against the session, and clones use that token.
type githubApp struct {
	clientID, clientSecret string
	slug                   string // optional; enables the "grant access to more repositories" link
	webURL, apiURL         string // github.com endpoints, overridden in tests
}

type githubAuth struct {
	token        string
	login        string
	expires      time.Time
	state        string
	stateExpires time.Time
}

var githubClient = &http.Client{Timeout: 15 * time.Second}

func githubAppFromEnv() githubApp {
	return githubApp{
		clientID:     os.Getenv("GITHUB_CLIENT_ID"),
		clientSecret: os.Getenv("GITHUB_CLIENT_SECRET"),
		slug:         os.Getenv("GITHUB_APP_SLUG"),
		webURL:       "https://github.com",
		apiURL:       "https://api.github.com",
	}
}

func (g githubApp) configured() bool { return g.clientID != "" && g.clientSecret != "" }

func (g githubApp) installURL() string {
	if g.slug == "" {
		return ""
	}
	return g.webURL + "/apps/" + url.PathEscape(g.slug) + "/installations/new"
}

func (s *Server) githubRoute(w http.ResponseWriter, r *http.Request, action string) {
	switch {
	case action == "status" && r.Method == http.MethodGet:
		auth := s.githubAuthFor(sessionID(nil, r))
		writeJSON(w, 200, map[string]any{
			"configured": s.github.configured(),
			"connected":  auth.token != "",
			"login":      auth.login,
			"installUrl": s.github.installURL(),
		})
	case !s.github.configured():
		writeError(w, 404, "GitHub sign-in is not configured")
	case action == "login" && r.Method == http.MethodGet:
		sid := sessionID(w, r)
		state := rand.Text()
		s.authMu.Lock()
		auth := s.auth[sid]
		if auth == nil {
			auth = &githubAuth{}
			s.auth[sid] = auth
		}
		auth.state, auth.stateExpires = state, time.Now().Add(10*time.Minute)
		s.authMu.Unlock()
		query := url.Values{"client_id": {s.github.clientID}, "state": {state}}
		http.Redirect(w, r, s.github.webURL+"/login/oauth/authorize?"+query.Encode(), http.StatusFound)
	case action == "callback" && r.Method == http.MethodGet:
		http.Redirect(w, r, "/?github="+s.githubCallback(r), http.StatusFound)
	case action == "logout" && r.Method == http.MethodPost:
		s.authMu.Lock()
		delete(s.auth, sessionID(nil, r))
		s.authMu.Unlock()
		writeJSON(w, 200, map[string]bool{"ok": true})
	default:
		writeError(w, 404, "Unknown GitHub route")
	}
}

// githubCallback completes sign-in and returns the outcome shown to the frontend.
func (s *Server) githubCallback(r *http.Request) string {
	sid, query := sessionID(nil, r), r.URL.Query()
	s.authMu.Lock()
	auth := s.auth[sid]
	valid := auth != nil && auth.state != "" && time.Now().Before(auth.stateExpires) &&
		subtle.ConstantTimeCompare([]byte(auth.state), []byte(query.Get("state"))) == 1
	if auth != nil {
		auth.state = "" // single use
	}
	s.authMu.Unlock()
	if !valid {
		return "error"
	}
	if query.Get("code") == "" {
		return "denied"
	}

	token, lifetime, err := s.github.exchangeCode(query.Get("code"))
	if err != nil {
		return "error"
	}
	login, err := s.github.userLogin(token)
	if err != nil {
		return "error"
	}
	s.authMu.Lock()
	s.auth[sid] = &githubAuth{token: token, login: login, expires: time.Now().Add(lifetime)}
	s.authMu.Unlock()
	return "connected"
}

func (g githubApp) exchangeCode(code string) (string, time.Duration, error) {
	form := url.Values{"client_id": {g.clientID}, "client_secret": {g.clientSecret}, "code": {code}}
	req, _ := http.NewRequest("POST", g.webURL+"/login/oauth/access_token", strings.NewReader(form.Encode()))
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Set("Accept", "application/json")
	res, err := githubClient.Do(req)
	if err != nil {
		return "", 0, err
	}
	defer res.Body.Close()
	var body struct {
		AccessToken string `json:"access_token"`
		ExpiresIn   int    `json:"expires_in"`
		Error       string `json:"error"`
	}
	if err := json.NewDecoder(res.Body).Decode(&body); err != nil {
		return "", 0, err
	}
	if body.AccessToken == "" {
		return "", 0, errors.New("token exchange failed: " + body.Error)
	}
	lifetime := time.Duration(body.ExpiresIn) * time.Second
	if lifetime <= 0 {
		lifetime = 8 * time.Hour // apps with token expiry disabled; still forget it eventually
	}
	return body.AccessToken, lifetime, nil
}

func (g githubApp) userLogin(token string) (string, error) {
	req, _ := http.NewRequest("GET", g.apiURL+"/user", nil)
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Accept", "application/vnd.github+json")
	req.Header.Set("X-GitHub-Api-Version", "2022-11-28")
	res, err := githubClient.Do(req)
	if err != nil {
		return "", err
	}
	defer res.Body.Close()
	var body struct{ Login string }
	if res.StatusCode != 200 || json.NewDecoder(res.Body).Decode(&body) != nil || body.Login == "" {
		return "", errors.New("could not read GitHub user")
	}
	return body.Login, nil
}

// githubAuthFor returns a copy of the session's GitHub sign-in; token is "" when absent or expired.
func (s *Server) githubAuthFor(sid string) githubAuth {
	s.authMu.Lock()
	defer s.authMu.Unlock()
	auth := s.auth[sid]
	if sid == "" || auth == nil || auth.token == "" || time.Now().After(auth.expires) {
		return githubAuth{}
	}
	return *auth
}

// gitClone builds the clone command. The token travels in environment-provided git config,
// never in the URL or arguments, so it cannot leak through process lists or error output.
func gitClone(repoURL, dest, token string) *exec.Cmd {
	cmd := exec.Command("git", "clone", "--depth=1", "--single-branch", "--quiet", "--", repoURL, dest)
	// Ignore the host's git config and credential helpers: only the visitor's own token may
	// ever authenticate a clone.
	cmd.Env = append(os.Environ(), "GIT_TERMINAL_PROMPT=0", "GIT_CONFIG_NOSYSTEM=1", "GIT_CONFIG_GLOBAL=/dev/null",
		"GIT_CONFIG_COUNT=1", "GIT_CONFIG_KEY_0=credential.helper", "GIT_CONFIG_VALUE_0=")
	if token != "" {
		basic := base64.StdEncoding.EncodeToString([]byte("x-access-token:" + token))
		cmd.Env = append(cmd.Env,
			"GIT_CONFIG_COUNT=2",
			"GIT_CONFIG_KEY_1=http.https://github.com/.extraHeader",
			"GIT_CONFIG_VALUE_1=Authorization: Basic "+basic)
	}
	return cmd
}

// cloneFailure turns git output into a user-facing message and whether signing in (or
// granting the app access) could fix it.
func (g githubApp) cloneFailure(output string, signedIn bool) (message string, authRequired string) {
	lower := strings.ToLower(output)
	needsAuth := false
	for _, hint := range []string{"could not read username", "authentication failed", "repository not found", "terminal prompts disabled", "returned error: 403", "returned error: 401"} {
		needsAuth = needsAuth || strings.Contains(lower, hint)
	}
	switch {
	case !needsAuth:
		return "git clone failed: " + strings.TrimSpace(output), ""
	case !g.configured():
		return "Repository not found. It may be private; load it with Open local folder instead.", ""
	case !signedIn:
		return "This repository is private or does not exist. Sign in with GitHub to open private repositories.", "signin"
	default:
		return "Your GitHub account cannot open this repository through CodeNavigator. Grant the app access to it, then try again.", "install"
	}
}