package fikenauth

import (
	"context"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"fmt"
	"html"
	"io"
	"net"
	"net/http"
	"net/url"
	"strings"
	"time"
)

// loginTimeout bounds how long the local server waits for the browser. Long
// enough to find a password, short enough that a forgotten terminal frees the
// port again.
const loginTimeout = 5 * time.Minute

// shutdownGrace is how long the local server is given to finish writing the
// page the browser is waiting on.
const shutdownGrace = 5 * time.Second

// NewState returns an unguessable value to tie the redirect back to this run.
func NewState() (string, error) {
	buf := make([]byte, 32)
	if _, err := rand.Read(buf); err != nil {
		return "", fmt.Errorf("generate an OAuth state: %w", err)
	}
	return base64.RawURLEncoding.EncodeToString(buf), nil
}

// callback is what the browser came back with.
type callback struct {
	code  string
	state string
	err   error
}

// Login runs the loopback flow: a local server listens on the redirect URL's
// port, announce is called with the URL to visit, and the token is exchanged
// once the browser returns.
//
// The redirect URL must be registered on the Fiken app exactly as configured.
func (c Config) Login(ctx context.Context, announce func(authURL string)) (Token, error) {
	if err := c.Validate(); err != nil {
		return Token{}, err
	}

	redirect, err := url.Parse(c.redirectURL())
	if err != nil {
		return Token{}, fmt.Errorf("fiken.redirect_url %q is not a URL: %w", c.redirectURL(), err)
	}
	if !isLoopback(redirect.Hostname()) {
		return Token{}, fmt.Errorf(
			"fiken.redirect_url %q does not point at this machine; "+
				"use a localhost URL, or log in with --manual", c.redirectURL())
	}

	listener, err := net.Listen("tcp", net.JoinHostPort(redirect.Hostname(), portOf(redirect)))
	if err != nil {
		return Token{}, fmt.Errorf(
			"listen on %s for the Fiken redirect: %w; "+
				"close whatever is using the port, or log in with --manual", redirect.Host, err)
	}

	state, err := NewState()
	if err != nil {
		return Token{}, err
	}

	results := make(chan callback, 1)
	server := &http.Server{
		Handler:           handler(redirect.Path, results),
		ReadHeaderTimeout: 10 * time.Second,
	}
	go server.Serve(listener) //nolint:errcheck // Serve always ends in an error on close

	// Shut down rather than close: the browser is still being sent the page
	// that says how it went, and closing the moment the code arrives leaves
	// the operator looking at a blank tab.
	defer func() {
		grace, cancel := context.WithTimeout(context.WithoutCancel(ctx), shutdownGrace)
		defer cancel()
		server.Shutdown(grace) //nolint:errcheck // nothing useful to do if it will not
	}()

	announce(c.AuthCodeURL(state))

	timeout := time.NewTimer(loginTimeout)
	defer timeout.Stop()

	select {
	case <-ctx.Done():
		return Token{}, ctx.Err()
	case <-timeout.C:
		return Token{}, fmt.Errorf("gave up waiting for the Fiken login after %s", loginTimeout)
	case result := <-results:
		if result.err != nil {
			return Token{}, result.err
		}
		if result.state != state {
			return Token{}, errors.New("the Fiken redirect carried the wrong state; the login was not completed here")
		}
		return c.Exchange(ctx, result.code, state)
	}
}

// handler answers the one request that matters and ignores the rest, so a
// browser fetching a favicon does not end the login.
func handler(path string, results chan<- callback) http.Handler {
	mux := http.NewServeMux()
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if path != "" && r.URL.Path != path {
			http.NotFound(w, r)
			return
		}

		result := fromQuery(r.URL.Query())
		w.Header().Set("Content-Type", "text/html; charset=utf-8")
		if result.err != nil {
			w.WriteHeader(http.StatusBadRequest)
			io.WriteString(w, page("Innlogging avbrutt", result.err.Error())) //nolint:errcheck
		} else {
			io.WriteString(w, page("Innlogging fullført", //nolint:errcheck
				"Du kan lukke dette vinduet og gå tilbake til terminalen."))
		}

		select {
		case results <- result:
		default: // a second request cannot change the outcome
		}
	})
	return mux
}

// fromQuery reads the redirect's parameters, turning Fiken's error reply into
// an error rather than an empty code.
func fromQuery(query url.Values) callback {
	if code := query.Get("error"); code != "" {
		return callback{err: &Error{Code: code, Description: query.Get("error_description")}}
	}
	code := query.Get("code")
	if code == "" {
		return callback{err: errors.New("the Fiken redirect carried no authorization code")}
	}
	return callback{code: code, state: query.Get("state")}
}

// page renders the one-shot browser response. The message can contain an error
// straight from Fiken, so it is escaped rather than trusted.
func page(title, message string) string {
	return "<!doctype html><meta charset=\"utf-8\"><title>" + html.EscapeString(title) +
		"</title><body style=\"font-family:system-ui;margin:4rem auto;max-width:32rem\">" +
		"<h1>" + html.EscapeString(title) + "</h1><p>" + html.EscapeString(message) + "</p>"
}

// ManualAuthCodeURL starts a login that does not need a local server: the user
// visits the URL, then pastes back where the browser ended up. This is the way
// in over SSH, or when the redirect port is unavailable.
func (c Config) ManualAuthCodeURL() (authURL, state string, err error) {
	if err := c.Validate(); err != nil {
		return "", "", err
	}
	state, err = NewState()
	if err != nil {
		return "", "", err
	}
	return c.AuthCodeURL(state), state, nil
}

// CompleteManual finishes a manual login from the URL the browser was
// redirected to. A bare authorization code is accepted too, for the case where
// the browser showed one instead of navigating.
func (c Config) CompleteManual(ctx context.Context, state, redirected string) (Token, error) {
	redirected = strings.TrimSpace(redirected)
	if redirected == "" {
		return Token{}, errors.New("nothing pasted")
	}

	// A bare authorization code is accepted as well as a whole URL, because
	// some browsers show the code rather than navigating anywhere.
	if !strings.Contains(redirected, "?") && !strings.Contains(redirected, "://") {
		return c.Exchange(ctx, redirected, state)
	}

	parsed, err := url.Parse(redirected)
	if err != nil {
		return Token{}, fmt.Errorf("that is not a URL: %w", err)
	}
	result := fromQuery(parsed.Query())
	if result.err != nil {
		return Token{}, result.err
	}
	if result.state != "" && result.state != state {
		return Token{}, errors.New("the pasted URL is from a different login attempt")
	}
	return c.Exchange(ctx, result.code, state)
}

func isLoopback(host string) bool {
	if host == "localhost" {
		return true
	}
	ip := net.ParseIP(host)
	return ip != nil && ip.IsLoopback()
}

func portOf(u *url.URL) string {
	if port := u.Port(); port != "" {
		return port
	}
	return "80"
}