// Package oauth implement an oauth client defined in e.g. rfc 6749
// However, we try to follow some recommendations from the v2.1 oauth draft RFC
// Some specific things we implement here:
// - PKCE (RFC 7636)
package oauth

import (
	"context"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"html/template"
	"net"
	"net/http"
	"net/url"
	"sync"
	"time"

	httpw "github.com/eduvpn/eduvpn-common/internal/http"
	"github.com/eduvpn/eduvpn-common/internal/util"
	"github.com/go-errors/errors"
)

// genState generates a random base64 string to be used for state
// https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-4.1.1
// "state":  OPTIONAL.  An opaque value used by the client to maintain
// state between the request and callback.  The authorization server
// includes this value when redirecting the user agent back to the
// client.
// We implement it similarly to the verifier.
func genState() (string, error) {
	bs, err := util.MakeRandomByteSlice(32)
	if err != nil {
		return "", err
	}

	// For consistency, we also use raw url encoding here
	return base64.RawURLEncoding.EncodeToString(bs), nil
}

// genChallengeS256 generates a sha256 base64 challenge from a verifier
// https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-7.8
func genChallengeS256(verifier string) string {
	hash := sha256.Sum256([]byte(verifier))

	// We use raw url encoding as the challenge does not accept padding
	return base64.RawURLEncoding.EncodeToString(hash[:])
}

// genVerifier generates a verifier
// https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-4.1.1
// The code_verifier is a unique high-entropy cryptographically random
// string generated for each authorization request, using the unreserved
// characters [A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~", with a
// minimum length of 43 characters and a maximum length of 128
// characters.
// We implement it according to the note:
//
//	NOTE: The code verifier SHOULD have enough entropy to make it
//	impractical to guess the value.  It is RECOMMENDED that the output of
//	a suitable random number generator be used to create a 32-octet
//	sequence.  The octet sequence is then base64url-encoded to produce a
//	43-octet URL safe string to use as the code verifier.
//
// See: https://datatracker.ietf.org/doc/html/rfc7636#section-4.1
func genVerifier() (string, error) {
	random, err := util.MakeRandomByteSlice(32)
	if err != nil {
		return "", err
	}

	return base64.RawURLEncoding.EncodeToString(random), nil
}

// OAuth defines the main structure for this package.
type OAuth struct {
	// The cached client id so we don't have to pass it around
	ClientID string `json:"client_id"`

	// The HTTP client that is used
	httpClient *httpw.Client

	// BaseAuthorizationURL is the URL where authorization should take place
	BaseAuthorizationURL string `json:"base_authorization_url"`

	// TokenURL is the URL where tokens should be obtained
	TokenURL string `json:"token_url"`

	// session is the internal in progress OAuth session
	session exchangeSession

	// Token is where the access and refresh tokens are stored along with the timestamps
	// It is protected by a lock
	token *tokenLock
}

// exchangeSession is a structure that gets passed to the callback for easy access to the current state.
type exchangeSession struct {
	// ClientID is the ID of the OAuth client
	ClientID string

	// State is the expected URL state parameter
	State string

	// Verifier is the preimage of the challenge
	Verifier string

	// Listener is the listener where the servers 'listens' on
	Listener net.Listener

	// ErrChan is used to send the error from the handler
	ErrChan chan error
}

// AccessToken gets the OAuth access token used for contacting the server API
// It returns the access token as a string, possibly obtained fresh using the Refresh Token
// If the token cannot be obtained, an error is returned and the token is an empty string.
func (oauth *OAuth) AccessToken() (string, error) {
	tl := oauth.token
	if tl == nil {
		return "", errors.New("No token structure available")
	}
	return tl.Access()
}

// setupListener sets up an OAuth listener
// If it was unsuccessful it returns an error.
// @see https://www.ietf.org/archive/id/draft-ietf-oauth-v2-1-07.html#section-8.4.2
// "Loopback Interface Redirection".
func (oauth *OAuth) setupListener() error {
	// create a listener
	lst, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		return errors.WrapPrefix(err, "net.Listen failed", 0)
	}
	oauth.session.Listener = lst
	return nil
}

// tokensWithCallback gets the OAuth tokens using a local web server
// If it was unsuccessful it returns an error.
func (oauth *OAuth) tokensWithCallback() error {
	if oauth.session.Listener == nil {
		return errors.New("failed getting tokens with callback: no listener")
	}
	mux := http.NewServeMux()
	// server /callback over the listener address
	s := &http.Server{
		Handler: mux,
		// Define a default 60 second header read timeout to protect against a Slowloris Attack
		// A bit overkill maybe for a local server but good to define anyways
		ReadHeaderTimeout: 60 * time.Second,
	}
	defer s.Shutdown(context.Background()) //nolint:errcheck

	// Use a sync.Once to only handle one request up until we shutdown the server
	var once sync.Once
	mux.HandleFunc("/callback", func(w http.ResponseWriter, r *http.Request) {
		once.Do(func() {
			oauth.Handler(w, r)
		})
	})

	go func() {
		if err := s.Serve(oauth.session.Listener); err != http.ErrServerClosed {
			oauth.session.ErrChan <- errors.WrapPrefix(err, "failed getting tokens with callback", 0)
		}
	}()
	return <-oauth.session.ErrChan
}

// tokenResponse fills the OAuth token response structure by the response
// The URL that is input here is used for additional context
// It returns this structure and an error if there is one
func (oauth *OAuth) tokenResponse(response []byte, url string) (*TokenResponse, error) {
	if oauth.token == nil {
		return nil, errors.New("No oauth structure when filling token")
	}
	res := TokenResponse{}

	err := json.Unmarshal(response, &res)
	if err != nil {
		return nil, errors.WrapPrefix(err, "failed filling OAuth tokens from "+url, 0)
	}

	return &res, nil
}

// SetTokenExpired marks the tokens as expired by setting the expired timestamp to the current time.
func (oauth *OAuth) SetTokenExpired() {
	if oauth.token != nil {
		oauth.token.SetExpired()
	}
}

// SetTokenRenew sets the tokens for renewal by completely clearing the structure.
func (oauth *OAuth) SetTokenRenew() {
	if oauth.token != nil {
		oauth.token.Update(Token{})
	}
}

func (oauth *OAuth) Token() Token {
	t := Token{}
	if oauth.token != nil {
		t = oauth.token.Get()
	}

	return t
}

// tokensWithAuthCode gets the access and refresh tokens using the authorization code
// Access tokens: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-1.4
// Refresh tokens: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-1.3.2
// If it was unsuccessful it returns an error.
func (oauth *OAuth) tokensWithAuthCode(authCode string) error {
	// Make sure the verifier is set as the parameter
	// so that the server can verify that we are the actual owner of the authorization code
	u := oauth.TokenURL

	port, err := oauth.ListenerPort()
	if err != nil {
		return err
	}

	data := url.Values{
		"client_id":     {oauth.session.ClientID},
		"code":          {authCode},
		"code_verifier": {oauth.session.Verifier},
		"grant_type":    {"authorization_code"},
		"redirect_uri":  {fmt.Sprintf("http://127.0.0.1:%d/callback", port)},
	}
	h := http.Header{
		"content-type": {"application/x-www-form-urlencoded"},
	}
	opts := &httpw.OptionalParams{Headers: h, Body: data}
	now := time.Now()

	// We are sure that we have a http client because we have initialized it when starting the exchange
	_, body, err := oauth.httpClient.PostWithOpts(u, opts)
	if err != nil {
		return err
	}

	tr, err := oauth.tokenResponse(body, u)
	if err != nil {
		return err
	}
	if tr == nil {
		return errors.New("No token response after authorization code")
	}

	oauth.token.UpdateResponse(*tr, now)
	return nil
}

func (oauth *OAuth) UpdateTokens(t Token) {
	if oauth.token == nil {
		oauth.token = &tokenLock{t: &tokenRefresher{Refresher: oauth.refreshResponse}}
	}
	oauth.token.Update(t)
}

// refreshResponse gets the refresh token response with a refresh token
// This response contains the access and refresh tokens, together with a timestamp
// Access tokens: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-1.4
// Refresh tokens: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-1.3.2
// If it was unsuccessful it returns an error.
func (oauth *OAuth) refreshResponse(r string) (*TokenResponse, time.Time, error) {
	u := oauth.TokenURL
	if oauth.token == nil {
		return nil, time.Time{}, errors.New("No oauth token structure in refresh")
	}
	if oauth.ClientID == "" {
		return nil, time.Time{}, errors.New("No client ID was cached for refresh")
	}
	data := url.Values{
		"client_id":     {oauth.ClientID},
		"refresh_token": {r},
		"grant_type":    {"refresh_token"},
	}
	h := http.Header{
		"content-type": {"application/x-www-form-urlencoded"},
	}
	opts := &httpw.OptionalParams{Headers: h, Body: data}
	now := time.Now()

	// Test if we have a http client and if not recreate one
	if oauth.httpClient == nil {
		oauth.httpClient = httpw.NewClient()
	}

	_, body, err := oauth.httpClient.PostWithOpts(u, opts)
	if err != nil {
		return nil, time.Time{}, err
	}

	tr, err := oauth.tokenResponse(body, u)
	return tr, now, err
}

// responseTemplate is the HTML template for the OAuth authorized response
// this template was dapted from: https://github.com/eduvpn/apple/blob/5b18f834be7aebfed00570ae0c2f7bcbaf1c69cc/EduVPN/Helpers/Mac/OAuthRedirectHTTPHandler.m#L25
const responseTemplate string = `
<!DOCTYPE html>
<html dir="ltr" xmlns="http://www.w3.org/1999/xhtml" lang="en"><head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<meta charset="utf-8">
<title>{{.Title}}</title>
<style>
body {
    font-family: arial;
    margin: 0;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background: #ccc;
    color: #252622;
}
main {
    padding: 1em 2em;
    text-align: center;
    border: 2pt solid #666;
    box-shadow: rgba(0, 0, 0, 0.6) 0px 1px 4px;
    border-color: #aaa;
    background: #ddd;
}
</style>
</head>
<body>
    <main>
        <h1>{{.Title}}</h1>
        <p>{{.Message}}</p>
    </main>
</body>
</html>
`

// oauthResponseHTML is a structure that is used to give back the OAuth response.
type oauthResponseHTML struct {
	Title   string
	Message string
}

// writeResponseHTML writes the OAuth response using a response writer and the title + message
// If it was unsuccessful it returns an error.
func writeResponseHTML(w http.ResponseWriter, title string, message string) error {
	t, err := template.New("oauth-response").Parse(responseTemplate)
	if err != nil {
		return errors.WrapPrefix(err, "failed writing response HTML", 0)
	}

	return t.Execute(w, oauthResponseHTML{Title: title, Message: message})
}

// Authcode gets the authorization code from the url
// It returns the code and an error if there is one
func (s *exchangeSession) Authcode(url *url.URL) (string, error) {
	q := url.Query()

	// Make sure the state is present and matches to protect against cross-site request forgeries
	// https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-7.15
	state := q.Get("state")
	if state == "" {
		return "", errors.Errorf("failed retrieving parameter 'state' from '%s'", url)
	}
	// The state is the first entry
	if state != s.State {
		return "", errors.Errorf("failed matching state; expected '%s' got '%s'", s.State, state)
	}

	// No authorization code
	code := q.Get("code")
	if code == "" {
		return "", errors.Errorf("failed retrieving parameter 'code' from '%s'", url)
	}

	return code, nil
}

// tokenHandler gets the tokens using the authorization code that is obtained through the url
// This function is called by the http handler and returns an error if the tokens cannot be obtained
func (oauth *OAuth) tokenHandler(url *url.URL) error {
	// Get the authorization code
	c, err := oauth.session.Authcode(url)
	if err != nil {
		return err
	}
	// Now that we have obtained the authorization code, we can move to the next step:
	// Obtaining the access and refresh tokens
	return oauth.tokensWithAuthCode(c)
}

// Handler is the function used to get the OAuth tokens using an authorization code callback
// The callback to retrieve the authorization code: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-04#section-1.3.1
// It sends an error to the session channel (can be nil)
func (oauth *OAuth) Handler(w http.ResponseWriter, req *http.Request) {
	err := oauth.tokenHandler(req.URL)
	if err != nil {
		_ = writeResponseHTML(
			w,
			"Authorization Failed",
			"The authorization has failed. See the log file for more information.",
		)
	} else {
		_ = writeResponseHTML(w, "Authorized", "The client has been successfully authorized. You can close this browser window.")
	}
	oauth.session.ErrChan <- err
}

// Init initializes OAuth with the following parameters:
// - The URL used for authorization
// - The URL to obtain new tokens.
func (oauth *OAuth) Init(baseAuthorizationURL string, tokenURL string) {
	oauth.BaseAuthorizationURL = baseAuthorizationURL
	oauth.TokenURL = tokenURL
}

// ListenerPort gets the listener for the OAuth web server
// It returns the port as an integer and an error if there is any.
func (oauth *OAuth) ListenerPort() (int, error) {
	if oauth.session.Listener == nil {
		return 0, errors.New("failed to get listener port")
	}
	return oauth.session.Listener.Addr().(*net.TCPAddr).Port, nil
}

// AuthURL gets the authorization url to start the OAuth procedure.
func (oauth *OAuth) AuthURL(name string, postProcessAuth func(string) string) (string, error) {
	// Update the client ID
	oauth.ClientID = name

	// Generate the verifier and challenge
	v, err := genVerifier()
	if err != nil {
		return "", errors.WrapPrefix(err, "genVerifier error", 0)
	}

	// Generate the state
	state, err := genState()
	if err != nil {
		return "", errors.WrapPrefix(err, "genState error", 0)
	}

	// Re-initialize the token structure
	oauth.UpdateTokens(Token{})

	// Fill the struct with the necessary fields filled for the next call to getting the HTTP client
	oauth.session = exchangeSession{
		ClientID: name,
		State:    state,
		Verifier: v,
		ErrChan:  make(chan error),
	}

	// set up the listener to get the redirect URI
	if err = oauth.setupListener(); err != nil {
		return "", errors.WrapPrefix(err, "oauth.setupListener error", 0)
	}

	// Get the listener port
	port, err := oauth.ListenerPort()
	if err != nil {
		return "", errors.WrapPrefix(err, "oauth.ListenerPort error", 0)
	}

	params := map[string]string{
		"client_id":             name,
		"code_challenge_method": "S256",
		"code_challenge":        genChallengeS256(v),
		"response_type":         "code",
		"scope":                 "config",
		"state":                 state,
		"redirect_uri":          fmt.Sprintf("http://127.0.0.1:%d/callback", port),
	}

	p, err := url.Parse(oauth.BaseAuthorizationURL)
	if err != nil {
		return "", errors.WrapPrefix(err, fmt.Sprintf("failed to parse OAuth base URL '%s'", oauth.BaseAuthorizationURL), 0)
	}

	u, err := httpw.ConstructURL(p, params)
	if err != nil {
		return "", errors.WrapPrefix(err, "httpw.ConstructURL error", 0)
	}

	// Return the url processed
	return postProcessAuth(u), nil
}

// Exchange starts the OAuth exchange by getting the tokens with the redirect callback
// If it was unsuccessful it returns an error.
func (oauth *OAuth) Exchange() error {
	// If there is no HTTP client defined, create a new one
	if oauth.httpClient == nil {
		oauth.httpClient = httpw.NewClient()
	}
	return oauth.tokensWithCallback()
}

// Cancel cancels the existing OAuth server by sending a cancel error to the channel
func (oauth *OAuth) Cancel() {
	oauth.session.ErrChan <- errors.Wrap(&CancelledCallbackError{}, 0)
}

type CancelledCallbackError struct{}

func (e *CancelledCallbackError) Error() string {
	return "client cancelled OAuth"
}

type TokensInvalidError struct {
	Cause string
}

func (e *TokensInvalidError) Error() string {
	return fmt.Sprintf("tokens are invalid due to: %s", e.Cause)
}