// Exchange converts an authorization code into a token.
//
// It is used after a resource provider redirects the user back
// to the Redirect URI (the URL obtained from AuthCodeURL).
//
// The HTTP client to use is derived from the context.
// If a client is not provided via the context, http.DefaultClient is used.
//
// The code will be in the *http.Request.FormValue("code"). Before
// calling Exchange, be sure to validate FormValue("state").
func (c *Config) Exchange(ctx context.Context, code string) (*Token, error) {
	return retrieveToken(ctx, c, url.Values{
		"grant_type":   {"authorization_code"},
		"code":         {code},
		"redirect_uri": internal.CondVal(c.RedirectURL),
		"scope":        internal.CondVal(strings.Join(c.Scopes, " ")),
	})
}

// PasswordCredentialsToken converts a resource owner username and password
// pair into a token.
//
// Per the RFC, this grant type should only be used "when there is a high
// degree of trust between the resource owner and the client (e.g., the client
// is part of the device operating system or a highly privileged application),
// and when other authorization grant types are not available."
// See https://tools.ietf.org/html/rfc6749#section-4.3 for more info.
//
// The HTTP client to use is derived from the context.
// If nil, http.DefaultClient is used.
func (c *Config) PasswordCredentialsToken(ctx context.Context, username, password string) (*Token, error) {
	return retrieveToken(ctx, c, url.Values{
		"grant_type": {"password"},
		"username":   {username},
		"password":   {password},
		"scope":      internal.CondVal(strings.Join(c.Scopes, " ")),
	})
}

// Token uses client credentials to retrieve a token.
// The HTTP client to use is derived from the context.
// If nil, http.DefaultClient is used.
func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
	var values = c.ExtraParams
	if values == nil {
		values = url.Values{}
	}
	values.Set("grant_type", "client_credentials")
	values.Set("scope", internal.CondVal(strings.Join(c.Scopes, " ")))

	return retrieveToken(ctx, c, values)
}

// AuthCodeURL returns a URL to OAuth 2.0 provider's consent page
// that asks for permissions for the required scopes explicitly.
//
// State is a token to protect the user from CSRF attacks. You must
// always provide a non-zero string and validate that it matches the
// the state query parameter on your redirect callback.
// See http://tools.ietf.org/html/rfc6749#section-10.12 for more info.
//
// Opts may include AccessTypeOnline or AccessTypeOffline, as well
// as ApprovalForce.
func (c *Config) AuthCodeURL(state string, opts ...AuthCodeOption) string {
	var buf bytes.Buffer
	buf.WriteString(c.Endpoint.AuthURL)
	v := url.Values{
		"response_type": {"code"},
		"client_id":     {c.ClientID},
		"redirect_uri":  internal.CondVal(c.RedirectURL),
		"scope":         internal.CondVal(strings.Join(c.Scopes, " ")),
		"state":         internal.CondVal(state),
	}
	for _, opt := range opts {
		opt.setValue(v)
	}
	if strings.Contains(c.Endpoint.AuthURL, "?") {
		buf.WriteByte('&')
	} else {
		buf.WriteByte('?')
	}
	buf.WriteString(v.Encode())
	return buf.String()
}

// Token refreshes the token by using a new client credentials request.
// tokens received this way do not include a refresh token
func (c *tokenSource) Token() (*oauth2.Token, error) {
	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, url.Values{
		"grant_type": {"client_credentials"},
		"scope":      internal.CondVal(strings.Join(c.conf.Scopes, " ")),
	})
	if err != nil {
		return nil, err
	}
	t := &oauth2.Token{
		AccessToken:  tk.AccessToken,
		TokenType:    tk.TokenType,
		RefreshToken: tk.RefreshToken,
		Expiry:       tk.Expiry,
	}
	return t.WithExtra(tk.Raw), nil
}

// Token uses client credentials to retreive a token.
// The HTTP client to use is derived from the context.
// If nil, http.DefaultClient is used.
func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
	return retrieveToken(ctx, c, url.Values{
		"grant_type": {"client_credentials"},
		"scope":      internal.CondVal(strings.Join(c.Scopes, " ")),
	})
}