/**
 * Configuration for OAuth client credentials flow
 */
export interface OAuthConfig {
  /** The URL to request the OAuth token from */
  authUrl: string;
  /** The client ID for OAuth authentication */
  clientId: string;
  /** The client secret for OAuth authentication */
  clientSecret: string;
  /** The scope for the OAuth token. Use empty string if no scope is needed. */
  scope: string;
}

/**
 * Standard OAuth 2.0 token response format as defined in RFC 6749
 */
export interface OAuthTokenResponse {
  /** The access token issued by the authorization server */
  access_token: string;
  /** The type of token issued (typically "Bearer") */
  token_type: string;
  /** The lifetime in seconds of the access token */
  expires_in: number;
  /** The refresh token, which can be used to obtain new access tokens (optional) */
  refresh_token?: string;
  /** The scope of the access token (optional) */
  scope?: string;
}

/**
 * A utility class for handling OAuth client credentials flow.
 * This is specifically for the client credentials grant type,
 * which is used for server-to-server authentication.
 */
export class OAuthClient {
  constructor(private config: OAuthConfig) {}

  /**
   * Requests an OAuth token using the client credentials flow.
   * The request is sent as application/x-www-form-urlencoded.
   * @returns A promise that resolves to the token response containing access_token and expires_in
   * @throws Error if the token request fails
   */
  async getToken(): Promise<OAuthTokenResponse> {
    const formData = new URLSearchParams({
      grant_type: "client_credentials",
      client_id: this.config.clientId,
      client_secret: this.config.clientSecret,
      scope: this.config.scope,
    });

    const response = await fetch(this.config.authUrl, {
      method: "POST",
      headers: {
        "Content-Type": "application/x-www-form-urlencoded",
      },
      body: formData,
    });

    if (!response.ok) {
      throw new Error(`OAuth token request failed: ${response.statusText}`);
    }

    const data = (await response.json()) as OAuthTokenResponse;
    if (data.token_type !== "Bearer") {
      throw new Error(`Unexpected token type: ${data.token_type}`);
    }

    return data;
  }
}