var AWS = require('../core');

/**
 * Creates a credential provider chain that searches for AWS credentials
 * in a list of credential providers specified by the {providers} property.
 *
 * By default, the chain will use the {defaultProviders} to resolve credentials.
 * These providers will look in the environment using the
 * {AWS.EnvironmentCredentials} class with the 'AWS' and 'AMAZON' prefixes.
 *
 * ## Setting Providers
 *
 * Each provider in the {providers} list should be a function that returns
 * a {AWS.Credentials} object, or a hardcoded credentials object. The function
 * form allows for delayed execution of the credential construction.
 *
 * ## Resolving Credentials from a Chain
 *
 * Call {resolve} to return the first valid credential object that can be
 * loaded by the provider chain.
 *
 * For example, to resolve a chain with a custom provider that checks a file
 * on disk after the set of {defaultProviders}:
 *
 * ```javascript
 * var diskProvider = new AWS.FileSystemCredentials('./creds.json');
 * var chain = new AWS.CredentialProviderChain();
 * chain.providers.push(diskProvider);
 * chain.resolve();
 * ```
 *
 * The above code will return the `diskProvider` object if the
 * file contains credentials and the `defaultProviders` do not contain
 * any credential settings.
 *
 * @!attribute providers
 *   @return [Array<AWS.Credentials, Function>]
 *     a list of credentials objects or functions that return credentials
 *     objects. If the provider is a function, the function will be
 *     executed lazily when the provider needs to be checked for valid
 *     credentials. By default, this object will be set to the
 *     {defaultProviders}.
 *   @see defaultProviders
 */
AWS.CredentialProviderChain = AWS.util.inherit(AWS.Credentials, {

  /**
   * Creates a new CredentialProviderChain with a default set of providers
   * specified by {defaultProviders}.
   */
  constructor: function CredentialProviderChain(providers) {
    if (providers) {
      this.providers = providers;
    } else {
      this.providers = AWS.CredentialProviderChain.defaultProviders.slice(0);
    }
  },

  /**
   * Resolves the provider chain by searching for the first set of
   * credentials in {providers}.
   *
   * @callback callback function(err, credentials)
   *   Called when the provider resolves the chain to a credentials object
   *   or null if no credentials can be found.
   *
   *   @param err [Error] the error object returned if no credentials are
   *     found.
   *   @param credentials [AWS.Credentials] the credentials object resolved
   *     by the provider chain.
   * @return [AWS.CredentialProviderChain] the provider, for chaining.
   */
  resolve: function resolve(callback) {
    if (this.providers.length === 0) {
      callback(new Error('No providers'));
      return;
    }

    var index = 0;
    var providers = this.providers.slice(0);

    function resolveNext(err, creds) {
      if ((!err && creds) || index === providers.length) {
        callback(err, creds);
        return;
      }

      var provider = providers[index++];
      if (typeof provider === 'function') {
        creds = provider.call();
      } else {
        creds = provider;
      }

      if (creds.get) {
        creds.get(function(err) {
          resolveNext(err, err ? null : creds);
        });
      } else {
        resolveNext(null, creds);
      }
    }

    resolveNext();
    return this;
  }

});

/**
 * The default set of providers used by a vanilla CredentialProviderChain.
 */
AWS.CredentialProviderChain.defaultProviders = [];
