<?php
/*
 * Copyright 2015 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace Google\Auth\Middleware;

use Google\Auth\CacheInterface;
use Google\Auth\CacheTrait;
use Google\Auth\FetchAuthTokenInterface;
use Psr\Http\Message\RequestInterface;

/**
 * ScopedAccessTokenMiddleware is a Guzzle Middleware that adds an Authorization
 * header provided by a closure.
 *
 * The closure returns an access token, taking the scope, either a single
 * string or an array of strings, as its value.  If provided, a cache will be
 * used to preserve the access token for a given lifetime.
 *
 * Requests will be accessed with the authorization header:
 *
 * 'Authorization' 'Bearer <value of auth_token>'
 */
class ScopedAccessTokenMiddleware
{
  use CacheTrait;

  const DEFAULT_CACHE_LIFETIME = 1500;

  /** @var An implementation of CacheInterface */
  private $cache;

  /** @var callback */
  private $httpHandler;

  /** @var An implementation of FetchAuthTokenInterface */
  private $fetcher;

  /** @var cache configuration */
  private $cacheConfig;

  /**
   * Creates a new ScopedAccessTokenMiddleware.
   *
   * @param callable $tokenFunc a token generator function
   * @param array|string $scopes the token authentication scopes
   * @param array $cacheConfig configuration for the cache when it's present
   * @param CacheInterface $cache an implementation of CacheInterface
   */
  public function __construct(
    callable $tokenFunc,
    $scopes,
    array $cacheConfig = null,
    CacheInterface $cache = null
  ) {
    $this->tokenFunc = $tokenFunc;
    if (!(is_string($scopes) || is_array($scopes))) {
      throw new \InvalidArgumentException(
          'wants scope should be string or array');
    }
    $this->scopes = $scopes;

    if (!is_null($cache)) {
      $this->cache = $cache;
      $this->cacheConfig = array_merge([
        'lifetime' => self::DEFAULT_CACHE_LIFETIME,
        'prefix'   => ''
      ], $cacheConfig);
    }
  }

  /**
   * Updates the request with an Authorization header when auth is 'scoped'.
   *
   *   E.g this could be used to authenticate using the AppEngine
   *   AppIdentityService.
   *
   *   use google\appengine\api\app_identity\AppIdentityService;
   *   use Google\Auth\Middleware\ScopedAccessTokenMiddleware;
   *   use GuzzleHttp\Client;
   *   use GuzzleHttp\HandlerStack;
   *
   *   $scope = 'https://www.googleapis.com/auth/taskqueue'
   *   $middleware = new ScopedAccessTokenMiddleware(
   *       'AppIdentityService::getAccessToken',
   *       $scope,
   *       [ 'prefix' => 'Google\Auth\ScopedAccessToken::' ],
   *       $cache = new Memcache()
   *   );
   *   $stack = HandlerStack::create();
   *   $stack->push($middleware);
   *
   *   $client = new Client([
   *       'handler' => $stack,
   *       'base_url' => 'https://www.googleapis.com/taskqueue/v1beta2/projects/',
   *       'auth' => 'google_auth' // authorize all requests
   *   ]);
   *
   *   $res = $client->get('myproject/taskqueues/myqueue');
   */
  public function __invoke(callable $handler)
  {
    return function (RequestInterface $request, array $options) use ($handler) {
      // Requests using "auth"="scoped" will be authorized.
      if (!isset($options['auth']) || $options['auth'] !== 'scoped') {
        return $handler($request, $options);
      }

      $request = $request->withHeader('Authorization', 'Bearer ' . $this->fetchToken());
      return $handler($request, $options);
    };
  }

  /**
   * @return string
   */
  private function getCacheKey()
  {
    $key = null;

    if (is_string($this->scopes)) {
      $key .= $this->scopes;
    } else if (is_array($this->scopes)) {
      $key .= implode(":", $this->scopes);
    }
    return $key;
  }

  /**
   * Determine if token is available in the cache, if not call tokenFunc to
   * fetch it.
   *
   * @return string
   */
  private function fetchToken()
  {
    $cached = $this->getCachedValue();

    if (!empty($cached)) {
      return $cached;
    }

    $token = call_user_func($this->tokenFunc, $this->scopes);
    $this->setCachedValue($token);
    return $token;
  }
}
