text: <?php
  namespace GuzzleHttp\Promise;
  
  /**
   * Promises/A+ implementation that avoids recursion when possible.
   *
   * @link https://promisesaplus.com/
   */
  class Promise implements PromiseInterface
  {
      private $state = self::PENDING;
      private $result;
      private $cancelFn;
      private $waitFn;
      private $waitList;
      private $handlers = [];
  
      /**
       * @param callable $waitFn   Fn that when invoked resolves the promise.
       * @param callable $cancelFn Fn that when invoked cancels the promise.
       */
      public function __construct(
          callable $waitFn = null,
          callable $cancelFn = null
      ) {
          $this->waitFn = $waitFn;
          $this->cancelFn = $cancelFn;
      }
  
      public function then(
          callable $onFulfilled = null,
          callable $onRejected = null
      ) {
          if ($this->state === self::PENDING) {
              $p = new Promise(null, [$this, 'cancel']);
              $this->handlers[] = [$p, $onFulfilled, $onRejected];
              $p->waitList = $this->waitList;
              $p->waitList[] = $this;
              return $p;
          }
  
          // Return a fulfilled promise and immediately invoke any callbacks.
          if ($this->state === self::FULFILLED) {
              return $onFulfilled
                  ? promise_for($this->result)->then($onFulfilled)
                  : promise_for($this->result);
          }
  
          // It's either cancelled or rejected, so return a rejected promise
          // and immediately invoke any callbacks.
          $rejection = rejection_for($this->result);
          return $onRejected ? $rejection->then(null, $onRejected) : $rejection;
      }
  
      public function otherwise(callable $onRejected)
      {
          return $this->then(null, $onRejected);
      }
  
      public function wait($unwrap = true)
      {
          $this->waitIfPending();
  
          $inner = $this->result instanceof PromiseInterface
              ? $this->result->wait($unwrap)
              : $this->result;
  
          if ($unwrap) {
              if ($this->result instanceof PromiseInterface
                  || $this->state === self::FULFILLED
              ) {
                  return $inner;
              } else {
                  // It's rejected so "unwrap" and throw an exception.
                  throw exception_for($inner);
              }
          }
      }
  
      public function getState()
      {
          return $this->state;
      }
  
      public function cancel()
      {
          if ($this->state !== self::PENDING) {
              return;
          }
  
          $this->waitFn = $this->waitList = null;
  
          if ($this->cancelFn) {
              $fn = $this->cancelFn;
              $this->cancelFn = null;
              try {
                  $fn();
              } catch (\Throwable $e) {
                  $this->reject($e);
              } catch (\Exception $e) {
                  $this->reject($e);
              }
          }
  
          // Reject the promise only if it wasn't rejected in a then callback.
          if ($this->state === self::PENDING) {
              $this->reject(new CancellationException('Promise has been cancelled'));
          }
      }
  
      public function resolve($value)
      {
          $this->settle(self::FULFILLED, $value);
      }
  
      public function reject($reason)
      {
          $this->settle(self::REJECTED, $reason);
      }
  
      private function settle($state, $value)
      {
          if ($this->state !== self::PENDING) {
              // Ignore calls with the same resolution.
              if ($state === $this->state && $value === $this->result) {
                  return;
              }
              throw $this->state === $state
                  ? new \LogicException("The promise is already {$state}.")
                  : new \LogicException("Cannot change a {$this->state} promise to {$state}");
          }
  
          if ($value === $this) {
              throw new \LogicException('Cannot fulfill or reject a promise with itself');
          }
  
          // Clear out the state of the promise but stash the handlers.
          $this->state = $state;
          $this->result = $value;
          $handlers = $this->handlers;
          $this->handlers = null;
          $this->waitList = $this->waitFn = null;
          $this->cancelFn = null;
  
          if (!$handlers) {
              return;
          }
  
          // If the value was not a settled promise or a thenable, then resolve
          // it in the task queue using the correct ID.
          if (!method_exists($value, 'then')) {
              $id = $state === self::FULFILLED ? 1 : 2;
              // It's a success, so resolve the handlers in the queue.
              queue()->add(static function () use ($id, $value, $handlers) {
                  foreach ($handlers as $handler) {
                      self::callHandler($id, $value, $handler);
                  }
              });
          } elseif ($value instanceof Promise
              && $value->getState() === self::PENDING
          ) {
              // We can just merge our handlers onto the next promise.
              $value->handlers = array_merge($value->handlers, $handlers);
          } else {
              // Resolve the handlers when the forwarded promise is resolved.
              $value->then(
                  static function ($value) use ($handlers) {
                      foreach ($handlers as $handler) {
                          self::callHandler(1, $value, $handler);
                      }
                  },
                  static function ($reason) use ($handlers) {
                      foreach ($handlers as $handler) {
                          self::callHandler(2, $reason, $handler);
                      }
                  }
              );
          }
      }
  
      /**
       * Call a stack of handlers using a specific callback index and value.
       *
       * @param int   $index   1 (resolve) or 2 (reject).
       * @param mixed $value   Value to pass to the callback.
       * @param array $handler Array of handler data (promise and callbacks).
       *
       * @return array Returns the next group to resolve.
       */
      private static function callHandler($index, $value, array $handler)
      {
          /** @var PromiseInterface $promise */
          $promise = $handler[0];
  
          // The promise may have been cancelled or resolved before placing
          // this thunk in the queue.
          if ($promise->getState() !== self::PENDING) {
              return;
          }
  
          try {
              if (isset($handler[$index])) {
                  $promise->resolve($handler[$index]($value));
              } elseif ($index === 1) {
                  // Forward resolution values as-is.
                  $promise->resolve($value);
              } else {
                  // Forward rejections down the chain.
                  $promise->reject($value);
              }
          } catch (\Throwable $reason) {
              $promise->reject($reason);
          } catch (\Exception $reason) {
              $promise->reject($reason);
          }
      }
  
      private function waitIfPending()
      {
          if ($this->state !== self::PENDING) {
              return;
          } elseif ($this->waitFn) {
              $this->invokeWaitFn();
          } elseif ($this->waitList) {
              $this->invokeWaitList();
          } else {
              // If there's not wait function, then reject the promise.
              $this->reject('Cannot wait on a promise that has '
                  . 'no internal wait function. You must provide a wait '
                  . 'function when constructing the promise to be able to '
                  . 'wait on a promise.');
          }
  
          queue()->run();
  
          if ($this->state === self::PENDING) {
              $this->reject('Invoking the wait callback did not resolve the promise');
          }
      }
  
      private function invokeWaitFn()
      {
          try {
              $wfn = $this->waitFn;
              $this->waitFn = null;
              $wfn(true);
          } catch (\Exception $reason) {
              if ($this->state === self::PENDING) {
                  // The promise has not been resolved yet, so reject the promise
                  // with the exception.
                  $this->reject($reason);
              } else {
                  // The promise was already resolved, so there's a problem in
                  // the application.
                  throw $reason;
              }
          }
      }
  
      private function invokeWaitList()
      {
          $waitList = $this->waitList;
          $this->waitList = null;
  
          foreach ($waitList as $result) {
              $result->waitIfPending();
              while ($result->result instanceof Promise) {
                  $result = $result->result;
                  $result->waitIfPending();
              }
          }
      }
  }
  