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<?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) {

            while (true) {

                $result->waitIfPending();

                if ($result->result instanceof Promise) {

                    $result = $result->result;

                } else {

                    if ($result->result instanceof PromiseInterface) {

                        $result->result->wait(false);

                    }

                    break;

                }

            }

        }

    }

}