<?php

namespace Amp\Internal;

use Amp\Future;
use Amp\Future\UnhandledFutureError;
use Revolt\EventLoop;

/**
 * @internal
 *
 * @template T
 */
final class FutureState
{
	private static string $nextId = 'a';
	private bool $complete = false;
	private bool $handled = false;

	/** @var array<string, \Closure(?\Throwable, ?T, string): void> */
	private array $callbacks = [];

	/** @var T|null */
	private mixed $result = null;
	private ?\Throwable $throwable = null;
	private ?string $origin = null;


	public function __construct()
	{
		\assert((function () {
		    if (isDebugEnabled()) {
		        $trace = \debug_backtrace(\DEBUG_BACKTRACE_IGNORE_ARGS);
		        \array_shift($trace); // remove current closure
		        $this->origin = formatStacktrace($trace);
		    }

		    return true;
		})());
	}


	public function __destruct()
	{
		if ($this->throwable && !$this->handled) {
		    $throwable = new UnhandledFutureError($this->throwable, $this->origin);
		    EventLoop::queue(static fn () => throw $throwable);
		}
	}


	/**
	 * Registers a callback to be notified once the operation is complete or errored.
	 *
	 * The callback is invoked directly from the event loop context, so suspension within the callback is not possible.
	 *
	 * @param \Closure(?\Throwable, ?T, string): void $callback Callback invoked on error / successful completion of
	 * the future.
	 *
	 * @return string Identifier that can be used to cancel interest for this future.
	 */
	public function subscribe(\Closure $callback): string
	{
		$id = self::$nextId++;

		$this->handled = true; // Even if unsubscribed later, consider the future handled.

		if ($this->complete) {
		    EventLoop::queue($callback, $this->throwable, $this->result, $id);
		} else {
		    $this->callbacks[$id] = $callback;
		}

		return $id;
	}


	/**
	 * Cancels a subscription.
	 *
	 * Cancellations are advisory only. The callback might still be called if it is already queued for execution.
	 *
	 * @param string $id Identifier returned from subscribe()
	 */
	public function unsubscribe(string $id): void
	{
		unset($this->callbacks[$id]);
	}


	/**
	 * Completes the operation with a result value.
	 *
	 * @param T $result Result of the operation.
	 */
	public function complete(mixed $result): void
	{
		if ($this->complete) {
		    throw new \Error('Operation is no longer pending');
		}

		if ($result instanceof Future) {
		    throw new \Error('Cannot complete with an instance of ' . Future::class);
		}

		$this->result = $result;
		$this->invokeCallbacks();
	}


	/**
	 * Marks the operation as failed.
	 *
	 * @param \Throwable $throwable Throwable to indicate the error.
	 */
	public function error(\Throwable $throwable): void
	{
		if ($this->complete) {
		    throw new \Error('Operation is no longer pending');
		}

		$this->throwable = $throwable;
		$this->invokeCallbacks();
	}


	/**
	 * @return bool True if the operation has completed.
	 */
	public function isComplete(): bool
	{
		return $this->complete;
	}


	/**
	 * Suppress the exception thrown to the loop error handler if and operation error is not handled by a callback.
	 */
	public function ignore(): void
	{
		$this->handled = true;
	}


	private function invokeCallbacks(): void
	{
		$this->complete = true;

		foreach ($this->callbacks as $id => $callback) {
		    EventLoop::queue($callback, $this->throwable, $this->result, $id);
		}

		$this->callbacks = [];
	}
}
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