<?php
namespace QWSpeedCache\Support;

defined( 'ABSPATH' ) || exit;

/** Own response buffers and close only the exposed QW stack before core shutdown. */
final class OutputBuffer {
	private static $buffers = array();
	private static $registered = false;
	private $callback;
	private $level;

	private function __construct( $callback ) {
		$this->callback = $callback;
		$this->level = ob_get_level() + 1;
	}

	public static function start( $callback ) {
		if ( ! is_callable( $callback ) ) {
			return false;
		}
		$buffer = new self( $callback );
		if ( ! ob_start( array( $buffer, 'process' ) ) ) {
			return false;
		}
		self::$buffers[ $buffer->level ] = $buffer;
		if ( ! self::$registered ) {
			self::$registered = true;
			// Core wp_ob_end_flush_all normally runs at priority 1.
			add_action( 'shutdown', array( __CLASS__, 'finish' ), 0 );
		}
		return true;
	}

	public function process( $html, $phase ) {
		if ( $phase & PHP_OUTPUT_HANDLER_FINAL ) {
			unset( self::$buffers[ $this->level ] );
		}
		// Discarded content must never be saved as a cache entry.
		if ( $phase & PHP_OUTPUT_HANDLER_CLEAN ) {
			return $html;
		}
		return call_user_func( $this->callback, $html );
	}

	public static function finish() {
		while ( isset( self::$buffers[ ob_get_level() ] ) ) {
			$status = ob_get_status();
			if ( empty( $status['name'] ) || __CLASS__ . '::process' !== $status['name']
				|| empty( $status['flags'] ) || ! ( $status['flags'] & PHP_OUTPUT_HANDLER_REMOVABLE ) ) {
				break;
			}
			// The FINAL callback removes this exact registration, including when
			// WordPress or another component already closed it before shutdown.
			if ( ! ob_end_flush() ) {
				break;
			}
		}
		// An unrelated top buffer belongs to its owner/core, never pop it here.
	}
}