<?php
/**
 * PSR-4 style class autoloader.
 *
 * @package Kordiam
 */

declare( strict_types=1 );

namespace Kordiam;

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Maps a namespace prefix onto a base directory and loads matching classes.
 *
 * Loaded by hand, so it must stay dependency-free.
 */
final class Autoloader {

	/**
	 * Namespace prefix this loader is responsible for, with a trailing separator.
	 *
	 * @var string
	 */
	private string $prefix;

	/**
	 * Directory the prefix maps onto, with a trailing slash.
	 *
	 * @var string
	 */
	private string $base_dir;

	/**
	 * Constructing a loader for one namespace prefix.
	 *
	 * @param string $prefix   Namespace prefix, with or without a trailing separator.
	 * @param string $base_dir Absolute path the prefix maps onto.
	 */
	private function __construct( string $prefix, string $base_dir ) {
		$this->prefix   = rtrim( $prefix, '\\' ) . '\\';
		$this->base_dir = rtrim( $base_dir, '/\\' ) . '/';
	}

	/**
	 * Registering a loader for the given prefix on the SPL autoload stack.
	 *
	 * @param string $prefix   Namespace prefix, with or without a trailing separator.
	 * @param string $base_dir Absolute path the prefix maps onto.
	 *
	 * @return self The registered loader.
	 */
	public static function register( string $prefix, string $base_dir ): self {
		$loader = new self( $prefix, $base_dir );

		spl_autoload_register( array( $loader, 'load' ) );

		return $loader;
	}

	/**
	 * Loading the file backing a class name, when this loader owns its prefix.
	 *
	 * @param string $class_name Fully qualified class name handed over by PHP.
	 */
	public function load( string $class_name ): void {
		if ( 0 !== strpos( $class_name, $this->prefix ) ) {
			return;
		}

		$relative = substr( $class_name, strlen( $this->prefix ) );

		// Defence in depth: keep the file path derived from a known-good string.
		if ( 1 !== preg_match( '/^[A-Za-z0-9_\\\\]+$/', $relative ) ) {
			return;
		}

		$path = $this->base_dir . str_replace( '\\', '/', $relative ) . '.php';

		if ( is_readable( $path ) ) {
			require_once $path;
		}
	}
}