private function findFileWithExtension($class, $ext)
    {
        // PSR-4 lookup
        $logicalPathPsr4 = strtr($class, '\\', DIRECTORY_SEPARATOR) . $ext;

        $first = $class[0];
        if (isset($this->prefixLengthsPsr4[$first])) {
            $subPath = $class;
            while (false !== $lastPos = strrpos($subPath, '\\')) {
                $subPath = substr($subPath, 0, $lastPos);
                $search = $subPath . '\\';
                if (isset($this->prefixDirsPsr4[$search])) {
                    $pathEnd = DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $lastPos + 1);
                    foreach ($this->prefixDirsPsr4[$search] as $dir) {
                        if (file_exists($file = $dir . $pathEnd)) {
                            return $file;
                        }
                    }
                }
            }
        }

        // PSR-4 fallback dirs
        foreach ($this->fallbackDirsPsr4 as $dir) {
            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr4)) {
                return $file;
            }
        }

        // PSR-0 lookup
        if (false !== $pos = strrpos($class, '\\')) {
            // namespaced class name
            $logicalPathPsr0 = substr($logicalPathPsr4, 0, $pos + 1)
                . strtr(substr($logicalPathPsr4, $pos + 1), '_', DIRECTORY_SEPARATOR);
        } else {
            // PEAR-like class name
            $logicalPathPsr0 = strtr($class, '_', DIRECTORY_SEPARATOR) . $ext;
        }

        if (isset($this->prefixesPsr0[$first])) {
            foreach ($this->prefixesPsr0[$first] as $prefix => $dirs) {
                if (0 === strpos($class, $prefix)) {
                    foreach ($dirs as $dir) {
                        if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
                            return $file;
                        }
                    }
                }
            }
        }

        // PSR-0 fallback dirs
        foreach ($this->fallbackDirsPsr0 as $dir) {
            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
                return $file;
            }
        }

        // PSR-0 include paths.
        if ($this->useIncludePath && $file = stream_resolve_include_path($logicalPathPsr0)) {
            return $file;
        }

        return false;
    }