$this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length 什么意思?

Question

对psr0 psr4不太了解
这是composer里面的一句,

$this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length

请问这句有什么作用

Answer 1

// 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[$search])) {
                    foreach ($this->prefixDirsPsr4[$search] as $dir) {
                        $length = $this->prefixLengthsPsr4[$first][$search];
                        if (file_exists($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
                            return $file;
                        }
                    }
                }
            }
        }

看这段你就明白了，主要是用类全名的第一个字母来做分类，方便后续的快速搜索