为什么switch（true）的NPath复杂度小于if（）elseif（）？

Question

我有这个函数负责将文件名和mime-type转换成更“人性化”的东西（例如file.png，image / png到[Image，PNG]）。我发现有趣的是if() elseif()语句组的NPath复杂度高于switch(true)语句。

使用以下代码，PHP Mess Detector输出一个4410的NPath：

public function humanKind()
{
    $typeRA = explode("/", strtolower($this->type));
    $fileRA = explode(".", $this->name);
    $fileType = strtoupper($fileRA[count($fileRA) - 1]);

    switch($typeRA[0]) {
        case "image":
            $humanType = "Image";
            break;
        case "video":
            $humanType = "Video";
            break;
        case "audio":
            $humanType = "Sound";
            break;
        case "font":
            $humanType = "Font";
            break;
        default:
            $humanType = "File";
    }

    switch ($this->type) {
        case "application/msword":
        case "application/pdf":
        case "applicaiton/wordperfect":
        case "text/plain":
        case "text/rtf":
        case "image/vnd.photoshop":
        case "image/psd":
        case "image/vnd.adobe.photoshop":
        case "image/x-photoshop":
        case "application/xml":
        case "application/x-mspublisher":
        case "text/html":
        case "application/xhtml+xml":
        case "text/richtext":
        case "application/rtf":
        case "application/x-iwork-pages-sffpages":
        case "application/vnd.apple.pages":
            $humanType = "Document";
            break;
        case "application/vnd.ms-excel":
        case "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet":
        case "application/x-iwork-numbers-sffnumbers":
        case "application/vnd.apple.numbers":
            $humanType = "Spreadsheet";
            break;
        case "application/vnd.ms-powerpoint":
        case "application/vnd.openxmlformats-officedocument.presentationml.presentation":
        case "application/vnd.openxmlformats-officedocument.presentationml.slideshow":
        case "application/x-iwork-keynote-sffkey":
        case "application/vnd.apple.keynote":
            $humanType = "Slideshow";
            break;
        case "application/zip":
        case "application/x-zip-compressed":
        case "application/x-compressed":
        case "application/x-compress":
        case "application/x-rar-compressed":
        case "applicaiton/x-7z-compressed":
        case "application/x-ace-compressed":
            $humanType = "Archive";
            break;
        case "text/x-vcard":
        case "text/x-ms-contact":
            $humanType = "Contact";
            break;
        case "text/x-php":
        case "application/x-dosexec":
        case "application/x-xpinstall":
        case "application/x-opera-extension":
        case "application/x-chrome-extension":
        case "application/x-perl":
        case "application/x-shockwave-flash":
        case "application/java-archive":
            $humanType = "Program";
            break;
        case "application/vnd.ms-fontobject":
        case "application/font-woff":
        case "application/x-font-truetype":
        case "application/x-font-opentype":
        case "application/x-font-ttf":
        case "application/font-sfnt":
            $humanType = "Font";
            break;
    }

    // Special Cases
    if ($humanType == "Archive" && $fileType == "APK") { // Android App
        $humanType = "App";
    } elseif ($humanType == "Archive" && $fileType == "XPS") {
        $humanType = "Document";
    } elseif ($this->type == "application/xml" && $fileType == "CONTACT") {
        $humanType = "Contact";
    } elseif ($this->type == "application/octet-stream" && $fileType == "JNT") {
        $humanType = "Document";
    }

    if (strlen($fileType) > 4) {
        $fileType = "";
    }

    return array($humanType, $fileType);

如果我们用以下内容替换特殊情况if elseif：

    // Special Cases
    switch(true) {
        case ($humanType == "Archive" && $fileType == "APK"): // Android App
            $humanType = "App";
            break;
        case ($humanType == "Archive" && $fileType == "XPS"):
            $humanType = "Document";
            break;
        case ($this->type == "application/xml" && $fileType == "CONTACT"):
            $humanType = "Contact";
            break;
        case ($this->type == "application/octet-stream" && $fileType == "JNT"):
            $humanType = "Document";
            break;
    }

PHP Mess Detector报告了1960年的NPath复杂性。

这是为什么？是什么让switch（true）不像我看来的那样复杂，只是控制结构相同？

Answer 1

由于NPath复杂性衡量的是完成coverage of your code所需的单元测试次数，应该在您的2个“特殊情况”实现之间没有区别。

但计算方面存在一些差异。让我们逐步完成2个“特殊情况”实现并手动计算NPath复杂度：

与if .. elseif ..

的NPath复杂性

if ($humanType == "Archive" && $fileType == "APK") { // Android App
    $humanType = "App";
} 
elseif ($humanType == "Archive" && $fileType == "XPS") {
    $humanType = "Document";
} 
elseif ($this->type == "application/xml" && $fileType == "CONTACT") {
    $humanType = "Contact";
} 
elseif ($this->type == "application/octet-stream" && $fileType == "JNT") {
    $humanType = "Document";
}

此声明导致 9 的NPath复杂度：if .. else为1点，每if(expr)为1点，每&&运算符为1点。 （1 + 4 + 4 = 9）

与switch(true)

的NPath复杂性

switch(true) {
    case ($humanType == "Archive" && $fileType == "APK"): // Android App
        $humanType = "App";
        break;
    case ($humanType == "Archive" && $fileType == "XPS"):
        $humanType = "Document";
        break;
    case ($this->type == "application/xml" && $fileType == "CONTACT"):
        $humanType = "Contact";
        break;
    case ($this->type == "application/octet-stream" && $fileType == "JNT"):
        $humanType = "Document";
        break;
}

此语句导致switch(true)的NPath复杂度仅为 4 ：0分，因为它不包含&&或||个运算符，每个运算符包含1个点case标签。 （0 + 4 = 4）

您的humanKind函数

的NPath复杂性

计算每个语句的NPath值，并将值乘以。没有“特殊情况”语句的函数的NPath复杂度是490.乘以if .. else if ..语句9的NPath值得到4410的NPath复杂度。并乘以{{1的NPath值4，你的复杂性只有1960年。这就是全部！

现在我们知道了： NPath复杂度不会衡量switch(true)语句中case标签的表达复杂性！

Answer 2

通常，switch可能比if / elseif更快，因为switch语句只评估一次条件，然后与每种情况进行比较。

据我所知，switch语句中的情况是内部索引的，因此你可能会因此而获得更好的性能（虽然我找不到原来的文章谈论这个，因此我无法证明它。）

我还想象一下，switch语句的AST比等效的if / elseif更简单。

编辑：

在基于C语言（很可能是其他语言）中，当switch语句变得超过4-5个案例时，它们将被实现为列表/哈希表。这意味着每个项目的访问时间变得相同。而在if / elseif块中，没有这样的优化。

编译器可以更轻松地处理这些类型的switch语句，因为它可以对不同的条件做出更多假设。因此，复杂性较低。任意情况的查找是O（1）。这再次链接到我之前关于如何更简单地转换AST的声明。

编辑＃2：

在更多CS术语中，编译器可以使用分支表（或跳转）来减少switch语句的cpu时间：http://en.wikipedia.org/wiki/Branch_table