export function _getDirectoryName()

in node/internal.ts [815:902]
38 lines of code
13 McCabe index (conditional complexity)

export function _getDirectoryName(p: string): string {
    // short-circuit if empty
    if (!p) {
        return '';
    }

    // normalize separators
    p = _normalizeSeparators(p);

    // on Windows, the goal of this function is to match the behavior of
    // [System.IO.Path]::GetDirectoryName(), e.g.
    //      C:/             =>
    //      C:/hello        => C:\
    //      C:/hello/       => C:\hello
    //      C:/hello/world  => C:\hello
    //      C:/hello/world/ => C:\hello\world
    //      C:              =>
    //      C:hello         => C:
    //      C:hello/        => C:hello
    //      /               =>
    //      /hello          => \
    //      /hello/         => \hello
    //      //hello         =>
    //      //hello/        =>
    //      //hello/world   =>
    //      //hello/world/  => \\hello\world
    //
    // unfortunately, path.dirname() can't simply be used. for example, on Windows
    // it yields different results from Path.GetDirectoryName:
    //      C:/             => C:/
    //      C:/hello        => C:/
    //      C:/hello/       => C:/
    //      C:/hello/world  => C:/hello
    //      C:/hello/world/ => C:/hello
    //      C:              => C:
    //      C:hello         => C:
    //      C:hello/        => C:
    //      /               => /
    //      /hello          => /
    //      /hello/         => /
    //      //hello         => /
    //      //hello/        => /
    //      //hello/world   => //hello/world
    //      //hello/world/  => //hello/world/
    //      //hello/world/again => //hello/world/
    //      //hello/world/again/ => //hello/world/
    //      //hello/world/again/again => //hello/world/again
    //      //hello/world/again/again/ => //hello/world/again
    if (process.platform == 'win32') {
        if (/^[A-Z]:\\?[^\\]+$/i.test(p)) { // e.g. C:\hello or C:hello
            return p.charAt(2) == '\\' ? p.substring(0, 3) : p.substring(0, 2);
        }
        else if (/^[A-Z]:\\?$/i.test(p)) { // e.g. C:\ or C:
            return '';
        }

        let lastSlashIndex = p.lastIndexOf('\\');
        if (lastSlashIndex < 0) { // file name only
            return '';
        }
        else if (p == '\\') { // relative root
            return '';
        }
        else if (lastSlashIndex == 0) { // e.g. \\hello
            return '\\';
        }
        else if (/^\\\\[^\\]+(\\[^\\]*)?$/.test(p)) { // UNC root, e.g. \\hello or \\hello\ or \\hello\world
            return '';
        }

        return p.substring(0, lastSlashIndex);  // e.g. hello\world => hello or hello\world\ => hello\world
        // note, this means trailing slashes for non-root directories
        // (i.e. not C:\, \, or \\unc\) will simply be removed.
    }

    // OSX/Linux
    if (p.indexOf('/') < 0) { // file name only
        return '';
    }
    else if (p == '/') {
        return '';
    }
    else if (_endsWith(p, '/')) {
        return p.substring(0, p.length - 1);
    }

    return path.dirname(p);
}