LibCore: Add FilePermissionsMask

This class parses UNIX file permissions definitions in numeric (octal)
or symbolic (ugoa+rwx) format and can apply them on a given file mode.

Userland/Libraries/LibCore/FilePermissionsMask.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Xavier Defrang <xavier.defrang@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/String.h>
+#include <sys/stat.h>
+
+namespace Core {
+
+class FilePermissionsMask {
+public:
+    static ErrorOr<FilePermissionsMask> parse(StringView string);
+    static ErrorOr<FilePermissionsMask> from_numeric_notation(StringView string);
+    static ErrorOr<FilePermissionsMask> from_symbolic_notation(StringView string);
+
+    FilePermissionsMask()
+        : m_clear_mask(0)
+        , m_write_mask(0)
+    {
+    }
+
+    FilePermissionsMask& assign_permissions(mode_t mode);
+    FilePermissionsMask& add_permissions(mode_t mode);
+    FilePermissionsMask& remove_permissions(mode_t mode);
+
+    mode_t apply(mode_t mode) const { return m_write_mask | (mode & ~m_clear_mask); }
+    mode_t clear_mask() const { return m_clear_mask; }
+    mode_t write_mask() const { return m_write_mask; }
+
+private:
+    mode_t m_clear_mask; // the bits that will be cleared
+    mode_t m_write_mask; // the bits that will be set
+};
+
+}