Bug 1452619 - Implement mozilla::IsAsciiAlpha. r=froydnj, a=lizzard

mfbt/TextUtils.h

@@ -0,0 +1,58 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Character/text operations. */
+
+#ifndef mozilla_TextUtils_h
+#define mozilla_TextUtils_h
+
+#include "mozilla/TypeTraits.h"
+
+namespace mozilla {
+
+namespace detail {
+
+template<typename Char>
+class MakeUnsignedChar
+  : public MakeUnsigned<Char>
+{};
+
+template<>
+class MakeUnsignedChar<char16_t>
+{
+public:
+  using Type = char16_t;
+};
+
+template<>
+class MakeUnsignedChar<char32_t>
+{
+public:
+  using Type = char32_t;
+};
+
+} // namespace detail
+
+/**
+ * Returns true iff |aChar| matches [a-zA-Z].
+ *
+ * This function is basically what you thought isalpha was, except its behavior
+ * doesn't depend on the user's current locale.
+ */
+template<typename Char>
+constexpr bool
+IsAsciiAlpha(Char aChar)
+{
+  using UnsignedChar = typename detail::MakeUnsignedChar<Char>::Type;
+  return ('a' <= static_cast<UnsignedChar>(aChar) &&
+          static_cast<UnsignedChar>(aChar) <= 'z') ||
+         ('A' <= static_cast<UnsignedChar>(aChar) &&
+          static_cast<UnsignedChar>(aChar) <= 'Z');
+}
+
+} // namespace mozilla
+
+#endif /* mozilla_TextUtils_h */