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diff --git a/xpcom/string/nsASCIIMask.h b/xpcom/string/nsASCIIMask.h
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+/* -*- 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/. */
+
+#ifndef nsASCIIMask_h_
+#define nsASCIIMask_h_
+
+#include <array>
+#include <utility>
+
+#include "mozilla/Attributes.h"
+
+typedef std::array<bool, 128> ASCIIMaskArray;
+
+namespace mozilla {
+
+// Boolean arrays, fixed size and filled in at compile time, meant to
+// record something about each of the (standard) ASCII characters.
+// No extended ASCII for now, there has been no use case.
+// If you have loops that go through a string character by character
+// and test for equality to a certain set of characters before deciding
+// on a course of action, chances are building up one of these arrays
+// and using it is going to be faster, especially if the set of
+// characters is more than one long, and known at compile time.
+class ASCIIMask {
+ public:
+  // Preset masks for some common character groups
+  // When testing, you must check if the index is < 128 or use IsMasked()
+  //
+  // if (someChar < 128 && MaskCRLF()[someChar]) this is \r or \n
+
+  static const ASCIIMaskArray& MaskCRLF();
+  static const ASCIIMaskArray& Mask0to9();
+  static const ASCIIMaskArray& MaskCRLFTab();
+  static const ASCIIMaskArray& MaskWhitespace();
+
+  static MOZ_ALWAYS_INLINE bool IsMasked(const ASCIIMaskArray& aMask,
+                                         uint32_t aChar) {
+    return aChar < 128 && aMask[aChar];
+  }
+};
+
+// Outside of the preset ones, use these templates to create more masks.
+//
+// The example creation will look like this:
+//
+// constexpr bool TestABC(char c) { return c == 'A' || c == 'B' || c == 'C'; }
+// constexpr std::array<bool, 128> sABCMask = CreateASCIIMask(TestABC);
+// ...
+// if (someChar < 128 && sABCMask[someChar]) this is A or B or C
+
+namespace asciimask_details {
+template <typename F, size_t... Indices>
+constexpr std::array<bool, 128> CreateASCIIMask(
+    F fun, std::index_sequence<Indices...>) {
+  return {{fun(Indices)...}};
+}
+}  // namespace asciimask_details
+
+template <typename F>
+constexpr std::array<bool, 128> CreateASCIIMask(F fun) {
+  return asciimask_details::CreateASCIIMask(fun,
+                                            std::make_index_sequence<128>{});
+}
+
+}  // namespace mozilla
+
+#endif  // nsASCIIMask_h_