1 files changed, 77 insertions, 0 deletions
diff --git a/js/src/builtin/intl/StringAsciiChars.h b/js/src/builtin/intl/StringAsciiChars.h
new file mode 100644
index 0000000000..3323544d8c
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+++ b/js/src/builtin/intl/StringAsciiChars.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 builtin_intl_StringAsciiChars_h
+#define builtin_intl_StringAsciiChars_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+
+#include <stddef.h>
+
+#include "js/GCAPI.h"
+#include "js/TypeDecls.h"
+#include "js/Vector.h"
+
+#include "vm/StringType.h"
+
+namespace js::intl {
+
+/**
+ * String view of an ASCII-only string.
+ *
+ * This holds a reference to a JSLinearString and can produce a string view
+ * into that string. If the string is represented by Latin1 characters, the
+ * span is returned directly. If the string is represented by UTF-16
+ * characters, it copies the char16_t characters into a char array, and then
+ * returns a span based on the copy.
+ *
+ * This allows us to avoid copying for the common use case that the ASCII
+ * characters are represented in Latin1.
+ */
+class MOZ_STACK_CLASS StringAsciiChars final {
+  // When copying string characters, use this many bytes of inline storage.
+  static const size_t InlineCapacity = 24;
+
+  JS::AutoCheckCannotGC nogc_;
+
+  JSLinearString* str_;
+
+  mozilla::Maybe<Vector<Latin1Char, InlineCapacity>> ownChars_;
+
+ public:
+  explicit StringAsciiChars(JSLinearString* str) : str_(str) {
+    MOZ_ASSERT(StringIsAscii(str));
+  }
+
+  operator mozilla::Span<const char>() const {
+    if (str_->hasLatin1Chars()) {
+      return mozilla::AsChars(str_->latin1Range(nogc_));
+    }
+    return mozilla::AsChars(mozilla::Span<const Latin1Char>(*ownChars_));
+  }
+
+  [[nodiscard]] bool init(JSContext* cx) {
+    if (str_->hasLatin1Chars()) {
+      return true;
+    }
+
+    ownChars_.emplace(cx);
+    if (!ownChars_->resize(str_->length())) {
+      return false;
+    }
+
+    js::CopyChars(ownChars_->begin(), *str_);
+
+    return true;
+  }
+};
+
+}  // namespace js::intl
+
+#endif  // builtin_intl_StringAsciiChars_h