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Pods/abseil/absl/strings/internal/resize_uninitialized.h

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+//
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
+#define ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
+
+#include <algorithm>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/port.h"
+#include "absl/meta/type_traits.h"  //  for void_t
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+// In this type trait, we look for a __resize_default_init member function, and
+// we use it if available, otherwise, we use resize. We provide HasMember to
+// indicate whether __resize_default_init is present.
+template <typename string_type, typename = void>
+struct ResizeUninitializedTraits {
+  using HasMember = std::false_type;
+  static void Resize(string_type* s, size_t new_size) { s->resize(new_size); }
+};
+
+// __resize_default_init is provided by libc++ >= 8.0
+template <typename string_type>
+struct ResizeUninitializedTraits<
+    string_type, absl::void_t<decltype(std::declval<string_type&>()
+                                           .__resize_default_init(237))> > {
+  using HasMember = std::true_type;
+  static void Resize(string_type* s, size_t new_size) {
+    s->__resize_default_init(new_size);
+  }
+};
+
+// Returns true if the std::string implementation supports a resize where
+// the new characters added to the std::string are left untouched.
+//
+// (A better name might be "STLStringSupportsUninitializedResize", alluding to
+// the previous function.)
+template <typename string_type>
+inline constexpr bool STLStringSupportsNontrashingResize(string_type*) {
+  return ResizeUninitializedTraits<string_type>::HasMember::value;
+}
+
+// Like str->resize(new_size), except any new characters added to "*str" as a
+// result of resizing may be left uninitialized, rather than being filled with
+// '0' bytes. Typically used when code is then going to overwrite the backing
+// store of the std::string with known data.
+template <typename string_type, typename = void>
+inline void STLStringResizeUninitialized(string_type* s, size_t new_size) {
+  ResizeUninitializedTraits<string_type>::Resize(s, new_size);
+}
+
+// Used to ensure exponential growth so that the amortized complexity of
+// increasing the string size by a small amount is O(1), in contrast to
+// O(str->size()) in the case of precise growth.
+template <typename string_type>
+void STLStringReserveAmortized(string_type* s, size_t new_size) {
+  const size_t cap = s->capacity();
+  if (new_size > cap) {
+    // Make sure to always grow by at least a factor of 2x.
+    s->reserve((std::max)(new_size, 2 * cap));
+  }
+}
+
+// In this type trait, we look for an __append_default_init member function, and
+// we use it if available, otherwise, we use append.
+template <typename string_type, typename = void>
+struct AppendUninitializedTraits {
+  static void Append(string_type* s, size_t n) {
+    s->append(n, typename string_type::value_type());
+  }
+};
+
+template <typename string_type>
+struct AppendUninitializedTraits<
+    string_type, absl::void_t<decltype(std::declval<string_type&>()
+                                           .__append_default_init(237))> > {
+  static void Append(string_type* s, size_t n) {
+    s->__append_default_init(n);
+  }
+};
+
+// Like STLStringResizeUninitialized(str, new_size), except guaranteed to use
+// exponential growth so that the amortized complexity of increasing the string
+// size by a small amount is O(1), in contrast to O(str->size()) in the case of
+// precise growth.
+template <typename string_type>
+void STLStringResizeUninitializedAmortized(string_type* s, size_t new_size) {
+  const size_t size = s->size();
+  if (new_size > size) {
+    AppendUninitializedTraits<string_type>::Append(s, new_size - size);
+  } else {
+    s->erase(new_size);
+  }
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_