1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
|
//
// 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 Y_ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
#define Y_ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
#include <algorithm>
#include <util/generic/string.h>
#include <type_traits>
#include <utility>
#include "y_absl/base/port.h"
#include "y_absl/meta/type_traits.h" // for void_t
namespace y_absl {
Y_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, y_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);
}
};
template <typename string_type>
struct ResizeUninitializedTraits<
string_type, y_absl::void_t<decltype(std::declval<string_type&>()
.ReserveAndResize(237))> > {
using HasMember = std::true_type;
static void Resize(string_type* s, size_t new_size) {
s->ReserveAndResize(new_size);
}
};
// Returns true if the TString implementation supports a resize where
// the new characters added to the TString 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 TString 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, y_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);
}
};
template <typename string_type>
struct AppendUninitializedTraits<
string_type, y_absl::void_t<decltype(std::declval<string_type&>()
.ReserveAndResize(237))> > {
static void Append(string_type* s, size_t n) {
s->ReserveAndResize(s->size() + 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
Y_ABSL_NAMESPACE_END
} // namespace y_absl
#endif // Y_ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
|
