1// Copyright 2018 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_STRINGS_CHAR_TRAITS_H_
6#define BASE_STRINGS_CHAR_TRAITS_H_
7
8#include <stddef.h>
9
10#include "base/compiler_specific.h"
11
12namespace base {
13
14// constexpr version of http://en.cppreference.com/w/cpp/string/char_traits.
15// This currently just implements the bits needed to support a (mostly)
16// constexpr StringPiece.
17//
18// TODO(dcheng): Once we switch to C++17, most methods will become constexpr and
19// we can switch over to using the one in the standard library.
20template <typename T>
21struct CharTraits {
22 // Performs a lexographical comparison of the first N characters of |s1| and
23 // |s2|. Returns 0 if equal, -1 if |s1| is less than |s2|, and 1 if |s1| is
24 // greater than |s2|.
25 static constexpr int compare(const T* s1, const T* s2, size_t n) noexcept;
26
27 // Returns the length of |s|, assuming null termination (and not including the
28 // terminating null).
29 static constexpr size_t length(const T* s) noexcept;
30};
31
32template <typename T>
33constexpr int CharTraits<T>::compare(const T* s1,
34 const T* s2,
35 size_t n) noexcept {
36 for (; n; --n, ++s1, ++s2) {
37 if (*s1 < *s2)
38 return -1;
39 if (*s1 > *s2)
40 return 1;
41 }
42 return 0;
43}
44
45template <typename T>
46constexpr size_t CharTraits<T>::length(const T* s) noexcept {
47 size_t i = 0;
48 for (; *s; ++s)
49 ++i;
50 return i;
51}
52
53// char specialization of CharTraits that can use clang's constexpr instrinsics,
54// where available.
55template <>
56struct CharTraits<char> {
57 static constexpr int compare(const char* s1,
58 const char* s2,
59 size_t n) noexcept;
60 static constexpr size_t length(const char* s) noexcept;
61};
62
63constexpr int CharTraits<char>::compare(const char* s1,
64 const char* s2,
65 size_t n) noexcept {
66#if HAS_FEATURE(cxx_constexpr_string_builtins)
67 return __builtin_memcmp(s1, s2, n);
68#else
69 for (; n; --n, ++s1, ++s2) {
70 if (*s1 < *s2)
71 return -1;
72 if (*s1 > *s2)
73 return 1;
74 }
75 return 0;
76#endif
77}
78
79constexpr size_t CharTraits<char>::length(const char* s) noexcept {
80#if defined(__clang__)
81 return __builtin_strlen(s);
82#else
83 size_t i = 0;
84 for (; *s; ++s)
85 ++i;
86 return i;
87#endif
88}
89
90} // namespace base
91
92#endif // BASE_STRINGS_CHAR_TRAITS_H_
93