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93 lines
2.5 KiB
C++
93 lines
2.5 KiB
C++
// Copyright 2018 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_STRINGS_CHAR_TRAITS_H_
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#define BASE_STRINGS_CHAR_TRAITS_H_
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#include <stddef.h>
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#include "base/compiler_specific.h"
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namespace base {
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// constexpr version of http://en.cppreference.com/w/cpp/string/char_traits.
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// This currently just implements the bits needed to support a (mostly)
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// constexpr StringPiece.
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//
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// TODO(dcheng): Once we switch to C++17, most methods will become constexpr and
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// we can switch over to using the one in the standard library.
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template <typename T>
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struct CharTraits {
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// Performs a lexographical comparison of the first N characters of |s1| and
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// |s2|. Returns 0 if equal, -1 if |s1| is less than |s2|, and 1 if |s1| is
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// greater than |s2|.
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static constexpr int compare(const T* s1, const T* s2, size_t n) noexcept;
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// Returns the length of |s|, assuming null termination (and not including the
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// terminating null).
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static constexpr size_t length(const T* s) noexcept;
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};
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template <typename T>
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constexpr int CharTraits<T>::compare(const T* s1,
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const T* s2,
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size_t n) noexcept {
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for (; n; --n, ++s1, ++s2) {
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if (*s1 < *s2)
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return -1;
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if (*s1 > *s2)
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return 1;
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}
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return 0;
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}
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template <typename T>
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constexpr size_t CharTraits<T>::length(const T* s) noexcept {
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size_t i = 0;
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for (; *s; ++s)
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++i;
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return i;
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}
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// char specialization of CharTraits that can use clang's constexpr instrinsics,
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// where available.
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template <>
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struct CharTraits<char> {
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static constexpr int compare(const char* s1,
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const char* s2,
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size_t n) noexcept;
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static constexpr size_t length(const char* s) noexcept;
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};
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constexpr int CharTraits<char>::compare(const char* s1,
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const char* s2,
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size_t n) noexcept {
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#if HAS_FEATURE(cxx_constexpr_string_builtins)
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return __builtin_memcmp(s1, s2, n);
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#else
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for (; n; --n, ++s1, ++s2) {
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if (*s1 < *s2)
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return -1;
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if (*s1 > *s2)
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return 1;
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}
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return 0;
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#endif
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}
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constexpr size_t CharTraits<char>::length(const char* s) noexcept {
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#if defined(__clang__)
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return __builtin_strlen(s);
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#else
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size_t i = 0;
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for (; *s; ++s)
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++i;
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return i;
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#endif
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}
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} // namespace base
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#endif // BASE_STRINGS_CHAR_TRAITS_H_
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