Home | History | Annotate | Download | only in strings 
      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 
     12 namespace 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.
     20 template <typename T>
     21 struct 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 
     32 template <typename T>
     33 constexpr 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 
     45 template <typename T>
     46 constexpr 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.
     55 template <>
     56 struct 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 
     63 constexpr 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 
     79 constexpr 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