arangodb/lib/Basics/short_alloc.h

#ifndef SHORT_ALLOC_H
#define SHORT_ALLOC_H

// The MIT License (MIT)
//
// Copyright (c) 2015 Howard Hinnant
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include <cassert>
#include <cstddef>

template <std::size_t N, std::size_t alignment = alignof(std::max_align_t)>
class arena {
  alignas(alignment) char buf_[N];
  char* ptr_;

 public:
  ~arena() { ptr_ = nullptr; }
  arena() noexcept : ptr_(buf_) {}
  arena(const arena&) = delete;
  arena& operator=(const arena&) = delete;

  template <std::size_t ReqAlign>
  char* allocate(std::size_t n);
  void deallocate(char* p, std::size_t n) noexcept;

  static constexpr std::size_t size() noexcept { return N; }
  std::size_t used() const noexcept {
    return static_cast<std::size_t>(ptr_ - buf_);
  }
  void reset() noexcept { ptr_ = buf_; }

 private:
  static std::size_t align_up(std::size_t n) noexcept {
    return (n + (alignment - 1)) & ~(alignment - 1);
  }

  bool pointer_in_buffer(char* p) noexcept {
    return buf_ <= p && p <= buf_ + N;
  }
};

template <std::size_t N, std::size_t alignment>
template <std::size_t ReqAlign>
char* arena<N, alignment>::allocate(std::size_t n) {
  static_assert(ReqAlign <= alignment, "alignment is too small for this arena");
  assert(pointer_in_buffer(ptr_) && "short_alloc has outlived arena");
  auto const aligned_n = align_up(n);
  if (static_cast<decltype(aligned_n)>(buf_ + N - ptr_) >= aligned_n) {
    char* r = ptr_;
    ptr_ += aligned_n;
    return r;
  }

  static_assert(alignment <= alignof(std::max_align_t),
                "you've chosen an "
                "alignment that is larger than alignof(std::max_align_t), and "
                "cannot be guaranteed by normal operator new");
  return static_cast<char*>(::operator new(n));
}

template <std::size_t N, std::size_t alignment>
void arena<N, alignment>::deallocate(char* p, std::size_t n) noexcept {
  assert(pointer_in_buffer(ptr_) && "short_alloc has outlived arena");
  if (pointer_in_buffer(p)) {
    n = align_up(n);
    if (p + n == ptr_) ptr_ = p;
  } else
    ::operator delete(p);
}

template <class T, std::size_t N, std::size_t Align = alignof(std::max_align_t)>
class short_alloc {
 public:
  using value_type = T;
  static auto constexpr alignment = Align;
  static auto constexpr size = N;
  using arena_type = arena<size, alignment>;

 private:
  arena_type& a_;

 public:
  short_alloc(const short_alloc&) = default;
  short_alloc& operator=(const short_alloc&) = delete;

  short_alloc(arena_type& a) noexcept : a_(a) {
    static_assert(size % alignment == 0,
                  "size N needs to be a multiple of alignment Align");
  }
  template <class U>
  short_alloc(const short_alloc<U, N, alignment>& a) noexcept : a_(a.a_) {}

  template <class _Up>
  struct rebind {
    using other = short_alloc<_Up, N, alignment>;
  };

  T* allocate(std::size_t n) {
    return reinterpret_cast<T*>(a_.template allocate<alignof(T)>(n * sizeof(T)));
  }
  void deallocate(T* p, std::size_t n) noexcept {
    a_.deallocate(reinterpret_cast<char*>(p), n * sizeof(T));
  }

  template <class T1, std::size_t N1, std::size_t A1, class U, std::size_t M, std::size_t A2>
  friend bool operator==(const short_alloc<T1, N1, A1>& x,
                         const short_alloc<U, M, A2>& y) noexcept;

  template <class U, std::size_t M, std::size_t A>
  friend class short_alloc;
};

template <class T, std::size_t N, std::size_t A1, class U, std::size_t M, std::size_t A2>
inline bool operator==(const short_alloc<T, N, A1>& x, const short_alloc<U, M, A2>& y) noexcept {
  return N == M && A1 == A2 && &x.a_ == &y.a_;
}

template <class T, std::size_t N, std::size_t A1, class U, std::size_t M, std::size_t A2>
inline bool operator!=(const short_alloc<T, N, A1>& x, const short_alloc<U, M, A2>& y) noexcept {
  return !(x == y);
}

#endif  // SHORT_ALLOC_H