/*
 * Copyright 2015 Facebook, Inc.
 *
 * 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
 *
 *   http://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 FOLLY_ARENA_H_
#define FOLLY_ARENA_H_

#include <cassert>
#include <limits>
#include <stdexcept>
#include <utility>
#include <boost/intrusive/slist.hpp>

#include <folly/Conv.h>
#include <folly/Likely.h>
#include <folly/Malloc.h>
#include <folly/Memory.h>

namespace folly {

/**
 * Simple arena: allocate memory which gets freed when the arena gets
 * destroyed.
 *
 * The arena itself allocates memory using a custom allocator which provides
 * the following interface (same as required by StlAllocator in StlAllocator.h)
 *
 *   void* allocate(size_t size);
 *      Allocate a block of size bytes, properly aligned to the maximum
 *      alignment required on your system; throw std::bad_alloc if the
 *      allocation can't be satisfied.
 *
 *   void deallocate(void* ptr);
 *      Deallocate a previously allocated block.
 *
 * You may also specialize ArenaAllocatorTraits for your allocator type to
 * provide:
 *
 *   size_t goodSize(const Allocator& alloc, size_t size) const;
 *      Return a size (>= the provided size) that is considered "good" for your
 *      allocator (for example, if your allocator allocates memory in 4MB
 *      chunks, size should be rounded up to 4MB).  The provided value is
 *      guaranteed to be rounded up to a multiple of the maximum alignment
 *      required on your system; the returned value must be also.
 *
 * An implementation that uses malloc() / free() is defined below, see SysArena.
 */
template <class Alloc> struct ArenaAllocatorTraits;
template <class Alloc>
class Arena {
 public:
  explicit Arena(const Alloc& alloc,
                 size_t minBlockSize = kDefaultMinBlockSize,
                 size_t sizeLimit = kNoSizeLimit,
                 size_t maxAlign = kDefaultMaxAlign)
    : allocAndSize_(alloc, minBlockSize)
    , ptr_(nullptr)
    , end_(nullptr)
    , totalAllocatedSize_(0)
    , bytesUsed_(0)
    , sizeLimit_(sizeLimit)
    , maxAlign_(maxAlign) {
    if ((maxAlign_ & (maxAlign_ - 1)) || maxAlign_ > alignof(Block)) {
      throw std::invalid_argument(
          folly::to<std::string>("Invalid maxAlign: ", maxAlign_));
    }
  }

  ~Arena();

  void* allocate(size_t size) {
    size = roundUp(size);
    bytesUsed_ += size;

    assert(ptr_ <= end_);
    if (LIKELY((size_t)(end_ - ptr_) >= size)) {
      // Fast path: there's enough room in the current block
      char* r = ptr_;
      ptr_ += size;
      assert(isAligned(r));
      return r;
    }

    // Not enough room in the current block
    void* r = allocateSlow(size);
    assert(isAligned(r));
    return r;
  }

  void deallocate(void* p) {
    // Deallocate? Never!
  }

  // Transfer ownership of all memory allocated from "other" to "this".
  void merge(Arena&& other);

  // Gets the total memory used by the arena
  size_t totalSize() const {
    return totalAllocatedSize_ + sizeof(Arena);
  }

  // Gets the total number of "used" bytes, i.e. bytes that the arena users
  // allocated via the calls to `allocate`. Doesn't include fragmentation, e.g.
  // if block size is 4KB and you allocate 2 objects of 3KB in size,
  // `bytesUsed()` will be 6KB, while `totalSize()` will be 8KB+.
  size_t bytesUsed() const {
    return bytesUsed_;
  }

 private:
  // not copyable
  Arena(const Arena&) = delete;
  Arena& operator=(const Arena&) = delete;

  // movable
  Arena(Arena&&) = default;
  Arena& operator=(Arena&&) = default;

  struct Block;
  typedef boost::intrusive::slist_member_hook<
    boost::intrusive::tag<Arena>> BlockLink;

  struct Block {
    BlockLink link;

    // Allocate a block with at least size bytes of storage.
    // If allowSlack is true, allocate more than size bytes if convenient
    // (via ArenaAllocatorTraits::goodSize()) as we'll try to pack small
    // allocations in this block.
    static std::pair<Block*, size_t> allocate(
        Alloc& alloc, size_t size, bool allowSlack);
    void deallocate(Alloc& alloc);

    char* start() {
      return reinterpret_cast<char*>(this + 1);
    }

   private:
    Block() { }
    ~Block() { }
  } __attribute__((__aligned__));
  // This should be alignas(std::max_align_t) but neither alignas nor
  // max_align_t are supported by gcc 4.6.2.

 public:
  static constexpr size_t kDefaultMinBlockSize = 4096 - sizeof(Block);
  static constexpr size_t kNoSizeLimit = 0;
  static constexpr size_t kDefaultMaxAlign = alignof(Block);
  static constexpr size_t kBlockOverhead = sizeof(Block);

 private:
  bool isAligned(uintptr_t address) const {
    return (address & (maxAlign_ - 1)) == 0;
  }
  bool isAligned(void* p) const {
    return isAligned(reinterpret_cast<uintptr_t>(p));
  }

  // Round up size so it's properly aligned
  size_t roundUp(size_t size) const {
    return (size + maxAlign_ - 1) & ~(maxAlign_ - 1);
  }

  // cache_last<true> makes the list keep a pointer to the last element, so we
  // have push_back() and constant time splice_after()
  typedef boost::intrusive::slist<
    Block,
    boost::intrusive::member_hook<Block, BlockLink, &Block::link>,
    boost::intrusive::constant_time_size<false>,
    boost::intrusive::cache_last<true>> BlockList;

  void* allocateSlow(size_t size);

  // Empty member optimization: package Alloc with a non-empty member
  // in case Alloc is empty (as it is in the case of SysAlloc).
  struct AllocAndSize : public Alloc {
    explicit AllocAndSize(const Alloc& a, size_t s)
      : Alloc(a), minBlockSize(s) {
    }

    size_t minBlockSize;
  };

  size_t minBlockSize() const {
    return allocAndSize_.minBlockSize;
  }
  Alloc& alloc() { return allocAndSize_; }
  const Alloc& alloc() const { return allocAndSize_; }

  AllocAndSize allocAndSize_;
  BlockList blocks_;
  char* ptr_;
  char* end_;
  size_t totalAllocatedSize_;
  size_t bytesUsed_;
  const size_t sizeLimit_;
  const size_t maxAlign_;
};

template <class Alloc>
struct IsArenaAllocator<Arena<Alloc>> : std::true_type { };

/**
 * By default, don't pad the given size.
 */
template <class Alloc>
struct ArenaAllocatorTraits {
  static size_t goodSize(const Alloc& alloc, size_t size) {
    return size;
  }
};

template <>
struct ArenaAllocatorTraits<SysAlloc> {
  static size_t goodSize(const SysAlloc& alloc, size_t size) {
    return goodMallocSize(size);
  }
};

/**
 * Arena that uses the system allocator (malloc / free)
 */
class SysArena : public Arena<SysAlloc> {
 public:
  explicit SysArena(size_t minBlockSize = kDefaultMinBlockSize,
                    size_t sizeLimit = kNoSizeLimit,
                    size_t maxAlign = kDefaultMaxAlign)
    : Arena<SysAlloc>(SysAlloc(), minBlockSize, sizeLimit, maxAlign) {
  }
};

template <>
struct IsArenaAllocator<SysArena> : std::true_type { };

}  // namespace folly

#include <folly/Arena-inl.h>

#endif /* FOLLY_ARENA_H_ */
