libcryfs/implementations/caching/cache/QueueMap.h

#pragma once
#ifndef MESSMER_BLOCKSTORE_IMPLEMENTATIONS_CACHING_CACHE_QUEUEMAP_H_
#define MESSMER_BLOCKSTORE_IMPLEMENTATIONS_CACHING_CACHE_QUEUEMAP_H_

#include <memory>
#include <unordered_map>
#include <cassert>
#include <boost/optional.hpp>
#include <messmer/cpp-utils/macros.h>
#include <messmer/cpp-utils/assert/assert.h>

namespace blockstore {
namespace caching {

//TODO FreeList for performance
//TODO Single linked list with pointer to last element (for insertion) should be enough for a queue. No double linked list needed.
//     But then, popping arbitrary elements needs to be rewritten so that _removeFromQueue() is _removeSuccessorFromQueue()
//     and the map doesn't store the element itself, but its predecessor. That is, popping might be a bit slower. Test with experiments!

// A class that is a queue and a map at the same time. We could also see it as an addressable queue.
template<class Key, class Value>
class QueueMap final {
public:
  QueueMap(): _entries(), _sentinel(&_sentinel, &_sentinel) {
  }
  ~QueueMap() {
    for (auto &entry : _entries) {
      entry.second.release();
    }
  }

  void push(const Key &key, Value value) {
    auto newEntry = _entries.emplace(std::piecewise_construct, std::forward_as_tuple(key), std::forward_as_tuple(_sentinel.prev, &_sentinel));
    if (!newEntry.second) {
      throw std::logic_error("There is already an element with this key");
    }
    newEntry.first->second.init(&newEntry.first->first, std::move(value));
    //The following is ok, because std::unordered_map never invalidates pointers to its entries
    _sentinel.prev->next = &newEntry.first->second;
    _sentinel.prev = &newEntry.first->second;
  }

  boost::optional<Value> pop(const Key &key) {
    auto found = _entries.find(key);
    if (found == _entries.end()) {
      return boost::none;
    }
    _removeFromQueue(found->second);
    auto value = found->second.release();
    _entries.erase(found);
    return std::move(value);
  }

  boost::optional<Value> pop() {
    if(_sentinel.next == &_sentinel) {
      return boost::none;
    }
    return pop(*_sentinel.next->key);
  }

  boost::optional<const Key &> peekKey() {
    if(_sentinel.next == &_sentinel) {
      return boost::none;
    }
    return *_sentinel.next->key;
  }

  boost::optional<const Value &> peek() {
    if(_sentinel.next == &_sentinel) {
      return boost::none;
    }
    return _sentinel.next->value();
  }

  uint32_t size() const {
    return _entries.size();
  }

private:
  class Entry final {
  public:
    Entry(Entry *prev_, Entry *next_): prev(prev_), next(next_), key(nullptr), __value() {
    }
    void init(const Key *key_, Value value_) {
      key = key_;
      new(__value) Value(std::move(value_));
    }
    Value release() {
      Value value = std::move(*_value());
      _value()->~Value();
      return value;
    }
    const Value &value() {
      return *_value();
    }
    Entry *prev;
    Entry *next;
    const Key *key;
  private:
    Value *_value() {
      return reinterpret_cast<Value*>(__value);
    }
    alignas(Value) char __value[sizeof(Value)];
    DISALLOW_COPY_AND_ASSIGN(Entry);
  };

  void _removeFromQueue(const Entry &entry) {
    entry.prev->next = entry.next;
    entry.next->prev = entry.prev;
  }

  std::unordered_map<Key, Entry> _entries;
  Entry _sentinel;

  DISALLOW_COPY_AND_ASSIGN(QueueMap);
};

}
}

#endif