harris_michael_hash_map.hpp

//
// Copyright (c) 2018-2020 Manuel Pöter.
// Licensed under the MIT License. See LICENSE file in the project root for full license information.
//
 
#ifndef XENIUM_HARRIS_MICHAEL_HASH_MAP_HPP
#define XENIUM_HARRIS_MICHAEL_HASH_MAP_HPP
 
#include <xenium/acquire_guard.hpp>
#include <xenium/backoff.hpp>
#include <xenium/hash.hpp>
#include <xenium/parameter.hpp>
#include <xenium/policy.hpp>
#include <xenium/utils.hpp>
 
#include <atomic>
#include <cassert>
 
namespace xenium {
 
namespace policy {
  template <std::size_t Value>
  struct buckets;
 
  template <class T>
  struct map_to_bucket;
 
  template <bool Value>
  struct memoize_hash;
} // namespace policy
 
template <class Key, class Value, class... Policies>
class harris_michael_hash_map {
public:
  using value_type = std::pair<const Key, Value>;
  using reclaimer = parameter::type_param_t<policy::reclaimer, parameter::nil, Policies...>;
  using hash = parameter::type_param_t<policy::hash, xenium::hash<Key>, Policies...>;
  using map_to_bucket = parameter::type_param_t<policy::map_to_bucket, utils::modulo<std::size_t>, Policies...>;
  using backoff = parameter::type_param_t<policy::backoff, no_backoff, Policies...>;
  static constexpr std::size_t num_buckets =
    parameter::value_param_t<std::size_t, policy::buckets, 512, Policies...>::value;
  static constexpr bool memoize_hash =
    parameter::value_param_t<bool, policy::memoize_hash, !std::is_scalar<Key>::value, Policies...>::value;
 
  template <class... NewPolicies>
  using with = harris_michael_hash_map<Key, Value, NewPolicies..., Policies...>;
 
  static_assert(parameter::is_set<reclaimer>::value, "reclaimer policy must be specified");
 
  class iterator;
  class accessor;
 
  harris_michael_hash_map() = default;
  ~harris_michael_hash_map();
 
  template <class... Args>
  bool emplace(Args&&... args);
 
  template <class... Args>
  std::pair<iterator, bool> emplace_or_get(Args&&... args);
 
  template <class... Args>
  std::pair<iterator, bool> get_or_emplace(Key key, Args&&... args);
 
  template <class Factory>
  std::pair<iterator, bool> get_or_emplace_lazy(Key key, Factory factory);
 
  bool erase(const Key& key);
 
  iterator erase(iterator pos);
 
  iterator find(const Key& key);
 
  bool contains(const Key& key);
 
  accessor operator[](const Key& key);
 
  iterator begin();
 
  iterator end();
 
private:
  struct node;
  using hash_t = std::size_t;
  using concurrent_ptr = typename reclaimer::template concurrent_ptr<node, 1>;
  using marked_ptr = typename concurrent_ptr::marked_ptr;
  using guard_ptr = typename concurrent_ptr::guard_ptr;
 
  template <typename Factory>
  std::pair<iterator, bool> do_get_or_emplace_lazy(Key key, Factory node_factory);
 
  struct construct_without_hash {};
 
  struct data_without_hash {
    value_type value;
    template <class... Args>
    explicit data_without_hash(hash_t /*hash*/, Args&&... args) : value(std::forward<Args>(args)...) {}
    template <class... Args>
    explicit data_without_hash(construct_without_hash, Args&&... args) : value(std::forward<Args>(args)...) {}
    [[nodiscard]] hash_t get_hash() const { return hash{}(value.first); }
    [[nodiscard]] bool greater_or_equal(hash_t /*h*/, const Key& key) const { return value.first >= key; }
  };
 
  struct data_with_hash {
    hash_t hash;
    value_type value;
    template <class... Args>
    explicit data_with_hash(hash_t hash, Args&&... args) : hash(hash), value(std::forward<Args>(args)...) {}
    template <class... Args>
    explicit data_with_hash(construct_without_hash, Args&&... args) : value(std::forward<Args>(args)...) {
      hash = harris_michael_hash_map::hash{}(value.first);
    }
    [[nodiscard]] hash_t get_hash() const { return hash; }
    [[nodiscard]] bool greater_or_equal(hash_t h, const Key& key) const { return hash >= h && value.first >= key; }
  };
 
  using data_t = std::conditional_t<memoize_hash, data_with_hash, data_without_hash>;
 
  struct node : reclaimer::template enable_concurrent_ptr<node, 1> {
    data_t data;
    concurrent_ptr next;
    template <class... Args>
    explicit node(Args&&... args) : data(std::forward<Args>(args)...), next() {}
  };
 
  struct find_info {
    concurrent_ptr* prev;
    marked_ptr next{};
    guard_ptr cur{};
    guard_ptr save{};
  };
 
  bool find(hash_t hash, const Key& key, std::size_t bucket, find_info& info, backoff& backoff);
 
  concurrent_ptr buckets[num_buckets];
};
 
template <class Key, class Value, class... Policies>
class harris_michael_hash_map<Key, Value, Policies...>::iterator {
public:
  using iterator_category = std::forward_iterator_tag;
  using value_type = harris_michael_hash_map::value_type;
  using difference_type = std::ptrdiff_t;
  using pointer = value_type*;
  using reference = value_type&;
 
  iterator(iterator&&) = default;
  iterator(const iterator&) = default;
 
  iterator& operator=(iterator&&) = default;
  iterator& operator=(const iterator&) = default;
 
  iterator& operator++() {
    assert(info.cur.get() != nullptr);
    auto next = info.cur->next.load(std::memory_order_relaxed);
    guard_ptr tmp_guard;
    // (1) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
    if (next.mark() == 0 && tmp_guard.acquire_if_equal(info.cur->next, next, std::memory_order_acquire)) {
      info.prev = &info.cur->next;
      info.save = std::move(info.cur);
      info.cur = std::move(tmp_guard);
    } else {
      // cur is marked for removal
      // -> use find to remove it and get to the next node with a key >= cur->key
      // Note: we have to copy key here!
      Key key = info.cur->data.value.first;
      hash_t h = info.cur->data.get_hash();
      backoff backoff;
      map->find(h, key, bucket, info, backoff);
    }
    assert(info.prev == &map->buckets[bucket] || info.cur.get() == nullptr ||
           (info.save.get() != nullptr && &info.save->next == info.prev));
 
    if (!info.cur) {
      move_to_next_bucket();
    }
 
    return *this;
  }
  iterator operator++(int) {
    iterator retval = *this;
    ++(*this);
    return retval;
  }
  bool operator==(const iterator& other) const { return info.cur.get() == other.info.cur.get(); }
  bool operator!=(const iterator& other) const { return !(*this == other); }
  reference operator*() const noexcept { return info.cur->data.value; }
  pointer operator->() const noexcept { return &info.cur->data.value; }
 
  void reset() {
    bucket = num_buckets;
    info.prev = nullptr;
    info.cur.reset();
    info.save.reset();
  }
 
private:
  friend harris_michael_hash_map;
 
  explicit iterator(harris_michael_hash_map* map) : map(map), bucket(num_buckets) {}
 
  explicit iterator(harris_michael_hash_map* map, std::size_t bucket) : map(map), bucket(bucket) {
    info.prev = &map->buckets[bucket];
    // (2) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
    info.cur.acquire(*info.prev, std::memory_order_acquire);
 
    if (!info.cur) {
      move_to_next_bucket();
    }
  }
 
  explicit iterator(harris_michael_hash_map* map, std::size_t bucket, find_info&& info) :
      map(map),
      bucket(bucket),
      info(std::move(info)) {}
 
  void move_to_next_bucket() {
    info.save.reset();
    while (!info.cur && bucket < num_buckets - 1) {
      ++bucket;
      info.prev = &map->buckets[bucket];
      // (3) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
      info.cur.acquire(*info.prev, std::memory_order_acquire);
    }
  }
 
  harris_michael_hash_map* map;
  std::size_t bucket;
  find_info info;
};
 
template <class Key, class Value, class... Policies>
class harris_michael_hash_map<Key, Value, Policies...>::accessor {
public:
  Value* operator->() const noexcept { return &guard->data.value.second; }
  Value& operator*() const noexcept { return guard->data.value.second; }
  void reset() { guard.reset(); }
 
private:
  explicit accessor(guard_ptr&& guard) : guard(std::move(guard)) {}
  guard_ptr guard;
  friend harris_michael_hash_map;
};
 
template <class Key, class Value, class... Policies>
harris_michael_hash_map<Key, Value, Policies...>::~harris_michael_hash_map() {
  for (std::size_t i = 0; i < num_buckets; ++i) {
    // (4) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
    auto p = buckets[i].load(std::memory_order_acquire);
    while (p) {
      // (5) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
      auto next = p->next.load(std::memory_order_acquire);
      delete p.get();
      p = next;
    }
  }
}
 
template <class Key, class Value, class... Policies>
bool harris_michael_hash_map<Key, Value, Policies...>::find(hash_t hash,
                                                            const Key& key,
                                                            std::size_t bucket,
                                                            find_info& info,
                                                            backoff& backoff) {
  auto& head = buckets[bucket];
  assert((info.save == nullptr && info.prev == &head) || &info.save->next == info.prev);
  concurrent_ptr* start = info.prev;
  guard_ptr start_guard = info.save; // we have to keep a guard_ptr to prevent start's node from getting reclaimed.
retry:
  info.prev = start;
  info.save = start_guard;
  info.next = info.prev->load(std::memory_order_relaxed);
  if (info.next.mark() != 0) {
    // our start node is marked for removal -> we have to restart from head
    start = &head;
    start_guard.reset();
    goto retry;
  }
 
  for (;;) {
    // (6) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
    if (!info.cur.acquire_if_equal(*info.prev, info.next, std::memory_order_acquire)) {
      goto retry;
    }
 
    if (!info.cur) {
      return false;
    }
 
    info.next = info.cur->next.load(std::memory_order_relaxed);
    if (info.next.mark() != 0) {
      // Node *cur is marked for deletion -> update the link and retire the element
 
      // (7) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
      info.next = info.cur->next.load(std::memory_order_acquire).get();
 
      // Try to splice out node
      marked_ptr expected = info.cur.get();
      // (8) - this release-CAS synchronizes with the acquire-load (1, 2, 3, 4, 5, 6, 7, 13)
      //       and the acquire-CAS (11, 14)
      //       it is the head of a potential release sequence containing (11, 14)
      if (!info.prev->compare_exchange_weak(
            expected, info.next, std::memory_order_release, std::memory_order_relaxed)) {
        backoff();
        goto retry;
      }
      info.cur.reclaim();
    } else {
      if (info.prev->load(std::memory_order_relaxed) != info.cur.get()) {
        goto retry; // cur might be cut from the hash_map.
      }
 
      const auto& data = info.cur->data;
      if (data.greater_or_equal(hash, key)) {
        return data.value.first == key;
      }
 
      info.prev = &info.cur->next;
      std::swap(info.save, info.cur);
    }
  }
}
 
template <class Key, class Value, class... Policies>
bool harris_michael_hash_map<Key, Value, Policies...>::contains(const Key& key) {
  auto h = hash{}(key);
  auto bucket = map_to_bucket{}(h, num_buckets);
  find_info info{&buckets[bucket]};
  backoff backoff;
  return find(h, key, bucket, info, backoff);
}
 
template <class Key, class Value, class... Policies>
auto harris_michael_hash_map<Key, Value, Policies...>::find(const Key& key) -> iterator {
  auto h = hash{}(key);
  auto bucket = map_to_bucket{}(h, num_buckets);
  find_info info{&buckets[bucket]};
  backoff backoff;
  if (find(h, key, bucket, info, backoff)) {
    return iterator(this, bucket, std::move(info));
  }
  return end();
}
 
template <class Key, class Value, class... Policies>
template <class... Args>
bool harris_michael_hash_map<Key, Value, Policies...>::emplace(Args&&... args) {
  auto result = emplace_or_get(std::forward<Args>(args)...);
  return result.second;
}
 
template <class Key, class Value, class... Policies>
template <class... Args>
auto harris_michael_hash_map<Key, Value, Policies...>::get_or_emplace(Key key, Args&&... args)
  -> std::pair<iterator, bool> {
  return do_get_or_emplace_lazy(std::move(key), [&args...](hash_t hash, Key key) {
    return new node(hash,
                    std::piecewise_construct,
                    std::forward_as_tuple(std::move(key)),
                    std::forward_as_tuple(std::forward<Args>(args)...));
  });
}
 
template <class Key, class Value, class... Policies>
template <typename Factory>
auto harris_michael_hash_map<Key, Value, Policies...>::get_or_emplace_lazy(Key key, Factory value_factory)
  -> std::pair<iterator, bool> {
  return do_get_or_emplace_lazy(
    std::move(key), [&value_factory](hash_t hash, Key key) { return new node(hash, std::move(key), value_factory()); });
}
 
template <class Key, class Value, class... Policies>
template <typename Factory>
auto harris_michael_hash_map<Key, Value, Policies...>::do_get_or_emplace_lazy(Key key, Factory node_factory)
  -> std::pair<iterator, bool> {
  node* n = nullptr;
  auto h = hash{}(key);
  auto bucket = map_to_bucket{}(h, num_buckets);
 
  const Key* pkey = &key;
  find_info info{&buckets[bucket]};
  backoff backoff;
  for (;;) {
    if (find(h, *pkey, bucket, info, backoff)) {
      delete n;
      return {iterator(this, bucket, std::move(info)), false};
    }
    if (n == nullptr) {
      n = node_factory(h, std::move(key)); // NOLINT (use-after-move)
      pkey = &n->data.value.first;
    }
 
    // Try to install new node
    marked_ptr cur = info.cur.get();
    info.cur.reset();
    info.cur = guard_ptr(n);
    n->next.store(cur, std::memory_order_relaxed);
 
    // (9) - this release-CAS synchronizes with the acquire-load (1, 2, 3, 4, 5, 6, 7, 13)
    //       and the acquire-CAS (11, 14)
    //       it is the head of a potential release sequence containing (11, 14)
    if (info.prev->compare_exchange_weak(cur, n, std::memory_order_release, std::memory_order_relaxed)) {
      return {iterator(this, bucket, std::move(info)), true};
    }
 
    backoff();
  }
}
 
template <class Key, class Value, class... Policies>
template <class... Args>
auto harris_michael_hash_map<Key, Value, Policies...>::emplace_or_get(Args&&... args) -> std::pair<iterator, bool> {
  node* n = new node(construct_without_hash{}, std::forward<Args>(args)...);
 
  auto h = n->data.get_hash();
  auto bucket = map_to_bucket{}(h, num_buckets);
 
  find_info info{&buckets[bucket]};
  backoff backoff;
  for (;;) {
    if (find(h, n->data.value.first, bucket, info, backoff)) {
      delete n;
      return {iterator(this, bucket, std::move(info)), false};
    }
    // Try to install new node
    marked_ptr expected = info.cur.get();
    n->next.store(expected, std::memory_order_relaxed);
    guard_ptr new_guard(n);
 
    // (10) - this release-CAS synchronizes with the acquire-load (1, 2, 3, 4, 5, 6, 7, 13)
    //        and the acquire-CAS (11, 14)
    //        it is the head of a potential release sequence containing (11, 14)
    if (info.prev->compare_exchange_weak(expected, n, std::memory_order_release, std::memory_order_relaxed)) {
      info.cur = std::move(new_guard);
      return {iterator(this, bucket, std::move(info)), true};
    }
 
    backoff();
  }
}
 
template <class Key, class Value, class... Policies>
bool harris_michael_hash_map<Key, Value, Policies...>::erase(const Key& key) {
  auto h = hash{}(key);
  auto bucket = map_to_bucket{}(h, num_buckets);
  backoff backoff;
  find_info info{&buckets[bucket]};
  // Find node in hash_map with matching key and mark it for erasure.
  do {
    if (!find(h, key, bucket, info, backoff)) {
      return false; // No such node in the hash_map
    }
    // (11) - this acquire-CAS synchronizes with the release-CAS (8, 9, 10, 12, 15)
    //        and is part of a release sequence headed by those operations
  } while (!info.cur->next.compare_exchange_weak(
    info.next, marked_ptr(info.next.get(), 1), std::memory_order_acquire, std::memory_order_relaxed));
 
  assert(info.next.mark() == 0);
  assert(info.cur.mark() == 0);
 
  // Try to splice out node
  marked_ptr expected = info.cur;
  // (12) - this release-CAS synchronizes with the acquire-load (1, 2, 3, 4, 5, 6, 7, 13)
  //        and the acquire-CAS (11, 14)
  //        it is the head of a potential release sequence containing (11, 14)
  if (info.prev->compare_exchange_weak(
        expected, info.next.get(), std::memory_order_release, std::memory_order_relaxed)) {
    info.cur.reclaim();
  } else {
    // Another thread interfered -> rewalk the bucket's list to ensure
    // reclamation of marked node before returning.
    find(h, key, bucket, info, backoff);
  }
 
  return true;
}
 
template <class Key, class Value, class... Policies>
auto harris_michael_hash_map<Key, Value, Policies...>::erase(iterator pos) -> iterator {
  backoff backoff;
  // (13) - this acquire-load synchronizes-with the release-CAS (8, 9, 10, 12, 15)
  auto next = pos.info.cur->next.load(std::memory_order_acquire);
  while (next.mark() == 0) {
    // (14) - this acquire-CAS synchronizes with the release-CAS (8, 9, 10, 12, 15)
    //        and is part of a release sequence headed by those operations
    if (pos.info.cur->next.compare_exchange_weak(next, marked_ptr(next.get(), 1), std::memory_order_acquire)) {
      break;
    }
 
    backoff();
  }
 
  guard_ptr next_guard(next.get());
  assert(pos.info.cur.mark() == 0);
 
  // Try to splice out node
  marked_ptr expected = pos.info.cur;
  // (15) - this release-CAS synchronizes with the acquire-load (1, 2, 3, 4, 5, 6, 7, 13)
  //        and the acquire-CAS (11, 14)
  //        it is the head of a potential release sequence containing (11, 14)
  if (pos.info.prev->compare_exchange_weak(
        expected, next_guard, std::memory_order_release, std::memory_order_relaxed)) {
    pos.info.cur.reclaim();
    pos.info.cur = std::move(next_guard);
  } else {
    next_guard.reset();
    Key key = pos.info.cur->data.value.first;
    hash_t h = pos.info.cur->data.get_hash();
 
    // Another thread interfered -> rewalk the list to ensure reclamation of marked node before returning.
    find(h, key, pos.bucket, pos.info, backoff);
  }
 
  if (!pos.info.cur) {
    pos.move_to_next_bucket();
  }
 
  return pos;
}
 
template <class Key, class Value, class... Policies>
auto harris_michael_hash_map<Key, Value, Policies...>::operator[](const Key& key) -> accessor {
  auto result = get_or_emplace_lazy(key, []() { return Value{}; });
  return accessor(std::move(result.first.info.cur));
}
 
template <class Key, class Value, class... Policies>
auto harris_michael_hash_map<Key, Value, Policies...>::begin() -> iterator {
  return iterator(this, 0);
}
 
template <class Key, class Value, class... Policies>
auto harris_michael_hash_map<Key, Value, Policies...>::end() -> iterator {
  return iterator(this);
}
} // namespace xenium
 
#endif