hashtable.hpp

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/*
 * This file is part of OpenTTD.
 * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
 * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <https://www.gnu.org/licenses/old-licenses/gpl-2.0>.
 */

 
#ifndef HASHTABLE_HPP
#define HASHTABLE_HPP
 
template <class TItem>
struct HashTableSlot {
  typedef typename TItem::Key Key; // make Titem::Key a property of HashTable
 
  TItem *first_item = nullptr;

  inline void Clear()
  {
    this->first_item = nullptr;
  }

  inline const TItem *Find(const Key &key) const
  {
    for (const TItem *item = this->first_item; item != nullptr; item = item->GetHashNext()) {
      if (item->GetKey() == key) {
        /* we have found the item, return it */
        return item;
      }
    }
    return nullptr;
  }

  inline TItem *Find(const Key &key)
  {
    for (TItem *item = this->first_item; item != nullptr; item = item->GetHashNext()) {
      if (item->GetKey() == key) {
        /* we have found the item, return it */
        return item;
      }
    }
    return nullptr;
  }

  inline void Attach(TItem &new_item)
  {
    assert(new_item.GetHashNext() == nullptr);
    new_item.SetHashNext(this->first_item);
    this->first_item = &new_item;
  }

  inline bool Detach(TItem &item_to_remove)
  {
    if (this->first_item == &item_to_remove) {
      this->first_item = item_to_remove.GetHashNext();
      item_to_remove.SetHashNext(nullptr);
      return true;
    }
    TItem *item = this->first_item;
    for (;;) {
      if (item == nullptr) {
        return false;
      }
      TItem *next_item = item->GetHashNext();
      if (next_item == &item_to_remove) break;
      item = next_item;
    }
    item->SetHashNext(item_to_remove.GetHashNext());
    item_to_remove.SetHashNext(nullptr);
    return true;
  }

  inline TItem *Detach(const Key &key)
  {
    /* do we have any items? */
    if (this->first_item == nullptr) {
      return nullptr;
    }
    /* is it our first item? */
    if (this->first_item->GetKey() == key) {
      TItem &ret_item = *this->first_item;
      this->first_item = this->first_item->GetHashNext();
      ret_item.SetHashNext(nullptr);
      return &ret_item;
    }
    /* find it in the following items */
    TItem *previous_item = this->first_item;
    for (TItem *item = this->first_item->GetHashNext(); item != nullptr; previous_item = item, item = item->GetHashNext()) {
      if (item->GetKey() == key) {
        /* we have found the item, unlink and return it */
        previous_item->SetHashNext(item->GetHashNext());
        item->SetHashNext(nullptr);
        return item;
      }
    }
    return nullptr;
  }
};

template <class Titem, int Thash_bits_>
class HashTable {
public:
  typedef typename Titem::Key Tkey; // make Titem::Key a property of HashTable
  static constexpr int HASH_BITS = Thash_bits_; // publish num of hash bits
  static constexpr int CAPACITY = 1 << HASH_BITS; // and num of slots 2^bits
 
protected:
  using Slot = HashTableSlot<Titem>;
 
  Slot slots[CAPACITY]; 
  int number_of_items = 0; 

  static inline int CalcHash(const Tkey &key)
  {
    uint32_t hash = key.CalcHash();
    hash -= (hash >> 17);          // hash * 131071 / 131072
    hash -= (hash >> 5);           //   * 31 / 32
    hash &= (1 << HASH_BITS) - 1; //   modulo slots
    return hash;
  }

  static inline int CalcHash(const Titem &item)
  {
    return CalcHash(item.GetKey());
  }
 
public:
  inline int Count() const
  {
    return this->number_of_items;
  }

  inline void Clear()
  {
    for (int i = 0; i < CAPACITY; i++) this->slots[i].Clear();
    this->number_of_items = 0;
  }

  const Titem *Find(const Tkey &key) const
  {
    int hash = CalcHash(key);
    const Slot &slot = this->slots[hash];
    const Titem *item = slot.Find(key);
    return item;
  }

  Titem *Find(const Tkey &key)
  {
    int hash = CalcHash(key);
    Slot &slot = this->slots[hash];
    Titem *item = slot.Find(key);
    return item;
  }

  Titem *TryPop(const Tkey &key)
  {
    int hash = CalcHash(key);
    Slot &slot = this->slots[hash];
    Titem *item = slot.Detach(key);
    if (item != nullptr) {
      this->number_of_items--;
    }
    return item;
  }

  Titem &Pop(const Tkey &key)
  {
    Titem *item = TryPop(key);
    assert(item != nullptr);
    return *item;
  }

  bool TryPop(Titem &item)
  {
    const Tkey &key = item.GetKey();
    int hash = CalcHash(key);
    Slot &slot = this->slots[hash];
    bool ret = slot.Detach(item);
    if (ret) {
      this->number_of_items--;
    }
    return ret;
  }

  void Pop(Titem &item)
  {
    [[maybe_unused]] bool ret = TryPop(item);
    assert(ret);
  }

  void Push(Titem &new_item)
  {
    int hash = CalcHash(new_item);
    Slot &slot = this->slots[hash];
    assert(slot.Find(new_item.GetKey()) == nullptr);
    slot.Attach(new_item);
    this->number_of_items++;
  }
};
 
#endif /* HASHTABLE_HPP */