gecko-dev/xpcom/ds/nsCheapSets.h

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef __nsCheapSets_h__
#define __nsCheapSets_h__

#include "nsTHashtable.h"
#include <stdint.h>

/**
 * A set that takes up minimal size when there are 0 or 1 entries in the set.
 * Use for cases where sizes of 0 and 1 are even slightly common.
 */
template<typename EntryType>
class nsCheapSet
{
public:
  typedef typename EntryType::KeyType KeyType;
  typedef PLDHashOperator (* Enumerator)(EntryType* aEntry, void* userArg);

  nsCheapSet() : mState(ZERO)
  {
  }
  ~nsCheapSet()
  {
    switch (mState) {
    case ZERO:
      break;
    case ONE:
      GetSingleEntry()->~EntryType();
      break;
    case MANY:
      delete mUnion.table;
      break;
    default:
      NS_NOTREACHED("bogus state");
      break;
    }
  }

  nsresult Put(const KeyType aVal);

  void Remove(const KeyType aVal);

  bool Contains(const KeyType aVal)
  {
    switch (mState) {
    case ZERO:
      return false;
    case ONE:
      return GetSingleEntry()->KeyEquals(EntryType::KeyToPointer(aVal));
    case MANY:
      return !!mUnion.table->GetEntry(aVal);
    default:
      NS_NOTREACHED("bogus state");
      return false;
    }
  }

  uint32_t EnumerateEntries(Enumerator enumFunc, void* userArg)
  {
    switch (mState) {
    case ZERO:
      return 0;
    case ONE:
      if (enumFunc(GetSingleEntry(), userArg) == PL_DHASH_REMOVE) {
        GetSingleEntry()->~EntryType();
        mState = ZERO;
      }
      return 1;
    case MANY:
      return mUnion.table->EnumerateEntries(enumFunc, userArg);
    default:
      NS_NOTREACHED("bogus state");
      return 0;
    }
  }

private:
  EntryType* GetSingleEntry()
  {
    return reinterpret_cast<EntryType*>(&mUnion.singleEntry[0]);
  }

  enum SetState {
    ZERO,
    ONE,
    MANY
  };

  union {
    nsTHashtable<EntryType> *table;
    char singleEntry[sizeof(EntryType)];
  } mUnion;
  enum SetState mState;
};

template<typename EntryType>
nsresult
nsCheapSet<EntryType>::Put(const KeyType aVal)
{
  switch (mState) {
  case ZERO:
    new (GetSingleEntry()) EntryType(EntryType::KeyToPointer(aVal));
    mState = ONE;
    return NS_OK;
  case ONE:
    {
      nsTHashtable<EntryType> *table = new nsTHashtable<EntryType>();
      EntryType *entry = GetSingleEntry();
      table->PutEntry(entry->GetKey());
      entry->~EntryType();
      mUnion.table = table;
      mState = MANY;
    }
    // Fall through.
  case MANY:
    mUnion.table->PutEntry(aVal);
    return NS_OK;
  default:
    NS_NOTREACHED("bogus state");
    return NS_OK;
  }
}

template<typename EntryType>
void
nsCheapSet<EntryType>::Remove(const KeyType aVal)
{
  switch (mState) {
  case ZERO:
    break;
  case ONE:
    if (Contains(aVal)) {
      GetSingleEntry()->~EntryType();
      mState = ZERO;
    }
    break;
  case MANY:
    mUnion.table->RemoveEntry(aVal);
    break;
  default:
    NS_NOTREACHED("bogus state");
    break;
  }
}

#endif