Revert "Convert HashSet, LinkedHashSet to factory methods and custom implementations."

runtime/lib/collection_patch.dart

Index: runtime/lib/collection_patch.dart
diff --git a/runtime/lib/collection_patch.dart b/runtime/lib/collection_patch.dart
index 33f6ac5b61d0ea47fd1355d9f9f4bbe763fdb1bc..3716cca786927354b2ce06b4ab104f620a5dcb9e 100644
--- a/runtime/lib/collection_patch.dart
+++ b/runtime/lib/collection_patch.dart
@@ -38,7 +38,7 @@ class _HashMap<K, V> implements HashMap<K, V> {
   Type get runtimeType => HashMap;
 
   int _elementCount = 0;
-  List<_HashMapEntry> _buckets = new List<_HashMapEntry>(_INITIAL_CAPACITY);
+  List<_HashMapEntry> _buckets = new List(_INITIAL_CAPACITY);
   int _modificationCount = 0;
 
   int get length => _elementCount;
@@ -170,7 +170,7 @@ class _HashMap<K, V> implements HashMap<K, V> {
 
   void clear() {
     _elementCount = 0;
-    _buckets = new List<_HashMapEntry>(_INITIAL_CAPACITY);
+    _buckets = new List(_INITIAL_CAPACITY);
     _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
   }
 
@@ -201,7 +201,7 @@ class _HashMap<K, V> implements HashMap<K, V> {
     List oldBuckets = _buckets;
     int oldLength = oldBuckets.length;
     int newLength = oldLength << 1;
-    List newBuckets = new List<_HashMapEntry>(newLength);
+    List newBuckets = new List(newLength);
     for (int i = 0; i < oldLength; i++) {
       _HashMapEntry entry = oldBuckets[i];
       while (entry != null) {
@@ -504,276 +504,86 @@ class _HashMapValueIterator<V> extends _HashMapIterator<V> {
 }
 
 patch class HashSet<E> {
-  /* patch */ factory HashSet({ bool equals(E e1, E e2),
-                                int hashCode(E e),
-                                bool isValidKey(potentialKey) }) {
-    if (isValidKey == null) {
-      if (hashCode == null) {
-        if (equals == null) {
-          return new _HashSet<E>();
-        }
-        if (identical(identical, equals)) {
-          return new _IdentityHashSet<E>();
-        }
-        _hashCode = _defaultHashCode;
-      } else if (equals == null) {
-        _equals = _defaultEquals;
-      }
-      isValidKey = new _TypeTest<E>().test;
-    } else {
-      if (hashCode == null) hashCode = _defaultHashCode;
-      if (equals == null) equals = _defaultEquals;
-    }
-    return new _CustomHashSet<E>(equals, hashCode, isValidKey);
-  }
-}
-
-class _HashSet<E> extends _HashSetBase<E> implements HashSet<E> {
   static const int _INITIAL_CAPACITY = 8;
+  final _HashTable<E> _table;
 
-  List<_HashSetEntry> _buckets = new List<_HashSetEntry>(_INITIAL_CAPACITY);
-  int _elementCount = 0;
-  int _modificationCount = 0;
+  /* patch */ HashSet() : _table = new _HashTable(_INITIAL_CAPACITY) {
+    _table._container = this;
+  }
 
-  bool _equals(e1, e2) => e1 == e2;
-  int _hashCode(e) => e.hashCode;
+  factory HashSet.from(Iterable<E> iterable) {
+    return new HashSet<E>()..addAll(iterable);
+  }
 
   // Iterable.
-  Iterator<E> get iterator => new _HashSetIterator<E>(this);
-
-  int get length => _elementCount;
+  /* patch */ Iterator<E> get iterator => new _HashTableKeyIterator<E>(_table);
 
-  bool get isEmpty => _elementCount == 0;
-
-  bool get isNotEmpty => _elementCount != 0;
+  /* patch */ int get length => _table._elementCount;
 
-  bool contains(Object object) {
-    int index = _hashCode(object) & (_buckets.length - 1);
-    HashSetEntry entry = _buckets[index];
-    while (entry != null) {
-      if (_equals(entry.key, object)) return true;
-      entry = entry.next;
-    }
-    return false;
-  }
+  /* patch */ bool get isEmpty => _table._elementCount == 0;
 
-  // Set
+  /* patch */ bool get isNotEmpty => !isEmpty;
 
-  void _add(E element) {
-    int hashCode = _hashCode(element);
-    int index = hashCode & (_buckets.length - 1);
-    HashSetEntry entry = _buckets[index];
-    while (entry != null) {
-      if (_equals(entry.key, element)) return;
-      entry = entry.next;
-    }
-    _addEntry(element, hashCode, index);
-  }
+  /* patch */ bool contains(Object object) => _table._get(object) >= 0;
 
-  void add(E element) {
-    _add(element);
+  // Collection.
+  /* patch */ void add(E element) {
+    _table._put(element);
+    _table._checkCapacity();
   }
 
-  void addAll(Iterable<E> objects) {
-    int ctr = 0;
+  /* patch */ void addAll(Iterable<E> objects) {
     for (E object in objects) {
-      ctr++;
-      _add(object);
+      _table._put(object);
+      _table._checkCapacity();
     }
   }
 
-  bool _remove(Object object, int hashCode) {
-    int index = hashCode & (_buckets.length - 1);
-    _HashSetEntry entry = _buckets[index];
-    _HashSetEntry previous = null;
-    while (entry != null) {
-      if (_equals(entry.key, object)) {
-        _HashSetEntry next = entry.remove();
-        if (previous == null) {
-          _buckets[index] = next;
-        } else {
-          previous.next = next;
-        }
-        _elementCount--;
-        _modificationCount =
-            (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
-        return true;
-      }
-      previous = entry;
-      entry = entry.next;
-    }
-    return false;
+  /* patch */ bool remove(Object object) {
+    int offset = _table._remove(object);
+    _table._checkCapacity();
+    return offset >= 0;
   }
 
-  bool remove(Object object) => _remove(object, _hashCode(object));
-
-  void removeAll(Iterable<Object> objectsToRemove) {
+  /* patch */ void removeAll(Iterable<Object> objectsToRemove) {
     for (Object object in objectsToRemove) {
-      _remove(object, _hashCode(object));
+      _table._remove(object);
+      _table._checkCapacity();
     }
   }
 
   void _filterWhere(bool test(E element), bool removeMatching) {
-    int length = _buckets.length;
-    for (int index =  0; index < length; index++) {
-      HashSetEntry entry = _buckets[index];
-      HashSetEntry previous = null;
-      while (entry != null) {
-        int modificationCount = _modificationCount;
-        bool testResult = test(entry.key);
-        if (modificationCount != _modificationCount) {
-          throw new ConcurrentModificationError(this);
-        }
-        if (testResult == removeMatching) {
-          HashSetEntry next = entry.remove();
-          if (previous == null) {
-            _buckets[index] = next;
-          } else {
-            previous.next = next;
-          }
-          _elementCount--;
-          _modificationCount =
-              (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
-          entry = next;
-        } else {
-          previous = entry;
-          entry = entry.next;
+    int entrySize = _table._entrySize;
+    int length = _table._table.length;
+    for (int offset =  0; offset < length; offset += entrySize) {
+      Object entry = _table._table[offset];
+      if (!_table._isFree(entry)) {
+        E key = identical(entry, _NULL) ? null : entry;
+        int modificationCount = _table._modificationCount;
+        bool shouldRemove = (removeMatching == test(key));
+        _table._checkModification(modificationCount);
+        if (shouldRemove) {
+          _table._deleteEntry(offset);
         }
       }
     }
+    _table._checkCapacity();
   }
 
-  void removeWhere(bool test(E element)) {
+  /* patch */ void removeWhere(bool test(E element)) {
     _filterWhere(test, true);
   }
 
-  void retainWhere(bool test(E element)) {
+  /* patch */ void retainWhere(bool test(E element)) {
     _filterWhere(test, false);
   }
 
-  void clear() {
-    _elementCount = 0;
-    _buckets = new List<HashSetEntry>(_INITIAL_CAPACITY);
-    _modificationCount++;
-  }
-
-  void _addEntry(E key, int hashCode, int index) {
-    _buckets[index] = new _HashSetEntry(key, hashCode, _buckets[index]);
-    int newElements = _elementCount + 1;
-    _elementCount = newElements;
-    int length = _buckets.length;
-    // If we end up with more than 75% non-empty entries, we
-    // resize the backing store.
-    if ((newElements << 2) > ((length << 1) + length)) _resize();
-    _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+  /* patch */ void clear() {
+    _table._clear();
   }
-
-  void _resize() {
-    int oldLength = _buckets.length;
-    int newLength = oldLength << 1;
-    List oldBuckets = _buckets;
-    List newBuckets = new List<_HashSetEntry>(newLength);
-    for (int i = 0; i < oldLength; i++) {
-      _HashSetEntry entry = oldBuckets[i];
-      while (entry != null) {
-        _HashSetEntry next = entry.next;
-        int newIndex = entry.hashCode & (newLength - 1);
-        entry.next = newBuckets[newIndex];
-        newBuckets[newIndex] = entry;
-        entry = next;
-      }
-    }
-    _buckets = newBuckets;
-  }
-
-  HashSet<E> _newSet() => new _HashSet<E>();
-}
-
-class _IdentityHashSet<E> extends _HashSet<E> {
-  bool _equals(e1, e2) => identical(e1, e2);
-  HashSet<E> _newSet() => new _IdentityHashSet<E>();
-}
-
-class _CustomHashSet<E> extends _HashSet<E> {
-  final _Equality<E> _equality;
-  final _Hasher<E> _hasher;
-  final _Predicate _validKey;
-  _CustomHashSet(this._equality, this._hasher, this._validKey);
-
-  E operator[](Object key) {
-    if (!_validKey(key)) return null;
-    return super[key];
-  }
-
-  bool remove(Object key) {
-    if (!_validKey(key)) return false;
-    return super.remove(key);
-  }
-
-  bool containsKey(Object key) {
-    if (!_validKey(key)) return false;
-    return super.containsKey(key);
-  }
-
-  bool _equals(e1, e2) => _equality(e1, e2);
-  int _hashCode(e) => _hasher(e);
-
-  HashSet<E> _newSet() => new _CustomHashSet<E>(_equality, _hasher, _validKey);
-}
-
-class _HashSetEntry {
-  final key;
-  final int hashCode;
-  _HashSetEntry next;
-  _HashSetEntry(this.key, this.hashCode, this.next);
-
-  _HashSetEntry remove() {
-    _HashSetEntry result = next;
-    next = null;
-    return result;
-  }
-}
-
-class _HashSetIterator<E> implements Iterator<E> {
-  final _HashSet _set;
-  final int _modificationCount;
-  int _index = 0;
-  _HashSetEntry _next = null;
-  E _current = null;
-
-  _HashSetIterator(_HashSet hashSet)
-      : _set = hashSet, _modificationCount = hashSet._modificationCount;
-
-  bool moveNext() {
-    if (_modificationCount != _set._modificationCount) {
-      throw new ConcurrentModificationError(_set);
-    }
-    if (_next != null) {
-      _current = _next.key;
-      _next = _next.next;
-      return true;
-    }
-    List<_HashSetEntry> buckets = _set._buckets;
-    while (_index < buckets.length) {
-      _next = buckets[_index];
-      _index = _index + 1;
-      if (_next != null) {
-        _current = _next.key;
-        _next = _next.next;
-        return true;
-      }
-    }
-    _current = null;
-    return false;
-  }
-
-  E get current => _current;
 }
 
 class _LinkedHashMapEntry extends _HashMapEntry {
-  /// Double-linked list of entries of a linked hash map.
-  /// The _LinkedHashMap itself is the head of the list, so the type is "var".
-  /// Both are initialized to `this` when initialized.
   var _nextEntry;
   var _previousEntry;
   _LinkedHashMapEntry(key, value, int hashCode, _LinkedHashMapEntry next,
@@ -849,11 +659,6 @@ class _LinkedHashMapValueIterator<V> extends _LinkedHashMapIterator<V> {
  * A hash-based map that iterates keys and values in key insertion order.
  */
 patch class LinkedHashMap<K, V> {
-  /// Holds a double-linked list of entries in insertion order.
-  /// The fields have the same name as the ones in [_LinkedHashMapEntry],
-  /// and this map is itself used as the head entry of the list.
-  /// Set to `this` when initialized, representing the empty list (containing
-  /// only the head entry itself).
   var _nextEntry;
   var _previousEntry;
 
@@ -933,7 +738,6 @@ abstract class _LinkedHashMapMixin<K, V> implements LinkedHashMap<K, V> {
     buckets[index] = entry;
     int newElements = _elementCount + 1;
     _elementCount = newElements;
-
     // If we end up with more than 75% non-empty entries, we
     // resize the backing store.
     if ((newElements << 2) > ((length << 1) + length)) _resize();
@@ -984,192 +788,703 @@ class _LinkedCustomHashMap<K, V> extends _CustomHashMap<K, V>
 }
 
 
-patch class LinkedHashSet<E> {
-  /* patch */ factory LinkedHashSet({ bool equals(E e1, E e2),
-                                      int hashCode(E e),
-                                      bool isValidKey(potentialKey) }) {
-    if (isValidKey == null) {
-      if (hashCode == null) {
-        if (equals == null) {
-          return new _LinkedHashSet<E>();
-        }
-        if (identical(identical, equals)) {
-          return new _LinkedIdentityHashSet<E>();
-        }
-        _hashCode = _defaultHashCode;
-      } else if (equals == null) {
-        _equals = _defaultEquals;
+patch class LinkedHashSet<E> extends _HashSetBase<E> {
+  static const int _INITIAL_CAPACITY = 8;
+  _LinkedHashTable<E> _table;
+
+  /* patch */ LinkedHashSet() {
+    _table = new _LinkedHashTable(_INITIAL_CAPACITY);
+    _table._container = this;
+  }
+
+  // Iterable.
+  /* patch */ Iterator<E> get iterator {
+    return new _LinkedHashTableKeyIterator<E>(_table);
+  }
+
+  /* patch */ int get length => _table._elementCount;
+
+  /* patch */ bool get isEmpty => _table._elementCount == 0;
+
+  /* patch */ bool get isNotEmpty => !isEmpty;
+
+  /* patch */ bool contains(Object object) => _table._get(object) >= 0;
+
+  /* patch */ void forEach(void action(E element)) {
+    int offset = _table._next(_LinkedHashTable._HEAD_OFFSET);
+    int modificationCount = _table._modificationCount;
+    while (offset != _LinkedHashTable._HEAD_OFFSET) {
+      E key = _table._key(offset);
+      action(key);
+      _table._checkModification(modificationCount);
+      offset = _table._next(offset);
+    }
+  }
+
+  /* patch */ E get first {
+    int firstOffset = _table._next(_LinkedHashTable._HEAD_OFFSET);
+    if (firstOffset == _LinkedHashTable._HEAD_OFFSET) {
+      throw new StateError("No elements");
+    }
+    return _table._key(firstOffset);
+  }
+
+  /* patch */ E get last {
+    int lastOffset = _table._prev(_LinkedHashTable._HEAD_OFFSET);
+    if (lastOffset == _LinkedHashTable._HEAD_OFFSET) {
+      throw new StateError("No elements");
+    }
+    return _table._key(lastOffset);
+  }
+
+  // Collection.
+  void _filterWhere(bool test(E element), bool removeMatching) {
+    int entrySize = _table._entrySize;
+    int length = _table._table.length;
+    int offset = _table._next(_LinkedHashTable._HEAD_OFFSET);
+    while (offset != _LinkedHashTable._HEAD_OFFSET) {
+      E key = _table._key(offset);
+      int nextOffset = _table._next(offset);
+      int modificationCount = _table._modificationCount;
+      bool shouldRemove = (removeMatching == test(key));
+      _table._checkModification(modificationCount);
+      if (shouldRemove) {
+        _table._deleteEntry(offset);
       }
-      isValidKey = new _TypeTest<E>().test;
-    } else {
-      if (hashCode == null) hashCode = _defaultHashCode;
-      if (equals == null) equals = _defaultEquals;
+      offset = nextOffset;
     }
-    return new _LinkedCustomHashSet<E>(equals, hashCode, isValidKey);
+    _table._checkCapacity();
   }
-}
 
-class _LinkedHashSetEntry extends _HashSetEntry {
-  /// Links this element into a double-linked list of elements of a hash set.
-  /// The hash set object itself is used as the head entry of the list, so
-  /// the field is typed as "var".
-  /// Both links are initialized to `this` when the object is created.
-  var _nextEntry;
-  var _previousEntry;
-  _LinkedHashSetEntry(var key, int hashCode, _LinkedHashSetEntry next,
-                      this._previousEntry, this._nextEntry)
-      : super(key, hashCode, next) {
-    _previousEntry._nextEntry = _nextEntry._previousEntry = this;
+  /* patch */ void add(E element) {
+    _table._put(element);
+    _table._checkCapacity();
   }
 
-  _LinkedHashSetEntry remove() {
-    _previousEntry._nextEntry = _nextEntry;
-    _nextEntry._previousEntry = _previousEntry;
-    _nextEntry = _previousEntry = this;
-    return super.remove();
+  /* patch */ void addAll(Iterable<E> objects) {
+    for (E object in objects) {
+      _table._put(object);
+      _table._checkCapacity();
+    }
+  }
+
+  /* patch */ bool remove(Object object) {
+    int offset = _table._remove(object);
+    if (offset >= 0) {
+      _table._checkCapacity();
+      return true;
+    }
+    return false;
+  }
+
+  /* patch */ void removeAll(Iterable objectsToRemove) {
+    for (Object object in objectsToRemove) {
+      if (_table._remove(object) >= 0) {
+        _table._checkCapacity();
+      }
+    }
+  }
+
+  /* patch */ void removeWhere(bool test(E element)) {
+    _filterWhere(test, true);
+  }
+
+  /* patch */ void retainWhere(bool test(E element)) {
+    _filterWhere(test, false);
+  }
+
+  /* patch */ void clear() {
+    _table._clear();
   }
 }
 
-class _LinkedHashSet<E> extends _HashSet<E>
-                        implements LinkedHashSet<E> {
-  /// Holds a double linked list of the element entries of the set in
-  /// insertion order.
-  /// The fields have the same names as the ones in [_LinkedHashSetEntry],
-  /// allowing this object to be used as the head entry of the list.
-  /// The fields are initialized to `this` when created, representing the
-  /// empty list that only contains the head entry.
-  var _nextEntry;
-  var _previousEntry;
+class _DeadEntry {
+  const _DeadEntry();
+}
 
-  _LinkedHashSet() {
-    _nextEntry = _previousEntry = this;
+class _NullKey {
+  const _NullKey();
+  int get hashCode => null.hashCode;
+}
+
+const _TOMBSTONE = const _DeadEntry();
+const _NULL = const _NullKey();
+
+class _HashTable<K> {
+  /**
+   * Table of entries with [_entrySize] slots per entry.
+   *
+   * Capacity in entries must be factor of two.
+   */
+  List _table;
+  /** Current capacity. Always equal to [:_table.length ~/ _entrySize:]. */
+  int _capacity;
+  /** Count of occupied entries, including deleted ones. */
+  int _entryCount = 0;
+  /** Count of deleted entries. */
+  int _deletedCount = 0;
+  /** Counter incremented when table is modified. */
+  int _modificationCount = 0;
+  /** If set, used as the source object for [ConcurrentModificationError]s. */
+  Object _container;
+
+  _HashTable(int initialCapacity) : _capacity = initialCapacity {
+    _table = _createTable(initialCapacity);
   }
 
-  // Iterable.
-  Iterator<E> get iterator => new _LinkedHashSetIterator<E>(this);
+  /** Reads key from table. Converts _NULL marker to null. */
+  Object _key(offset) {
+    assert(!_isFree(_table[offset]));
+    Object key = _table[offset];
+    if (!identical(key, _NULL)) return key;
+    return null;
+  }
 
-  void forEach(void action(E element)) {
-    var cursor = _nextEntry;
-    int modificationCount = _modificationCount;
-    while (!identical(cursor, this)) {
-      _LinkedHashSetEntry entry = cursor;
-      action(entry.key);
-      if (_modificationCount != modificationCount) {
-        throw new ConcurrentModificationError(this);
+  /** Writes key to table. Converts null to _NULL marker. */
+  void _setKey(int offset, Object key) {
+    if (key == null) key = _NULL;
+    _table[offset] = key;
+  }
+
+  int get _elementCount => _entryCount - _deletedCount;
+
+  /** Size of each entry. */
+  int get _entrySize => 1;
+
+  void _checkModification(int expectedModificationCount) {
+    if (_modificationCount != expectedModificationCount) {
+      throw new ConcurrentModificationError(_container);
+    }
+  }
+
+  void _recordModification() {
+    // Value cycles after 2^30 modifications. If you keep hold of an
+    // iterator for that long, you might miss a modification detection,
+    // and iteration can go sour. Don't do that.
+    _modificationCount = (_modificationCount + 1) & (0x3FFFFFFF);
+  }
+
+  /**
+   * Create an empty table.
+   */
+  List _createTable(int capacity) {
+    List table = new List(capacity * _entrySize);
+    return table;
+  }
+
+  /** First table probe. */
+  int _firstProbe(int hashCode, int capacity) {
+    return hashCode & (capacity - 1);
+  }
+
+  /** Following table probes. */
+  int _nextProbe(int previousIndex, int probeCount, int capacity) {
+    // When capacity is a power of 2, this probing algorithm (the triangular
+    // number sequence modulo capacity) is guaranteed to hit all indices exactly
+    // once before repeating.
+    return (previousIndex + probeCount) & (capacity - 1);
+  }
+
+  /** Whether an object is a free-marker (either tombstone or free). */
+  bool _isFree(Object marker) =>
+      marker == null || identical(marker, _TOMBSTONE);
+
+  /**
+   * Look up the offset for an object in the table.
+   *
+   * Finds the offset of the object in the table, if it is there,
+   * or the first free offset for its hashCode.
+   */
+  int _probeForAdd(int hashCode, Object object) {
+    int entrySize = _entrySize;
+    int index = _firstProbe(hashCode, _capacity);
+    int firstTombstone = -1;
+    int probeCount = 0;
+    while (true) {
+      int offset = index * entrySize;
+      Object entry = _table[offset];
+      if (identical(entry, _TOMBSTONE)) {
+        if (firstTombstone < 0) firstTombstone = offset;
+      } else if (entry == null) {
+        if (firstTombstone < 0) return offset;
+        return firstTombstone;
+      } else if (identical(_NULL, entry) ? _equals(null, object)
+                                         : _equals(entry, object)) {
+        return offset;
       }
-      cursor = entry._nextEntry;
+      // The _nextProbe is designed so that it hits
+      // every index eventually.
+      index = _nextProbe(index, ++probeCount, _capacity);
     }
   }
 
-  E get first {
-    if (identical(_nextEntry, this)) {
-      throw new StateError("No elements");
+  /**
+   * Look up the offset for an object in the table.
+   *
+   * If the object is in the table, its offset is returned.
+   *
+   * If the object is not in the table, Otherwise a negative value is returned.
+   */
+  int _probeForLookup(int hashCode, Object object) {
+    int entrySize = _entrySize;
+    int index = _firstProbe(hashCode, _capacity);
+    int probeCount = 0;
+    while (true) {
+      int offset = index * entrySize;
+      Object entry = _table[offset];
+      if (entry == null) {
+        return -1;
+      } else if (!identical(_TOMBSTONE, entry)) {
+        if (identical(_NULL, entry) ? _equals(null, object)
+                                    : _equals(entry, object)) {
+          return offset;
+        }
+      }
+      // The _nextProbe is designed so that it hits
+      // every index eventually.
+      index = _nextProbe(index, ++probeCount, _capacity);
     }
-    _LinkedHashSetEntry entry = _nextEntry;
-    return entry.key;
   }
 
-  E get last {
-    if (identical(_previousEntry, this)) {
-      throw new StateError("No elements");
+  // Override the following two to change equality/hashCode computations
+
+  /**
+   * Compare two object for equality.
+   *
+   * The first object is the one already in the table,
+   * and the second is the one being searched for.
+   */
+  bool _equals(Object element, Object other) {
+    return element == other;
+  }
+
+  /**
+   * Compute hash-code for an object.
+   */
+  int _hashCodeOf(Object object) => object.hashCode;
+
+  /**
+   * Ensure that the table isn't too full for its own good.
+   *
+   * Call this after adding an element.
+   */
+  int _checkCapacity() {
+    // Compute everything in multiples of entrySize to avoid division.
+    int freeCount = _capacity - _entryCount;
+    if (freeCount * 4 < _capacity ||
+        freeCount < _deletedCount) {
+      // Less than 25% free or more deleted entries than free entries.
+      _grow(_entryCount - _deletedCount);
     }
-    _LinkedHashSetEntry entry = _previousEntry;
-    return entry.key;
   }
 
-  // Set.
-  void _filterWhere(bool test(E element), bool removeMatching) {
-    var cursor = _nextEntry;
-    while (!identical(cursor, this)) {
-      _LinkedHashSetEntry entry = cursor;
-      int modificationCount = _modificationCount;
-      bool testResult = test(entry.key);
-      if (modificationCount != _modificationCount) {
-        throw new ConcurrentModificationError(this);
+  void _grow(int contentCount) {
+    int capacity = _capacity;
+    // Don't grow to less than twice the needed capacity.
+    int minCapacity = contentCount * 2;
+    while (capacity < minCapacity) {
+      capacity *= 2;
+    }
+    // Reset to another table and add all existing elements.
+    List oldTable = _table;
+    _table = _createTable(capacity);
+    _capacity = capacity;
+    _entryCount = 0;
+    _deletedCount = 0;
+    _addAllEntries(oldTable);
+    _recordModification();
+  }
+
+  /**
+   * Copies all non-free entries from the old table to the new empty table.
+   */
+  void _addAllEntries(List oldTable) {
+    for (int i = 0; i < oldTable.length; i += _entrySize) {
+      Object object = oldTable[i];
+      if (!_isFree(object)) {
+        int toOffset = _put(object);
+        _copyEntry(oldTable, i, toOffset);
       }
-      cursor = entry._nextEntry;
-      if (testResult == removeMatching) {
-        _remove(entry.key, entry.hashCode);
+    }
+  }
+
+  /**
+   * Copies everything but the key element from one entry to another.
+   *
+   * Called while growing the base array.
+   *
+   * Override this if any non-key fields need copying.
+   */
+  void _copyEntry(List fromTable, int fromOffset, int toOffset) {}
+
+  // The following three methods are for simple get/set/remove operations.
+  // They only affect the key of an entry. The remaining fields must be
+  // filled by the caller.
+
+  /**
+   * Returns the offset of a key in [_table], or negative if it's not there.
+   */
+  int _get(Object key) {
+    return _probeForLookup(_hashCodeOf(key), key);
+  }
+
+  /**
+   * Puts the key into the table and returns its offset into [_table].
+   *
+   * If [_entrySize] is greater than 1, the caller should fill the
+   * remaining fields.
+   *
+   * Remember to call [_checkCapacity] after using this method.
+   */
+  int _put(K key) {
+    int offset = _probeForAdd(_hashCodeOf(key), key);
+    Object oldEntry = _table[offset];
+    if (oldEntry == null) {
+      _entryCount++;
+    } else if (identical(oldEntry, _TOMBSTONE)) {
+      _deletedCount--;
+    } else {
+      return offset;
+    }
+    _setKey(offset, key);
+    _recordModification();
+    return offset;
+  }
+
+  /**
+   * Removes a key from the table and returns its offset into [_table].
+   *
+   * Returns null if the key was not in the table.
+   * If [_entrySize] is greater than 1, the caller should clean up the
+   * remaining fields.
+   */
+  int _remove(Object key) {
+    int offset = _probeForLookup(_hashCodeOf(key), key);
+    if (offset >= 0) {
+      _deleteEntry(offset);
+    }
+    return offset;
+  }
+
+  /** Clears the table completely, leaving it empty. */
+  void _clear() {
+    if (_elementCount == 0) return;
+    for (int i = 0; i < _table.length; i++) {
+      _table[i] = null;
+    }
+    _entryCount = _deletedCount = 0;
+    _recordModification();
+  }
+
+  /** Clears an entry in the table. */
+  void _deleteEntry(int offset) {
+    assert(!_isFree(_table[offset]));
+    _setKey(offset, _TOMBSTONE);
+    _deletedCount++;
+    _recordModification();
+  }
+}
+
+/**
+ * Generic iterable based on a [_HashTable].
+ */
+abstract class _HashTableIterable<E> extends IterableBase<E> {
+  final _HashTable _hashTable;
+  _HashTableIterable(this._hashTable);
+
+  Iterator<E> get iterator;
+
+  /**
+   * Return the iterated value for a given entry.
+   */
+  E _valueAt(int offset, Object key);
+
+  int get length => _hashTable._elementCount;
+
+  bool get isEmpty => _hashTable._elementCount == 0;
+
+  void forEach(void action(E element)) {
+    int entrySize = _hashTable._entrySize;
+    List table = _hashTable._table;
+    int modificationCount = _hashTable._modificationCount;
+    for (int offset = 0; offset < table.length; offset += entrySize) {
+      Object entry = table[offset];
+      if (!_hashTable._isFree(entry)) {
+        E value = _valueAt(offset, entry);
+        action(value);
       }
+      _hashTable._checkModification(modificationCount);
     }
   }
+}
 
-  void _addEntry(E key, int hashCode, int index) {
-    _buckets[index] =
-        new _LinkedHashSetEntry(key, hashCode, _buckets[index],
-                                _previousEntry, this);
-    int newElements = _elementCount + 1;
-    _elementCount = newElements;
-    int length = _buckets.length;
-    // If we end up with more than 75% non-empty entries, we
-    // resize the backing store.
-    if ((newElements << 2) > ((length << 1) + length)) _resize();
-    _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+abstract class _HashTableIterator<E> implements Iterator<E> {
+  final _HashTable _hashTable;
+  final int _modificationCount;
+  /** Location right after last found element. */
+  int _offset = 0;
+  E _current = null;
+
+  _HashTableIterator(_HashTable hashTable)
+      : _hashTable = hashTable,
+        _modificationCount = hashTable._modificationCount;
+
+  bool moveNext() {
+    _hashTable._checkModification(_modificationCount);
+
+    List table = _hashTable._table;
+    int entrySize = _hashTable._entrySize;
+
+    while (_offset < table.length) {
+      int currentOffset = _offset;
+      Object entry = table[currentOffset];
+      _offset = currentOffset + entrySize;
+      if (!_hashTable._isFree(entry)) {
+        _current = _valueAt(currentOffset, entry);
+        return true;
+      }
+    }
+    _current = null;
+    return false;
   }
 
-  HashSet<E> _newSet() => new _LinkedHashSet<E>();
+  E get current => _current;
+
+  E _valueAt(int offset, Object key);
 }
 
-class _LinkedIdentityHashSet<E> extends _LinkedHashSet<E> {
-  bool _equals(e1, e2) => identical(e1, e2);
-  HashSet<E> _newSet() => new _LinkedIdentityHashSet<E>();
+class _HashTableKeyIterable<K> extends _HashTableIterable<K> {
+  _HashTableKeyIterable(_HashTable<K> hashTable) : super(hashTable);
+
+  Iterator<K> get iterator => new _HashTableKeyIterator<K>(_hashTable);
+
+  K _valueAt(int offset, Object key) {
+    if (identical(key, _NULL)) return null;
+    return key;
+  }
+
+  bool contains(Object value) => _hashTable._get(value) >= 0;
 }
 
-class _LinkedCustomHashSet<E> extends _LinkedHashSet<E> {
-  final _Equality<E> _equality;
-  final _Hasher<E> _hasher;
-  final _Predicate _validKey;
+class _HashTableKeyIterator<K> extends _HashTableIterator<K> {
+  _HashTableKeyIterator(_HashTable hashTable) : super(hashTable);
+
+  K _valueAt(int offset, Object key) {
+    if (identical(key, _NULL)) return null;
+    return key;
+  }
+}
+
+class _HashTableValueIterable<V> extends _HashTableIterable<V> {
+  final int _entryIndex;
+
+  _HashTableValueIterable(_HashTable hashTable, this._entryIndex)
+      : super(hashTable);
+
+  Iterator<V> get iterator {
+    return new _HashTableValueIterator<V>(_hashTable, _entryIndex);
+  }
+
+  V _valueAt(int offset, Object key) => _hashTable._table[offset + _entryIndex];
+}
+
+class _HashTableValueIterator<V> extends _HashTableIterator<V> {
+  final int _entryIndex;
+
+  _HashTableValueIterator(_HashTable hashTable, this._entryIndex)
+      : super(hashTable);
+
+  V _valueAt(int offset, Object key) => _hashTable._table[offset + _entryIndex];
+}
+
+class _HashMapTable<K, V> extends _HashTable<K> {
+  static const int _INITIAL_CAPACITY = 8;
+  static const int _VALUE_INDEX = 1;
+
+  _HashMapTable() : super(_INITIAL_CAPACITY);
+
+  int get _entrySize => 2;
+
+  V _value(int offset) => _table[offset + _VALUE_INDEX];
+  void _setValue(int offset, V value) { _table[offset + _VALUE_INDEX] = value; }
+
+  _copyEntry(List fromTable, int fromOffset, int toOffset) {
+    _table[toOffset + _VALUE_INDEX] = fromTable[fromOffset + _VALUE_INDEX];
+  }
+}
 
-  _LinkedCustomHashSet(this._equality, this._hasher, bool validKey(object))
-      : _validKey = (validKey != null) ? validKey : new _TypeTest<E>().test;
+/** Unique marker object for the head of a linked list of entries. */
+class _LinkedHashTableHeadMarker {
+  const _LinkedHashTableHeadMarker();
+}
+
+const _LinkedHashTableHeadMarker _HEAD_MARKER =
+    const _LinkedHashTableHeadMarker();
+
+class _LinkedHashTable<K> extends _HashTable<K> {
+  static const _NEXT_INDEX = 1;
+  static const _PREV_INDEX = 2;
+  static const _HEAD_OFFSET = 0;
+
+  _LinkedHashTable(int initialCapacity) : super(initialCapacity);
+
+  int get _entrySize => 3;
 
-  bool _equals(e1, e2) => _equality(e1, e2);
+  List _createTable(int capacity) {
+    List result = new List(capacity * _entrySize);
+    result[_HEAD_OFFSET] = _HEAD_MARKER;
+    result[_HEAD_OFFSET + _NEXT_INDEX] = _HEAD_OFFSET;
+    result[_HEAD_OFFSET + _PREV_INDEX] = _HEAD_OFFSET;
+    return result;
+  }
 
-  int _hashCode(e) => _hasher(e);
+  int _next(int offset) => _table[offset + _NEXT_INDEX];
+  void _setNext(int offset, int to) { _table[offset + _NEXT_INDEX] = to; }
+
+  int _prev(int offset) => _table[offset + _PREV_INDEX];
+  void _setPrev(int offset, int to) { _table[offset + _PREV_INDEX] = to; }
+
+  void _linkLast(int offset) {
+    // Add entry at offset at end of double-linked list.
+    int last = _prev(_HEAD_OFFSET);
+    _setNext(offset, _HEAD_OFFSET);
+    _setPrev(offset, last);
+    _setNext(last, offset);
+    _setPrev(_HEAD_OFFSET, offset);
+  }
+
+  void _unlink(int offset) {
+    assert(offset != _HEAD_OFFSET);
+    int next = _next(offset);
+    int prev = _prev(offset);
+    _setNext(offset, null);
+    _setPrev(offset, null);
+    _setNext(prev, next);
+    _setPrev(next, prev);
+  }
+
+  /**
+   * Copies all non-free entries from the old table to the new empty table.
+   */
+  void _addAllEntries(List oldTable) {
+    int offset = oldTable[_HEAD_OFFSET + _NEXT_INDEX];
+    while (offset != _HEAD_OFFSET) {
+      Object object = oldTable[offset];
+      int nextOffset = oldTable[offset + _NEXT_INDEX];
+      int toOffset = _put(object);
+      _copyEntry(oldTable, offset, toOffset);
+      offset = nextOffset;
+    }
+  }
 
-  bool contains(Object o) {
-    if (!_validKey(o)) return false;
-    return super.contains(o);
+  void _clear() {
+    if (_elementCount == 0) return;
+    _setNext(_HEAD_OFFSET, _HEAD_OFFSET);
+    _setPrev(_HEAD_OFFSET, _HEAD_OFFSET);
+    for (int i = _entrySize; i < _table.length; i++) {
+      _table[i] = null;
+    }
+    _entryCount = _deletedCount = 0;
+    _recordModification();
   }
 
-  bool remove(Object o) {
-    if (!_validKey(o)) return false;
-    return super.remove(o);
+  int _put(K key) {
+    int offset = _probeForAdd(_hashCodeOf(key), key);
+    Object oldEntry = _table[offset];
+    if (identical(oldEntry, _TOMBSTONE)) {
+      _deletedCount--;
+    } else if (oldEntry == null) {
+      _entryCount++;
+    } else {
+      return offset;
+    }
+    _recordModification();
+    _setKey(offset, key);
+    _linkLast(offset);
+    return offset;
   }
 
-  E operator[](Object o) {
-    if (!_validKey(o)) return null;
-    return super[o];
+  void _deleteEntry(int offset) {
+    _unlink(offset);
+    _setKey(offset, _TOMBSTONE);
+    _deletedCount++;
+    _recordModification();
   }
+}
+
+class _LinkedHashTableKeyIterable<K> extends IterableBase<K> {
+  final _LinkedHashTable<K> _table;
+  _LinkedHashTableKeyIterable(this._table);
+  Iterator<K> get iterator => new _LinkedHashTableKeyIterator<K>(_table);
+
+  bool contains(Object value) => _table._get(value) >= 0;
+
+  int get length => _table._elementCount;
+}
+
+class _LinkedHashTableKeyIterator<K> extends _LinkedHashTableIterator<K> {
+  _LinkedHashTableKeyIterator(_LinkedHashTable<K> hashTable): super(hashTable);
+
+  K _getCurrent(int offset) => _hashTable._key(offset);
+}
 
-  HashSet<E> _newSet() =>
-      new _LinkedCustomHashSet<E>(_equality, _hasher, _validKey);
+class _LinkedHashTableValueIterable<V> extends IterableBase<V> {
+  final _LinkedHashTable _hashTable;
+  final int _valueIndex;
+  _LinkedHashTableValueIterable(this._hashTable, this._valueIndex);
+  Iterator<V> get iterator =>
+      new _LinkedHashTableValueIterator<V>(_hashTable, _valueIndex);
+  int get length => _hashTable._elementCount;
 }
 
-class _LinkedHashSetIterator<E> implements Iterator<E> {
-  final _LinkedHashSet _set;
+class _LinkedHashTableValueIterator<V> extends _LinkedHashTableIterator<V> {
+  final int _valueIndex;
+
+  _LinkedHashTableValueIterator(_LinkedHashTable hashTable, this._valueIndex)
+      : super(hashTable);
+
+  V _getCurrent(int offset) => _hashTable._table[offset + _valueIndex];
+}
+
+abstract class _LinkedHashTableIterator<T> implements Iterator<T> {
+  final _LinkedHashTable _hashTable;
   final int _modificationCount;
-  var _next;
-  E _current;
+  int _offset;
+  T _current;
 
-  _LinkedHashSetIterator(_LinkedHashSet hashSet)
-      : _set = hashSet,
-        _modificationCount = hashSet._modificationCount,
-        _next = hashSet._nextEntry;
+  _LinkedHashTableIterator(_LinkedHashTable table)
+      : _hashTable = table,
+        _modificationCount = table._modificationCount,
+        _offset = table._next(_LinkedHashTable._HEAD_OFFSET);
 
   bool moveNext() {
-    if (_modificationCount != _set._modificationCount) {
-      throw new ConcurrentModificationError(_set);
-    }
-    if (identical(_set, _next)) {
+    _hashTable._checkModification(_modificationCount);
+    if (_offset == _LinkedHashTable._HEAD_OFFSET) {
       _current = null;
       return false;
     }
-    _LinkedHashSetEntry entry = _next;
-    _current = entry.key;
-    _next = entry._nextEntry;
+    _current = _getCurrent(_offset);
+    _offset = _hashTable._next(_offset);
     return true;
   }
 
-  E get current => _current;
+  T _getCurrent(int offset);
+
+  T get current => _current;
+}
+
+class _LinkedHashMapTable<K, V> extends _LinkedHashTable<K> {
+  static const int _INITIAL_CAPACITY = 8;
+  static const int _VALUE_INDEX = 3;
+
+  int get _entrySize => 4;
+
+  _LinkedHashMapTable() : super(_INITIAL_CAPACITY);
+
+  V _value(int offset) => _table[offset + _VALUE_INDEX];
+  void _setValue(int offset, V value) { _table[offset + _VALUE_INDEX] = value; }
+
+  _copyEntry(List oldTable, int fromOffset, int toOffset) {
+    _table[toOffset + _VALUE_INDEX] = oldTable[fromOffset + _VALUE_INDEX];
+  }
 }