Added corresponding IdentitySet

src/dorkbox/collections/IdentitySet.java

@@ -0,0 +1,171 @@
+/*
+ * Copyright 2023 dorkbox, llc
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package dorkbox.collections;
+
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.ConcurrentModificationException;
+import java.util.Iterator;
+import java.util.Set;
+
+public
+class IdentitySet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable {
+    private transient IdentityMap<E, Object> map;
+
+    // Dummy value to associate with an Object in the backing Map
+    private static final Object PRESENT = new Object();
+
+    /**
+     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
+     * default initial capacity (16) and load factor (0.75).
+     */
+    public
+    IdentitySet() {
+        map = new IdentityMap<>();
+    }
+
+    /**
+     * Constructs a new set containing the elements in the specified
+     * collection.  The <tt>HashMap</tt> is created with default load factor
+     * (0.75) and an initial capacity sufficient to contain the elements in
+     * the specified collection.
+     *
+     * @param c the collection whose elements are to be placed into this set
+     *
+     * @throws NullPointerException if the specified collection is null
+     */
+    public
+    IdentitySet(Collection<? extends E> c) {
+        map = new IdentityMap<>(Math.max((int) (c.size() / .75f) + 1, 16));
+        addAll(c);
+    }
+
+    /**
+     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
+     * the specified initial capacity and default load factor (0.75).
+     *
+     * @param initialCapacity the initial capacity of the hash table
+     *
+     * @throws IllegalArgumentException if the initial capacity is less
+     *         than zero
+     */
+    public
+    IdentitySet(int initialCapacity) {
+        map = new IdentityMap<>(initialCapacity);
+    }
+
+    /**
+     * Returns an iterator over the elements in this set.  The elements
+     * are returned in no particular order.
+     *
+     * @return an Iterator over the elements in this set
+     *
+     * @see ConcurrentModificationException
+     */
+    @Override
+    public
+    Iterator<E> iterator() {
+        return map.keySet()
+                  .iterator();
+    }
+
+    /**
+     * Returns the number of elements in this set (its cardinality).
+     *
+     * @return the number of elements in this set (its cardinality)
+     */
+    @Override
+    public
+    int size() {
+        return map.size();
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains no elements.
+     *
+     * @return <tt>true</tt> if this set contains no elements
+     */
+    @Override
+    public
+    boolean isEmpty() {
+        return map.isEmpty();
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this set
+     * contains an element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
+     *
+     * @param o element whose presence in this set is to be tested
+     *
+     * @return <tt>true</tt> if this set contains the specified element
+     */
+    @Override
+    public
+    boolean contains(Object o) {
+        return map.containsKey(o);
+    }
+
+    /**
+     * Adds the specified element to this set if it is not already present.
+     * More formally, adds the specified element <tt>e</tt> to this set if
+     * this set contains no element <tt>e2</tt> such that
+     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
+     * If this set already contains the element, the call leaves the set
+     * unchanged and returns <tt>false</tt>.
+     *
+     * @param e element to be added to this set
+     *
+     * @return <tt>true</tt> if this set did not already contain the specified
+     *         element
+     */
+    @Override
+    public
+    boolean add(E e) {
+        return map.put(e, PRESENT) == null;
+    }
+
+    /**
+     * Removes the specified element from this set if it is present.
+     * More formally, removes an element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
+     * if this set contains such an element.  Returns <tt>true</tt> if
+     * this set contained the element (or equivalently, if this set
+     * changed as a result of the call).  (This set will not contain the
+     * element once the call returns.)
+     *
+     * @param o object to be removed from this set, if present
+     *
+     * @return <tt>true</tt> if the set contained the specified element
+     */
+    @Override
+    public
+    boolean remove(Object o) {
+        return map.remove(o) == PRESENT;
+    }
+
+    /**
+     * Removes all of the elements from this set.
+     * The set will be empty after this call returns.
+     */
+    @Override
+    public
+    void clear() {
+        map.clear();
+    }
+}