add intersection for unsorted lists

Changed the way the intersection is computed. An intersection does
not return duplicate values. So the result is like a set.

primitiveCollections/src/main/java/org/lucares/collections/IntList.java

@@ -22,7 +22,6 @@ public final class IntList implements Serializable, Cloneable {
 	// TODO support sublists
 	// TODO add lastIndexOf
 	// TODO remove bounds checks that are handled by java, e.g. negative indices
-	// TODO intersection for unsorted lists
 
 	private static final long serialVersionUID = 2622570032686034909L;
 
@@ -361,12 +360,19 @@ public final class IntList implements Serializable, Cloneable {
 	 * <p>
 	 * This method does not release any memory. Call {@link #trim()} to free unused
 	 * memory.
+	 * <p>
+	 * For a method that computes the intersection of two lists and also removes
+	 * duplicate values, see {@link #intersection(IntList, IntList)}.
+	 * <p>
+	 * The algorithm has a complexity of O(n*log(m)), where n is the length of
+	 * {@code this} and m the length of {@code retain}.
 	 *
 	 * @param retain
 	 *            the elements to retain
 	 * @throws NullPointerException
 	 *             if the specified {@link IntList} is null
 	 * @see #trim()
+	 * @see #intersection(IntList, IntList)
 	 */
 	public void retainAll(final IntList retain) {
 
@@ -747,8 +753,17 @@ public final class IntList implements Serializable, Cloneable {
 	/**
 	 * Returns a list with all elements that are in {@code a} and {@code b}.
 	 * <p>
-	 * The cardinality of each element will be equal to the minimum of the
-	 * cardinality of each element in the given lists.
+	 * The cardinality of each element will be equal to one (like in a set). This
+	 * method returns a new list. The list can have unused capacity. Consider using
+	 * {@link #trim()}.
+	 * <p>
+	 * See {@link #retainAll(IntList)} for a method that modifies the list and keeps
+	 * duplicate values.
+	 * <p>
+	 * If both lists are sorted, then the complexity is O(n+m), where n is the
+	 * length of the first list and m the length of the second list. If at least one
+	 * list is not sorted, then the complexity is O(n*log(m)), where n is the length
+	 * of the shorter list and m the length of the longer list.
 	 *
 	 * @param a
 	 *            a sorted {@link IntList}
@@ -758,33 +773,99 @@ public final class IntList implements Serializable, Cloneable {
 	 *         {@code b}
 	 * @throws NullPointerException
 	 *             if {@code a} or {@code b} is null
+	 * @see #retainAll(IntList)
+	 * @see #trim()
 	 */
 	public static IntList intersection(final IntList a, final IntList b) {
-		final IntList result = new IntList(Math.min(a.size(), b.size()));
+		final IntList result;
 
 		if (a.isSorted() && b.isSorted()) {
-			int l = 0;
-			int r = 0;
-
-			while (l < a.size() && r < b.size()) {
-
-				final int lv = a.get(l);
-				final int rv = b.get(r);
-
-				if (lv < rv) {
-					l++;
-				} else if (lv > rv) {
-					r++;
-				} else {
-					result.add(lv);
-					l++;
-					r++;
-				}
-			}
+			result = intersectionSorted(a, b);
 		} else {
-			throw new UnsupportedOperationException("retainAll on unsorted lists is not yet supported");
+			result = intersectionUnsorted(a, b);
 		}
 		return result;
 	}
 
+	/**
+	 * Implements an intersection algorithm with O(n+m), where n is the length of
+	 * the first list and m the length of the second list.
+	 *
+	 * @param a
+	 *            first list
+	 * @param b
+	 *            second list
+	 * @return the intersection
+	 */
+	private static IntList intersectionSorted(final IntList a, final IntList b) {
+		final int aSize = a.size();
+		final int bSize = b.size();
+		final IntList result = new IntList(Math.min(aSize, bSize));
+
+		assert a.isSorted() : "first list must be sorted";
+		assert b.isSorted() : "second list must be sorted";
+
+		int l = 0;
+		int r = 0;
+
+		while (l < aSize && r < bSize) {
+
+			final int lv = a.get(l);
+			final int rv = b.get(r);
+
+			if (lv < rv) {
+				l++;
+			} else if (lv > rv) {
+				r++;
+			} else {
+				result.add(lv);
+				do {
+					l++;
+				} while (l < aSize && lv == a.get(l));
+				do {
+					r++;
+				} while (r < bSize && rv == b.get(r));
+			}
+		}
+		return result;
+	}
+
+	/**
+	 * Implements an algorithm with O(n*log(m)), where n is the length of the
+	 * shorter list and m the length of the longer list.
+	 *
+	 * @param a
+	 *            first list
+	 * @param b
+	 *            second list
+	 * @return the intersection
+	 */
+	private static IntList intersectionUnsorted(final IntList a, final IntList b) {
+		final int aSize = a.size();
+		final int bSize = b.size();
+		final IntList result;
+
+		if (aSize < bSize) {
+			result = new IntList(Math.min(aSize, bSize));
+
+			assert !a.isSorted() || !b.isSorted() : "at least one list must be unsorted";
+
+			for (int l = 0; l < aSize; l++) {
+				final int lv = a.get(l);
+
+				if (b.indexOf(lv) >= 0) {
+					result.add(lv);
+				}
+
+				while (l + 1 < aSize && lv == a.get(l + 1)) {
+					l++;
+				}
+			}
+		} else {
+			result = intersectionUnsorted(b, a);
+		}
+
+		return result;
+	}
+
 }