modify TreeSet.MyIterator to support both ascending and descending iteration

This also fixes a bug such that the remove() method left the iterator
in an inconsistent state.

classpath/avian/PersistentSet.java

@@ -358,6 +358,18 @@ public class PersistentSet <T> implements Iterable <T> {
     return new Cell(n, ancestors);
   }
 
+  private static <T> Cell<Node<T>> maximum(Node<T> n,
+                                           Cell<Node<T>> ancestors)
+  {
+    while (n.right != NullNode) {
+      n.right = new Node(n.right);
+      ancestors = new Cell(n, ancestors);
+      n = n.right;
+    }
+
+    return new Cell(n, ancestors);
+  }
+
   private static <T> Cell<Node<T>> successor(Node<T> n,
                                              Cell<Node<T>> ancestors)
   {
@@ -374,6 +386,22 @@ public class PersistentSet <T> implements Iterable <T> {
     return ancestors;
   }
 
+  private static <T> Cell<Node<T>> predecessor(Node<T> n,
+                                               Cell<Node<T>> ancestors)
+  {
+    if (n.left != NullNode) {
+      n.left = new Node(n.left);
+      return maximum(n.left, new Cell(n, ancestors));
+    }
+
+    while (ancestors != null && n == ancestors.value.left) {
+      n = ancestors.value;
+      ancestors = ancestors.next;
+    }
+
+    return ancestors;
+  }
+
   public Path<T> find(T value) {
     Node<T> newRoot = new Node(root);
     Cell<Node<T>> ancestors = null;
@@ -456,6 +484,15 @@ public class PersistentSet <T> implements Iterable <T> {
     }
   }
 
+  private Path<T> predecessor(Path<T> p) {
+    Cell<Node<T>> s = predecessor(p.node, p.ancestors);
+    if (s == null) {
+      return null;
+    } else {
+      return new Path(false, s.value, p.root, s.next);
+    }
+  }
+
   private static class Node <T> {
     public T value;
     public Node left;
@@ -503,6 +540,10 @@ public class PersistentSet <T> implements Iterable <T> {
       return root.successor(this);
     }
 
+    public Path<T> predecessor() {
+      return root.predecessor(this);
+    }
+
     public PersistentSet<T> remove() {
       if (fresh) throw new IllegalStateException();
 

classpath/java/util/TreeSet.java

@@ -57,9 +57,7 @@ public class TreeSet<T> extends AbstractSet<T> implements Collection<T> {
   }
 
   public Iterator<T> descendingIterator() {
-    ArrayList<T> iterable = new ArrayList<T>(this);
-    Collections.reverse(iterable);
-    return iterable.iterator();
+    return new MyIterator<T>(set.last(), true);
   }
 
   public String toString() {
@@ -141,12 +139,18 @@ public class TreeSet<T> extends AbstractSet<T> implements Collection<T> {
     private Cell<T> prevCell;
     private Cell<T> prevPrevCell;
     private boolean canRemove = false;
+    private final boolean reversed;
 
     private MyIterator(PersistentSet.Path<Cell<T>> path) {
+      this(path, false);
+    }
+
+    private MyIterator(PersistentSet.Path<Cell<T>> path, boolean reversed) {
       this.path = path;
+      this.reversed = reversed;
       if (path != null) {
         cell = path.value();
-        nextPath = path.successor();
+        nextPath = nextPath();
       }
     }
 
@@ -157,6 +161,7 @@ public class TreeSet<T> extends AbstractSet<T> implements Collection<T> {
       prevCell = start.prevCell;
       prevPrevCell = start.prevPrevCell;
       canRemove = start.canRemove;
+      reversed = start.reversed;
     }
 
     public boolean hasNext() {
@@ -166,7 +171,7 @@ public class TreeSet<T> extends AbstractSet<T> implements Collection<T> {
     public T next() {
       if (cell == null) {
         path = nextPath;
-        nextPath = path.successor();
+        nextPath = nextPath();
         cell = path.value();
       }
       prevPrevCell = prevCell;
@@ -176,6 +181,10 @@ public class TreeSet<T> extends AbstractSet<T> implements Collection<T> {
       return prevCell.value;
     }
 
+    private PersistentSet.Path nextPath() {
+      return reversed ? path.predecessor() : path.successor();
+    }
+
     public void remove() {
       if (! canRemove) throw new IllegalStateException();
 
@@ -190,11 +199,12 @@ public class TreeSet<T> extends AbstractSet<T> implements Collection<T> {
       } else {
         // cell is alone in the list.
         set = (PersistentSet) path.remove();
-        path = path.successor();
+        path = nextPath;
         if (path != null) {
           prevCell = null;
           cell = path.value();
           path = (PersistentSet.Path) set.find((Cell) cell);
+          nextPath = nextPath();
         }
       }
 

test/Tree.java

@@ -48,7 +48,39 @@ public class Tree {
     }
   }
 
+  private static void ascendingIterator() {
+    TreeSet<Integer> t = new TreeSet<Integer>();
+    t.add(7);
+    t.add(2);
+    t.add(9);
+    t.add(2);
+    Iterator<Integer> iter = t.iterator();
+    expect(2 == (int)iter.next());
+    expect(7 == (int)iter.next());
+    iter.remove();
+    expect(9 == (int)iter.next());
+    expect(!iter.hasNext());
+    isEqual(printList(t), "2, 9");
+  }
+
+  private static void descendingIterator() {
+    TreeSet<Integer> t = new TreeSet<Integer>();
+    t.add(7);
+    t.add(2);
+    t.add(9);
+    t.add(2);
+    Iterator<Integer> iter = t.descendingIterator();
+    expect(9 == (int)iter.next());
+    expect(7 == (int)iter.next());
+    iter.remove();
+    expect(2 == (int)iter.next());
+    expect(!iter.hasNext());
+    isEqual(printList(t), "2, 9");
+  }
+
   public static void main(String args[]) {
+    ascendingIterator();
+    descendingIterator();
     TreeSet<Integer> t1 = new TreeSet<Integer>(new MyCompare());
     t1.add(5); t1.add(2); t1.add(1); t1.add(8); t1.add(3);
     isEqual(printList(t1), "1, 2, 3, 5, 8");