Add a clear() method to bforest::{Set,Map}.

This is a lot more efficient that removing entries one by one.

lib/cretonne/src/bforest/map.rs

@@ -118,6 +118,13 @@ where
         }
     }
 
+    /// Remove all entries.
+    pub fn clear(&mut self, forest: &mut MapForest<K, V, C>) {
+        if let Some(root) = self.root.take() {
+            forest.nodes.free_tree(root);
+        }
+    }
+
     /// Create a cursor for navigating this map. The cursor is initially positioned off the end of
     /// the map.
     pub fn cursor<'a>(
@@ -322,6 +329,7 @@ mod test {
 
         let mut m = Map::<u32, f32, ()>::new();
         assert!(m.is_empty());
+        m.clear(&mut f);
 
         assert_eq!(m.get(7, &f, &()), None);
 
@@ -459,6 +467,9 @@ mod test {
         m.insert(45, 4.2, f, &());
         m.verify(f, &());
         assert_eq!(m.get(45, f, &()), Some(4.2));
+
+        m.clear(f);
+        assert!(m.is_empty());
     }
 
     #[test]
@@ -579,6 +590,9 @@ mod test {
         m.insert(805, 4.2, f, &());
         m.verify(f, &());
         assert_eq!(m.get(805, f, &()), Some(4.2));
+
+        m.clear(f);
+        m.verify(f, &());
     }
 
     // Make a tree with two barely healthy leaf nodes:

lib/cretonne/src/bforest/pool.rs

@@ -52,6 +52,20 @@ impl<F: Forest> NodePool<F> {
         self.nodes[node] = NodeData::Free { next: self.freelist };
         self.freelist = Some(node);
     }
+
+    /// Free the entire tree rooted at `node`.
+    pub fn free_tree(&mut self, node: Node) {
+        if let NodeData::Inner { size, tree, .. } = self[node] {
+            // Note that we have to capture `tree` by value to avoid borrow checker trouble.
+            for i in 0..usize::from(size + 1) {
+                // Recursively free sub-trees. This recursion can never be deeper than `MAX_PATH`,
+                // and since most trees have less than a handful of nodes, it is worthwhile to
+                // avoid the heap allocation for an iterative tree traversal.
+                self.free_tree(tree[i]);
+            }
+        }
+        self.free_node(node);
+    }
 }
 
 #[cfg(test)]

lib/cretonne/src/bforest/set.rs

@@ -115,6 +115,13 @@ where
         }
     }
 
+    /// Remove all entries.
+    pub fn clear(&mut self, forest: &mut SetForest<K, C>) {
+        if let Some(root) = self.root.take() {
+            forest.nodes.free_tree(root);
+        }
+    }
+
     /// Create a cursor for navigating this set. The cursor is initially positioned off the end of
     /// the set.
     pub fn cursor<'a>(
@@ -283,6 +290,7 @@ mod test {
 
         let mut s = Set::<u32, ()>::new();
         assert!(s.is_empty());
+        s.clear(&mut f);
         assert!(!s.contains(7, &f, &()));
 
         let c = SetCursor::new(&mut s, &mut f, &());
@@ -483,4 +491,10 @@ mod test {
         assert!(c.is_empty());
     }
 
+    #[test]
+    fn four_level_clear() {
+        let mut f = SetForest::<i32, ()>::new();
+        let mut s = dense4l(&mut f);
+        s.clear(&mut f);
+    }
 }