Merge pull request #11 from azizkayumov/cut

Cut

src/connected.rs

@@ -10,7 +10,7 @@ pub fn connected(forest: &mut Vec<Node>, v: usize, w: usize) -> bool {
     );
     access(forest, v); // v is now the root of the tree
     access(forest, w);
-    // if v is not the root of the tree anymore, then v and w are connected:
+    // if access(w) messed with the root of the tree, then v and w are connected:
     !matches!(forest[v].parent, Parent::Root) || v == w
 }
 

src/cut.rs

@@ -0,0 +1,94 @@
+use crate::{
+    access::access,
+    node::{Node, Parent},
+};
+
+pub fn cut(forest: &mut Vec<Node>, v: usize) {
+    self::access(forest, v);
+    if let Some(left) = forest[v].left {
+        forest[left].parent = Parent::Root;
+        forest[v].left = None;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        connected::connected,
+        cut::cut,
+        link::link,
+        node::{Node, Parent},
+    };
+
+    fn create_nodes(n: usize) -> Vec<Node> {
+        (0..n).map(|i| Node::new(i, 0.0)).collect()
+    }
+
+    #[test]
+    pub fn base_case() {
+        let mut forest = create_nodes(2);
+        assert!(!connected(&mut forest, 0, 1)); // not connected yet
+
+        link(&mut forest, 0, 1);
+        assert!(matches!(forest[1].parent, Parent::Root));
+        assert_eq!(forest[1].right, Some(0));
+        assert_eq!(forest[1].left, None);
+        assert!(matches!(forest[0].parent, Parent::Node(1)));
+        //  1
+        //   \       <= link(0, 1)
+        //    0
+
+        assert!(connected(&mut forest, 0, 1)); // now connected
+        assert!(matches!(forest[1].parent, Parent::Root));
+        assert_eq!(forest[1].right, None);
+        assert_eq!(forest[1].left, None);
+        assert!(matches!(forest[0].parent, Parent::Path(1)));
+        //    0             1
+        //   /      =>      |
+        //  1               0
+
+        super::cut(&mut forest, 0);
+        assert!(matches!(forest[0].parent, Parent::Root));
+        assert_eq!(forest[0].right, None);
+        //    0           0
+        //   /     =>
+        //  1           1
+        assert!(!connected(&mut forest, 0, 1)); // now disconnected
+    }
+
+    #[test]
+    pub fn cut_into_two_subtrees() {
+        let mut forest = create_nodes(5);
+        forest[0].left = Some(1);
+        forest[1].parent = Parent::Node(0);
+        forest[1].left = Some(2);
+        forest[2].parent = Parent::Node(1);
+        forest[3].right = Some(4);
+        forest[4].parent = Parent::Node(3);
+        // Given two trees:
+        //       0       3
+        //      /         \
+        //     1           4
+        //    /
+        //   2
+        link(&mut forest, 2, 3);
+        // link(2, 3) should result in:
+        //      3
+        //      | \
+        //      4  2
+        //         |
+        //         1
+        //          \
+        //           0
+        assert!(matches!(forest[3].parent, Parent::Root));
+        assert!(matches!(forest[4].parent, Parent::Path(3)));
+        assert_eq!(forest[3].right, Some(2));
+        assert!(matches!(forest[2].parent, Parent::Node(3)));
+        assert!(matches!(forest[1].parent, Parent::Path(2)));
+        assert_eq!(forest[1].right, Some(0));
+        assert!(connected(&mut forest, 2, 3));
+        cut(&mut forest, 2);
+        assert!(!connected(&mut forest, 2, 3));
+        assert!(!connected(&mut forest, 2, 4));
+    }
+}

src/lib.rs

@@ -1,6 +1,7 @@
 #![allow(dead_code, clippy::module_name_repetitions)] // yes, I want to name my structs with the same name as the file
 mod access;
 mod connected;
+mod cut;
 mod link;
 mod node;
 mod splay;