Add Rope::rebalance()

src/core/rope.rs

@@ -1,4 +1,4 @@
-use std::rc::Rc;
+use {super::fibbonacci::fibbonacci, std::rc::Rc};
 
 /// [Rope](https://en.wikipedia.org/wiki/Rope_(data_structure)) data structure
 /// implementation.
@@ -43,6 +43,21 @@ impl Rope {
         Rc::new(Rope::Leaf { text })
     }
 
+    /// Concatenate two Ropes without rebalancing.
+    ///
+    /// Combines to ropes by creating a new parent node and adding `left` and
+    /// `right` as children. If `right` is [None] then the resulting node will
+    /// only have one child.
+    fn join(left: Rc<Rope>, right: Option<Rc<Rope>>) -> Rc<Self> {
+        Rc::new(Rope::Branch {
+            bytes_weight: left.total_bytes(),
+            chars_weight: left.total_chars(),
+            lines_weight: left.total_lines(),
+            left,
+            right,
+        })
+    }
+
     /// Return the total number of bytes in the text.
     pub fn total_bytes(&self) -> usize {
         match self {
@@ -94,21 +109,6 @@ impl Rope {
         }
     }
 
-    /// Concatenate two Ropes without rebalancing.
-    ///
-    /// Combines to ropes by creating a new parent node and adding `left` and
-    /// `right` as children. If `right` is [None] then the resulting node will
-    /// only have one child.
-    fn join(left: Rc<Rope>, right: Option<Rc<Rope>>) -> Rc<Self> {
-        Rc::new(Rope::Branch {
-            bytes_weight: left.total_bytes(),
-            chars_weight: left.total_chars(),
-            lines_weight: left.total_lines(),
-            left,
-            right,
-        })
-    }
-
     /// Split the rope in two at character `index`.
     ///
     /// The result is two Ropes—one containing the first 'i' characters of the
@@ -152,9 +152,19 @@ impl Rope {
         }
     }
 
+    fn is_balanced(&self) -> bool {
+        match self {
+            Rope::Branch { bytes_weight, .. } => fibbonacci(self.depth() + 2) <= *bytes_weight,
+            Rope::Leaf { .. } => true,
+        }
+    }
+
     fn rebalance(self: Rc<Rope>) -> Rc<Rope> {
-        // TODO
-        self
+        if self.is_balanced() {
+            return self;
+        }
+        let leaf_nodes: Vec<_> = self.iter_nodes().collect();
+        merge(&leaf_nodes)
     }
 
     /// Number of steps between this node and it's most distance descendant
@@ -191,6 +201,18 @@ impl Rope {
     }
 }
 
+fn merge(leaf_nodes: &[Rc<Rope>]) -> Rc<Rope> {
+    match leaf_nodes.len() {
+        0 => panic!("Attempt to merge empty list"),
+        1 => leaf_nodes[0].clone(),
+        2 => Rope::join(leaf_nodes[0].clone(), Some(leaf_nodes[1].clone())),
+        3.. => {
+            let mid = leaf_nodes.len() / 2;
+            Rope::join(merge(&leaf_nodes[..mid]), Some(merge(&leaf_nodes[mid..])))
+        }
+    }
+}
+
 struct NodeIterator {
     stack: Vec<Rc<Rope>>,
 }