Somewhat working binary expression tree

src/expr/bexpr_tree.rs

@@ -1,19 +1,103 @@
-pub struct BExprTree<T> {
-    children: Box<Vec<Node<T>>>
+use std::mem;
+
+pub struct BExprTree<T: Default> {
+    root: Box<Node<T>>
 }
 
-type BinaryOp<T> = Box<dyn Fn(T, T) -> T>;
-type UnaryOp<T> = Box<dyn Fn(T) -> T>;
+type BinaryOp<T> = dyn Fn(T, T) -> T;
+type UnaryOp<T> = dyn Fn(T) -> T;
 
-enum Node<T> {
-    Binary(BinaryOp<T>, Box<Node<T>>, Box<Node<T>>),
-    Unary(UnaryOp<T>, Box<Node<T>>),
+enum Node<T: Default> {
+    Binary(Box<BinaryOp<T>>, Box<Node<T>>, Box<Node<T>>),
+    Unary(Box<UnaryOp<T>>, Box<Node<T>>),
     Token(T)
 }
 
+impl<T: Default> Node<T> {
+    fn new() -> Self {
+        Self::Token(T::default())
+    }
+
+    fn new_token(token: T) -> Self {
+        Self::Token(token)
+    }
+
+    fn new_binary(op: &'static BinaryOp<T>, left: Node<T>, right: Node<T>) -> Self {
+        Self::Binary(
+            Box::new(op), Box::new(left), Box::new(right)
+        )
+    }
+
+    fn new_unary(op: &'static UnaryOp<T>, node: Node<T>) -> Self {
+        Self::Unary(
+            Box::new(op), Box::new(node)
+        )
+    }
+
+    // TODO: replace recursive solution with an iterative one
+    fn eval(node: Node<T>) -> T {
+        return match node {
+            Node::Token(token) => token,
+            Node::Unary(op, next) => {
+                op(Self::eval(*next))
+            },
+            Node::Binary(op, left, right) => {
+                op(Self::eval(*left), Self::eval(*right))
+            }
+        };
+    }
+}
+
+impl<T: Default> BExprTree<T> {
+    pub fn new() -> Self {
+        Self { root: Box::new(Node::new()) }
+    }
+
+    pub fn new_token(token: T) -> Self {
+        Self { root: Box::new(Node::new_token(token)) }
+    }
+
+    fn insert_bin_left(&mut self, op: &'static BinaryOp<T>, left: BExprTree<T>) {
+        let root = mem::take(self).root;
+        self.root = Box::new(Node::new_binary(op, *left.root, *root));
+    }
+
+    fn insert_bin_right(&mut self, op: &'static BinaryOp<T>, right: BExprTree<T>) {
+        let root = mem::take(self).root;
+        self.root = Box::new(Node::new_binary(op, *root, *right.root));
+    }
+
+    fn insert_unary(&mut self, op: &'static UnaryOp<T>) {
+        let root = mem::take(self).root;
+        self.root = Box::new(Node::new_unary(op, *root));
+    }
+
+    pub fn eval(self) -> T {
+        Node::eval(*self.root)
+    }
+}
+
+impl<T: Default> Default for BExprTree<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    const ADD: &'static BinaryOp<i32> = &|x, y| x + y;
+    const SUB: &'static BinaryOp<i32> = &|x, y| x - y;
+    const NEG: &'static UnaryOp<i32>= &|x| -x;
 
+    #[test]
+    fn test_insert() {
+        let mut tree = BExprTree::new_token(5);
+        tree.insert_bin_left(ADD, BExprTree::new_token(2));
+        tree.insert_bin_right(SUB, BExprTree::new_token(3));
+        tree.insert_unary(NEG);
+
+        assert_eq!(tree.eval(), -4);
+    }
 }