# Solution

``````use std::cmp::Ordering;

/// A node in the binary tree.
#[derive(Debug)]
struct Node<T: Ord> {
value: T,
left: Subtree<T>,
right: Subtree<T>,
}

/// A possibly-empty subtree.
#[derive(Debug)]
struct Subtree<T: Ord>(Option<Box<Node<T>>>);

/// A container storing a set of values, using a binary tree.
///
/// If the same value is added multiple times, it is only stored once.
#[derive(Debug)]
pub struct BinaryTree<T: Ord> {
root: Subtree<T>,
}

impl<T: Ord> BinaryTree<T> {
fn new() -> Self {
Self { root: Subtree::new() }
}

fn insert(&mut self, value: T) {
self.root.insert(value);
}

fn has(&self, value: &T) -> bool {
self.root.has(value)
}

fn len(&self) -> usize {
self.root.len()
}
}

impl<T: Ord> Subtree<T> {
fn new() -> Self {
Self(None)
}

fn insert(&mut self, value: T) {
match &mut self.0 {
None => self.0 = Some(Box::new(Node::new(value))),
Some(n) => match value.cmp(&n.value) {
Ordering::Less => n.left.insert(value),
Ordering::Equal => {}
Ordering::Greater => n.right.insert(value),
},
}
}

fn has(&self, value: &T) -> bool {
match &self.0 {
None => false,
Some(n) => match value.cmp(&n.value) {
Ordering::Less => n.left.has(value),
Ordering::Equal => true,
Ordering::Greater => n.right.has(value),
},
}
}

fn len(&self) -> usize {
match &self.0 {
None => 0,
Some(n) => 1 + n.left.len() + n.right.len(),
}
}
}

impl<T: Ord> Node<T> {
fn new(value: T) -> Self {
Self { value, left: Subtree::new(), right: Subtree::new() }
}
}

fn main() {
let mut tree = BinaryTree::new();
tree.insert("foo");
assert_eq!(tree.len(), 1);
tree.insert("bar");
assert!(tree.has(&"foo"));
}

#[cfg(test)]
mod tests {
use super::*;

#[test]
fn len() {
let mut tree = BinaryTree::new();
assert_eq!(tree.len(), 0);
tree.insert(2);
assert_eq!(tree.len(), 1);
tree.insert(1);
assert_eq!(tree.len(), 2);
tree.insert(2); // not a unique item
assert_eq!(tree.len(), 2);
}

#[test]
fn has() {
let mut tree = BinaryTree::new();
fn check_has(tree: &BinaryTree<i32>, exp: &[bool]) {
let got: Vec<bool> =
(0..exp.len()).map(|i| tree.has(&(i as i32))).collect();
assert_eq!(&got, exp);
}

check_has(&tree, &[false, false, false, false, false]);
tree.insert(0);
check_has(&tree, &[true, false, false, false, false]);
tree.insert(4);
check_has(&tree, &[true, false, false, false, true]);
tree.insert(4);
check_has(&tree, &[true, false, false, false, true]);
tree.insert(3);
check_has(&tree, &[true, false, false, true, true]);
}

#[test]
fn unbalanced() {
let mut tree = BinaryTree::new();
for i in 0..100 {
tree.insert(i);
}
assert_eq!(tree.len(), 100);
assert!(tree.has(&50));
}
}``````