use std::collections::HashMap;
use std::hash::Hash;

/// Layered environment that lets us push and pop frames and lookup across
/// layers.
#[derive(Debug)]
pub struct LayeredEnv<K, V> {
  inner: Option<InnerEnv<K, V>>,
}

impl<K: Hash + Eq, V> Default for LayeredEnv<K, V> {
  fn default() -> Self {
    LayeredEnv {
      inner: Some(InnerEnv::new()),
    }
  }
}

impl<K: Hash + Eq, V> LayeredEnv<K, V> {
  pub fn insert(&mut self, key: K, value: V) {
    self.inner.as_mut().unwrap().map.insert(key, value);
  }

  pub fn push(&mut self) {
    let inner = self.inner.take().unwrap();
    self.inner = Some(InnerEnv::with_parent(inner));
  }

  pub fn pop(&mut self) {
    let inner = self.inner.take().unwrap();
    self.inner = Some(*inner.parent.unwrap());
  }

  /// Look up the key within the environment.
  ///
  /// If a key exists in multiple layers at once, the top-most one (of the
  /// stack) will shadow all the others.
  ///
  /// TODO: Make a "lookup" function that traverses all environments.
  pub fn lookup(&self, key: &K) -> Option<&V> {
    self.inner.as_ref().unwrap().lookup(key)
  }
}

#[derive(Debug)]
struct InnerEnv<K, V> {
  parent: Option<Box<InnerEnv<K, V>>>,
  map: HashMap<K, V>,
}

impl<K: Hash + Eq, V> InnerEnv<K, V> {
  fn new() -> Self {
    InnerEnv {
      parent: None,
      map: HashMap::new(),
    }
  }

  fn with_parent(parent: InnerEnv<K, V>) -> Self {
    InnerEnv {
      parent: Some(Box::new(parent)),
      map: HashMap::new(),
    }
  }

  fn lookup(&self, key: &K) -> Option<&V> {
    match self.map.get(key) {
      Some(v) => Some(v),
      None => match self.parent.as_ref() {
        Some(p) => p.lookup(key),
        None => None,
      },
    }
  }
}