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

#[derive(Default)]
pub struct LayeredEnv<K, V> {
  parent: Option<Box<LayeredEnv<K, V>>>,
  inner_map: HashMap<K, V>,
}

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

  pub fn push(self) -> LayeredEnv<K, V> {
    LayeredEnv {
      parent: Some(Box::new(self)),
      inner_map: HashMap::new(),
    }
  }

  pub fn pop(self) -> LayeredEnv<K, V> {
    *self.parent.unwrap()
  }

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