use std::collections::{HashMap, VecDeque};
use std::hash::Hash;

/// A small, safe LRU cache.
///
/// The most recently used key is stored at the back of `order`; when capacity is
/// exceeded, the least recently used key is evicted from the front.
#[derive(Debug, Clone)]
pub struct LruCache<K, V> {
    capacity: usize,
    map: HashMap<K, V>,
    order: VecDeque<K>,
}

impl<K, V> LruCache<K, V>
where
    K: Eq + Hash + Clone,
{
    /// Creates an empty cache with the given maximum capacity.
    pub fn new(capacity: usize) -> Self {
        Self {
            capacity,
            map: HashMap::new(),
            order: VecDeque::new(),
        }
    }

    pub fn capacity(&self) -> usize {
        self.capacity
    }

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

    pub fn is_empty(&self) -> bool {
        self.map.is_empty()
    }

    pub fn contains_key(&self, key: &K) -> bool {
        self.map.contains_key(key)
    }

    /// Returns a value by key and marks it as recently used.
    pub fn get(&mut self, key: &K) -> Option<&V> {
        if !self.map.contains_key(key) {
            return None;
        }

        self.touch(key);
        self.map.get(key)
    }

    /// Inserts or updates a value.
    ///
    /// Returns the previous value when updating an existing key.
    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        if self.capacity == 0 {
            return None;
        }

        if self.map.contains_key(&key) {
            self.touch(&key);
            return self.map.insert(key, value);
        }

        if self.map.len() == self.capacity {
            if let Some(oldest) = self.order.pop_front() {
                self.map.remove(&oldest);
            }
        }

        self.order.push_back(key.clone());
        self.map.insert(key, value)
    }

    /// Removes a key from the cache.
    pub fn remove(&mut self, key: &K) -> Option<V> {
        self.order.retain(|existing| existing != key);
        self.map.remove(key)
    }

    pub fn clear(&mut self) {
        self.map.clear();
        self.order.clear();
    }

    fn touch(&mut self, key: &K) {
        self.order.retain(|existing| existing != key);
        self.order.push_back(key.clone());
    }
}

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

    #[test]
    fn evicts_least_recently_used_entry() {
        let mut cache = LruCache::new(2);

        cache.put("a", 1);
        cache.put("b", 2);
        cache.get(&"a");
        cache.put("c", 3);

        assert_eq!(cache.get(&"a"), Some(&1));
        assert_eq!(cache.get(&"b"), None);
        assert_eq!(cache.get(&"c"), Some(&3));
    }

    #[test]
    fn updating_existing_key_preserves_capacity_and_refreshes_recency() {
        let mut cache = LruCache::new(2);

        cache.put(1, "one");
        cache.put(2, "two");
        assert_eq!(cache.put(1, "ONE"), Some("one"));
        cache.put(3, "three");

        assert_eq!(cache.len(), 2);
        assert_eq!(cache.get(&1), Some(&"ONE"));
        assert_eq!(cache.get(&2), None);
        assert_eq!(cache.get(&3), Some(&"three"));
    }
}