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

struct Node<K, V> {
    value: V,
    prev: Option<K>,
    next: Option<K>,
}

/// A small LRU cache with fixed capacity.
///
/// The head of the linked list is the most recently used entry; the tail is the
/// least recently used entry and is evicted first when capacity is exceeded.
pub struct LruCache<K, V> {
    capacity: usize,
    map: HashMap<K, Node<K, V>>,
    head: Option<K>,
    tail: Option<K>,
}

impl<K, V> LruCache<K, V>
where
    K: Eq + Hash + Clone,
{
    pub fn new(capacity: usize) -> Self {
        Self {
            capacity,
            map: HashMap::new(),
            head: None,
            tail: None,
        }
    }

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

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

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

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

    pub fn clear(&mut self) {
        self.map.clear();
        self.head = None;
        self.tail = None;
    }

    pub fn get(&mut self, key: &K) -> Option<&V> {
        if self.map.contains_key(key) {
            self.move_to_front(key);
            self.map.get(key).map(|node| &node.value)
        } else {
            None
        }
    }

    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        if self.capacity == 0 {
            return None;
        }

        if self.map.contains_key(&key) {
            let old = {
                let node = self.map.get_mut(&key).expect("key was just checked");
                std::mem::replace(&mut node.value, value)
            };
            self.move_to_front(&key);
            return Some(old);
        }

        self.insert_front(key, value);

        if self.map.len() > self.capacity {
            self.pop_lru();
        }

        None
    }

    pub fn remove(&mut self, key: &K) -> Option<V> {
        self.detach(key);
        self.map.remove(key).map(|node| node.value)
    }

    fn insert_front(&mut self, key: K, value: V) {
        let old_head = self.head.clone();

        self.map.insert(
            key.clone(),
            Node {
                value,
                prev: None,
                next: old_head.clone(),
            },
        );

        if let Some(head_key) = old_head {
            if let Some(head) = self.map.get_mut(&head_key) {
                head.prev = Some(key.clone());
            }
        } else {
            self.tail = Some(key.clone());
        }

        self.head = Some(key);
    }

    fn move_to_front(&mut self, key: &K) {
        if self.head.as_ref() == Some(key) {
            return;
        }

        self.detach(key);

        let old_head = self.head.clone();

        if let Some(node) = self.map.get_mut(key) {
            node.prev = None;
            node.next = old_head.clone();
        }

        if let Some(head_key) = old_head {
            if let Some(head) = self.map.get_mut(&head_key) {
                head.prev = Some(key.clone());
            }
        } else {
            self.tail = Some(key.clone());
        }

        self.head = Some(key.clone());
    }

    fn detach(&mut self, key: &K) {
        let (prev, next) = match self.map.get(key) {
            Some(node) => (node.prev.clone(), node.next.clone()),
            None => return,
        };

        match prev.as_ref() {
            Some(prev_key) => {
                if let Some(prev_node) = self.map.get_mut(prev_key) {
                    prev_node.next = next.clone();
                }
            }
            None => {
                self.head = next.clone();
            }
        }

        match next.as_ref() {
            Some(next_key) => {
                if let Some(next_node) = self.map.get_mut(next_key) {
                    next_node.prev = prev.clone();
                }
            }
            None => {
                self.tail = prev.clone();
            }
        }

        if let Some(node) = self.map.get_mut(key) {
            node.prev = None;
            node.next = None;
        }
    }

    fn pop_lru(&mut self) -> Option<(K, V)> {
        let key = self.tail.clone()?;
        self.detach(&key);
        self.map.remove_entry(&key).map(|(key, node)| (key, node.value))
    }
}

#[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);
        assert_eq!(cache.get(&"a"), Some(&1));

        cache.put("c", 3);

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

    #[test]
    fn updating_existing_key_refreshes_it() {
        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.get(&2), None);
        assert_eq!(cache.get(&1), Some(&"ONE"));
        assert_eq!(cache.get(&3), Some(&"three"));
    }
}