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

/// An LRU cache with O(1) lookup, insertion, and eviction.
pub struct LruCache<K, V> {
    capacity: usize,
    map: HashMap<K, usize>,
    nodes: Vec<Option<Node<K, V>>>,
    free: Vec<usize>,
    head: Option<usize>, // Most recently used.
    tail: Option<usize>, // Least recently used.
}

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

impl<K: Eq + Hash + Clone, V> LruCache<K, V> {
    pub fn new(capacity: usize) -> Self {
        Self {
            capacity,
            map: HashMap::with_capacity(capacity),
            nodes: Vec::with_capacity(capacity),
            free: Vec::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()
    }

    /// Returns a value and marks it as most recently used.
    pub fn get(&mut self, key: &K) -> Option<&V> {
        let index = *self.map.get(key)?;
        self.touch(index);
        Some(&self.node(index).value)
    }

    /// Returns a mutable value and marks it as most recently used.
    pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
        let index = *self.map.get(key)?;
        self.touch(index);
        Some(&mut self.node_mut(index).value)
    }

    /// Inserts a value, returning the previous value for the same key.
    ///
    /// If the cache is full, the least recently used entry is evicted.
    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        if let Some(&index) = self.map.get(&key) {
            let old = std::mem::replace(&mut self.node_mut(index).value, value);
            self.touch(index);
            return Some(old);
        }

        if self.capacity == 0 {
            return None;
        }

        if self.len() == self.capacity {
            let index = self.tail.expect("a full cache must have a tail");
            self.remove_index(index);
        }

        let index = if let Some(index) = self.free.pop() {
            self.nodes[index] = Some(Node {
                key: key.clone(),
                value,
                prev: None,
                next: None,
            });
            index
        } else {
            self.nodes.push(Some(Node {
                key: key.clone(),
                value,
                prev: None,
                next: None,
            }));
            self.nodes.len() - 1
        };

        self.map.insert(key, index);
        self.attach_front(index);
        None
    }

    pub fn remove(&mut self, key: &K) -> Option<V> {
        let index = *self.map.get(key)?;
        Some(self.remove_index(index).value)
    }

    fn touch(&mut self, index: usize) {
        if self.head != Some(index) {
            self.detach(index);
            self.attach_front(index);
        }
    }

    fn detach(&mut self, index: usize) {
        let (prev, next) = {
            let node = self.node(index);
            (node.prev, node.next)
        };

        match prev {
            Some(prev) => self.node_mut(prev).next = next,
            None => self.head = next,
        }

        match next {
            Some(next) => self.node_mut(next).prev = prev,
            None => self.tail = prev,
        }

        let node = self.node_mut(index);
        node.prev = None;
        node.next = None;
    }

    fn attach_front(&mut self, index: usize) {
        let old_head = self.head;

        {
            let node = self.node_mut(index);
            node.prev = None;
            node.next = old_head;
        }

        if let Some(head) = old_head {
            self.node_mut(head).prev = Some(index);
        } else {
            self.tail = Some(index);
        }

        self.head = Some(index);
    }

    fn remove_index(&mut self, index: usize) -> Node<K, V> {
        self.detach(index);
        let node = self.nodes[index]
            .take()
            .expect("cache index must reference a node");
        self.map.remove(&node.key);
        self.free.push(index);
        node
    }

    fn node(&self, index: usize) -> &Node<K, V> {
        self.nodes[index]
            .as_ref()
            .expect("cache index must reference a node")
    }

    fn node_mut(&mut self, index: usize) -> &mut Node<K, V> {
        self.nodes[index]
            .as_mut()
            .expect("cache index must reference a node")
    }
}

#[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 updates_and_removes_entries() {
        let mut cache = LruCache::new(2);

        assert_eq!(cache.put("a", 1), None);
        assert_eq!(cache.put("a", 7), Some(1));
        assert_eq!(cache.remove(&"a"), Some(7));
        assert!(cache.is_empty());

        let mut disabled = LruCache::new(0);
        disabled.put("ignored", 1);
        assert!(disabled.is_empty());
    }
}