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

/// A least-recently-used cache with a fixed capacity.
///
/// Both `get` and `get_mut` mark an entry as recently used.
pub struct LruCache<K, V> {
    capacity: usize,
    entries: 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,
    previous: Option<usize>,
    next: Option<usize>,
}

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

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

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

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

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

    pub fn get(&mut self, key: &K) -> Option<&V> {
        let index = *self.entries.get(key)?;
        self.mark_recent(index);
        Some(&self.nodes[index].as_ref().unwrap().value)
    }

    pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
        let index = *self.entries.get(key)?;
        self.mark_recent(index);
        Some(&mut self.nodes[index].as_mut().unwrap().value)
    }

    /// Inserts a value, returning the previous value for the key if present.
    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        if let Some(&index) = self.entries.get(&key) {
            let old_value = {
                let node = self.nodes[index].as_mut().unwrap();
                std::mem::replace(&mut node.value, value)
            };
            self.mark_recent(index);
            return Some(old_value);
        }

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

        if self.len() == self.capacity {
            let least_recent = self.tail.unwrap();
            self.remove_index(least_recent);
        }

        let index = if let Some(index) = self.free.pop() {
            index
        } else {
            self.nodes.push(None);
            self.nodes.len() - 1
        };

        self.nodes[index] = Some(Node {
            key: key.clone(),
            value,
            previous: None,
            next: self.head,
        });

        if let Some(old_head) = self.head {
            self.nodes[old_head].as_mut().unwrap().previous = Some(index);
        } else {
            self.tail = Some(index);
        }

        self.head = Some(index);
        self.entries.insert(key, index);
        None
    }

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

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

    fn mark_recent(&mut self, index: usize) {
        if self.head == Some(index) {
            return;
        }

        self.detach(index);

        let old_head = self.head;
        {
            let node = self.nodes[index].as_mut().unwrap();
            node.previous = None;
            node.next = old_head;
        }

        if let Some(old_head) = old_head {
            self.nodes[old_head].as_mut().unwrap().previous = Some(index);
        } else {
            self.tail = Some(index);
        }

        self.head = Some(index);
    }

    fn detach(&mut self, index: usize) {
        let (previous, next) = {
            let node = self.nodes[index].as_ref().unwrap();
            (node.previous, node.next)
        };

        if let Some(previous) = previous {
            self.nodes[previous].as_mut().unwrap().next = next;
        } else {
            self.head = next;
        }

        if let Some(next) = next {
            self.nodes[next].as_mut().unwrap().previous = previous;
        } else {
            self.tail = previous;
        }
    }

    fn remove_index(&mut self, index: usize) -> Node<K, V> {
        self.detach(index);
        let node = self.nodes[index].take().unwrap();
        self.entries.remove(&node.key);
        self.free.push(index);
        node
    }
}

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

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

        cache.put("a", 1);
        cache.put("b", 2);
        assert_eq!(cache.put("a", 10), Some(1));

        cache.put("c", 3);

        assert_eq!(cache.len(), 2);
        assert_eq!(cache.get(&"a"), Some(&10));
        assert_eq!(cache.get(&"b"), None);
    }

    #[test]
    fn zero_capacity_never_stores_entries() {
        let mut cache = LruCache::new(0);
        cache.put("a", 1);

        assert!(cache.is_empty());
        assert_eq!(cache.get(&"a"), None);
    }
}