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

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

/// A least-recently-used cache with a fixed capacity.
///
/// Lookup and insertion are O(1) on average.
pub struct LruCache<K, V> {
    capacity: usize,
    entries: HashMap<K, usize>,
    nodes: Vec<Option<Node<K, V>>>,
    free: Vec<usize>,
    most_recent: Option<usize>,
    least_recent: Option<usize>,
}

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

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

    /// Inserts a value, evicting the least recently used entry if necessary.
    /// Returns the previous value when the key already existed.
    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        if let Some(&index) = self.entries.get(&key) {
            let old = std::mem::replace(
                &mut self.nodes[index].as_mut().unwrap().value,
                value,
            );
            self.touch(index);
            return Some(old);
        }

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

        if self.len() == self.capacity {
            self.evict_least_recent();
        }

        let index = self.free.pop().unwrap_or_else(|| {
            self.nodes.push(None);
            self.nodes.len() - 1
        });

        self.nodes[index] = Some(Node {
            key: key.clone(),
            value,
            prev: None,
            next: None,
        });
        self.entries.insert(key, index);
        self.attach_front(index);
        None
    }

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

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

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

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

        match prev {
            Some(i) => self.nodes[i].as_mut().unwrap().next = next,
            None => self.most_recent = next,
        }

        match next {
            Some(i) => self.nodes[i].as_mut().unwrap().prev = prev,
            None => self.least_recent = prev,
        }

        let node = self.nodes[index].as_mut().unwrap();
        node.prev = None;
        node.next = None;
    }

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

        {
            let node = self.nodes[index].as_mut().unwrap();
            node.prev = None;
            node.next = old_front;
        }

        if let Some(front) = old_front {
            self.nodes[front].as_mut().unwrap().prev = Some(index);
        } else {
            self.least_recent = Some(index);
        }

        self.most_recent = Some(index);
    }

    fn evict_least_recent(&mut self) {
        if let Some(index) = self.least_recent {
            self.detach(index);
            let node = self.nodes[index].take().unwrap();
            self.entries.remove(&node.key);
            self.free.push(index);
        }
    }
}

fn main() {}

#[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_an_entry_refreshes_it() {
        let mut cache = LruCache::new(2);
        cache.put(1, "old");
        cache.put(2, "two");

        assert_eq!(cache.put(1, "new"), Some("old"));
        cache.put(3, "three");

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