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

struct Node<K, V> {
    key: Rc<K>,
    value: V,
    newer: Option<usize>,
    older: Option<usize>,
}

/// An O(1) least-recently-used cache.
///
/// Reading or updating an entry marks it as most recently used. Inserting beyond
/// the configured capacity evicts the least recently used entry.
pub struct LruCache<K, V> {
    capacity: usize,
    entries: HashMap<Rc<K>, usize>,
    nodes: Vec<Option<Node<K, V>>>,
    free: Vec<usize>,
    newest: Option<usize>,
    oldest: Option<usize>,
}

impl<K: Eq + Hash, 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(),
            newest: None,
            oldest: 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 promotes its entry to most recently used.
    pub fn get(&mut self, key: &K) -> Option<&V> {
        let index = *self.entries.get(key)?;
        self.promote(index);
        self.nodes[index].as_ref().map(|node| &node.value)
    }

    /// Returns a mutable value and promotes its entry to most recently used.
    pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
        let index = *self.entries.get(key)?;
        self.promote(index);
        self.nodes[index].as_mut().map(|node| &mut node.value)
    }

    /// Inserts a value. 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.promote(index);
            return Some(old);
        }

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

        let key = Rc::new(key);
        let index = self.allocate(Node {
            key: Rc::clone(&key),
            value,
            newer: None,
            older: self.newest,
        });

        if let Some(previous_newest) = self.newest {
            self.nodes[previous_newest].as_mut().unwrap().newer = Some(index);
        } else {
            self.oldest = Some(index);
        }

        self.newest = Some(index);
        self.entries.insert(key, index);

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

        None
    }

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

    /// Removes and returns the least recently used entry.
    pub fn pop_lru(&mut self) -> Option<(K, V)> {
        self.oldest.map(|index| self.remove_index(index))
    }

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

    fn allocate(&mut self, node: Node<K, V>) -> usize {
        if let Some(index) = self.free.pop() {
            self.nodes[index] = Some(node);
            index
        } else {
            self.nodes.push(Some(node));
            self.nodes.len() - 1
        }
    }

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

        let (newer, older) = {
            let node = self.nodes[index].as_ref().unwrap();
            (node.newer, node.older)
        };

        if let Some(newer_index) = newer {
            self.nodes[newer_index].as_mut().unwrap().older = older;
        }

        if let Some(older_index) = older {
            self.nodes[older_index].as_mut().unwrap().newer = newer;
        } else {
            self.oldest = newer;
        }

        if let Some(current_newest) = self.newest {
            self.nodes[current_newest].as_mut().unwrap().newer = Some(index);
        }

        let node = self.nodes[index].as_mut().unwrap();
        node.newer = None;
        node.older = self.newest;
        self.newest = Some(index);
    }

    fn remove_index(&mut self, index: usize) -> (K, V) {
        let node = self.nodes[index].take().unwrap();

        if let Some(newer) = node.newer {
            self.nodes[newer].as_mut().unwrap().older = node.older;
        } else {
            self.newest = node.older;
        }

        if let Some(older) = node.older {
            self.nodes[older].as_mut().unwrap().newer = node.newer;
        } else {
            self.oldest = node.newer;
        }

        self.entries.remove(node.key.as_ref());
        self.free.push(index);

        let key = match Rc::try_unwrap(node.key) {
            Ok(key) => key,
            Err(_) => unreachable!("cache retained an unexpected key reference"),
        };

        (key, node.value)
    }
}

fn main() {}

#[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_preserves_capacity_and_promotes_the_entry() {
        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);
    }
}