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

/// A fixed-capacity least-recently-used cache.
///
/// Lookups and insertions are expected O(1). Accessing an entry marks it as
/// most recently used.
pub struct LruCache<K, V> {
    capacity: usize,
    entries: HashMap<K, usize>,
    nodes: Vec<Option<Node<K, V>>>,
    free: Vec<usize>,
    least_recent: Option<usize>,
    most_recent: Option<usize>,
}

struct Node<K, V> {
    key: K,
    value: V,
    previous: Option<usize>,
    next: 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(),
            least_recent: None,
            most_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)
    }

    pub fn get(&mut self, key: &K) -> Option<&V> {
        let index = *self.entries.get(key)?;
        self.mark_most_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_most_recent(index);
        Some(&mut self.nodes[index].as_mut().unwrap().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 insert(&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.mark_most_recent(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,
            previous: self.most_recent,
            next: None,
        });

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

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

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

        self.detach(index);

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

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

        self.most_recent = 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.least_recent = next;
        }

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

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

    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);
        }
    }
}

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

    #[test]
    fn evicts_least_recently_used_entry() {
        let mut cache = LruCache::new(2);
        cache.insert("a", 1);
        cache.insert("b", 2);

        assert_eq!(cache.get(&"a"), Some(&1));
        cache.insert("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.insert("a", 1), None);
        assert_eq!(cache.insert("a", 10), Some(1));
        assert_eq!(cache.remove(&"a"), Some(10));
        assert!(cache.is_empty());
    }
}