use std::collections::{HashMap, VecDeque};
use std::hash::Hash;

pub struct LruCache<K, V> {
    capacity: usize,
    clock: u64,
    entries: HashMap<K, Entry<V>>,
    order: VecDeque<(K, u64)>,
}

struct Entry<V> {
    value: V,
    generation: u64,
}

impl<K, V> LruCache<K, V>
where
    K: Eq + Hash + Clone,
{
    pub fn new(capacity: usize) -> Self {
        Self {
            capacity,
            clock: 0,
            entries: HashMap::new(),
            order: VecDeque::new(),
        }
    }

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

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

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

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

    pub fn get(&mut self, key: &K) -> Option<&V> {
        let generation = self.next_generation();
        let entry = self.entries.get_mut(key)?;
        entry.generation = generation;
        self.order.push_back((key.clone(), generation));
        Some(&entry.value)
    }

    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        if self.capacity == 0 {
            return None;
        }

        let generation = self.next_generation();

        // Insert or refresh the key as the most recently used item.
        let old = match self.entries.get_mut(&key) {
            Some(entry) => {
                entry.generation = generation;
                Some(std::mem::replace(&mut entry.value, value))
            }
            None => {
                self.entries.insert(key.clone(), Entry { value, generation });
                None
            }
        };

        self.order.push_back((key, generation));
        self.evict_until_within_capacity();
        self.compact_if_needed();

        old
    }

    pub fn remove(&mut self, key: &K) -> Option<V> {
        self.entries.remove(key).map(|entry| entry.value)
    }

    pub fn clear(&mut self) {
        self.entries.clear();
        self.order.clear();
    }

    fn next_generation(&mut self) -> u64 {
        self.clock = self.clock.wrapping_add(1);
        self.clock
    }

    fn evict_until_within_capacity(&mut self) {
        while self.entries.len() > self.capacity {
            if let Some((key, generation)) = self.order.pop_front() {
                let is_current = self
                    .entries
                    .get(&key)
                    .is_some_and(|entry| entry.generation == generation);

                if is_current {
                    self.entries.remove(&key);
                }
            } else {
                break;
            }
        }
    }

    fn compact_if_needed(&mut self) {
        if self.order.len() <= self.entries.len().saturating_mul(4).max(16) {
            return;
        }

        // Drop stale order entries left behind by repeated accesses.
        self.order.retain(|(key, generation)| {
            self.entries
                .get(key)
                .is_some_and(|entry| entry.generation == *generation)
        });
    }
}

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

    #[test]
    fn evicts_least_recently_used_item() {
        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_existing_key_keeps_capacity_and_refreshes_recency() {
        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);
        assert_eq!(cache.get(&"c"), Some(&3));
    }

    #[test]
    fn zero_capacity_never_stores_values() {
        let mut cache = LruCache::new(0);

        cache.put("a", 1);

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