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

/// A simple LRU cache. Eviction is O(n); lookup and insertion are O(1) on average.
pub struct LruCache<K, V> {
    capacity: usize,
    clock: u128,
    entries: HashMap<K, Entry<V>>,
}

struct Entry<V> {
    value: V,
    last_used: u128,
}

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

    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
    }

    /// Returns a value and marks it as most recently used.
    pub fn get(&mut self, key: &K) -> Option<&V> {
        self.clock += 1;
        let now = self.clock;
        let entry = self.entries.get_mut(key)?;
        entry.last_used = now;
        Some(&entry.value)
    }

    /// Inserts or replaces a value, returning the old value when present.
    pub fn insert(&mut self, key: K, value: V) -> Option<V> {
        if self.capacity == 0 {
            return None;
        }

        self.clock += 1;
        let now = self.clock;

        if let Some(entry) = self.entries.get_mut(&key) {
            entry.last_used = now;
            return Some(std::mem::replace(&mut entry.value, value));
        }

        if self.entries.len() == self.capacity {
            self.evict_lru();
        }

        self.entries.insert(
            key,
            Entry {
                value,
                last_used: now,
            },
        );
        None
    }

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

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

    fn evict_lru(&mut self) {
        if let Some(key) = self
            .entries
            .iter()
            .min_by_key(|(_, entry)| entry.last_used)
            .map(|(key, _)| key)
        {
            self.entries.remove(key);
        }
    }
}

fn main() {}

#[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!(!cache.contains_key(&"b"));
        assert_eq!(cache.get(&"a"), Some(&1));
        assert_eq!(cache.get(&"c"), Some(&3));
    }

    #[test]
    fn replacing_an_entry_does_not_evict_another() {
        let mut cache = LruCache::new(2);
        cache.insert(1, "old");
        cache.insert(2, "second");

        assert_eq!(cache.insert(1, "new"), Some("old"));
        assert_eq!(cache.len(), 2);

        cache.insert(3, "third");
        assert!(!cache.contains_key(&2));
        assert_eq!(cache.get(&1), Some(&"new"));
    }

    #[test]
    fn zero_capacity_never_stores_entries() {
        let mut cache = LruCache::new(0);
        cache.insert("key", "value");

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