use dashmap::DashMap;
use std::collections::HashMap;
use std::hash::Hash;
use std::sync::Mutex;
use std::time::{Duration, Instant};

struct CacheEntry<V> {
    value: V,
    expires_at: Instant,
}

struct LruNode<K> {
    key: Option<K>,
    prev: usize,
    next: usize,
}

struct LruTracker<K> {
    nodes: Vec<LruNode<K>>,
    key_to_idx: HashMap<K, usize>,
    head: usize,
    tail: usize,
}

impl<K: Eq + Hash + Clone> LruTracker<K> {
    fn new() -> Self {
        let sentinel = LruNode {
            key: None,
            prev: 0,
            next: 0,
        };
        Self {
            nodes: vec![sentinel],
            key_to_idx: HashMap::new(),
            head: 0,
            tail: 0,
        }
    }

    fn touch(&mut self, key: &K) {
        if let Some(&idx) = self.key_to_idx.get(key) {
            self.detach(idx);
            self.attach_front(idx);
        }
    }

    fn push_front(&mut self, key: K) -> usize {
        let idx = self.nodes.len();
        self.nodes.push(LruNode {
            key: Some(key.clone()),
            prev: 0,
            next: 0,
        });
        self.key_to_idx.insert(key, idx);
        self.attach_front(idx);
        idx
    }

    fn pop_back(&mut self) -> Option<K> {
        if self.tail == 0 {
            return None;
        }
        let lru = self.tail;
        let key = self.nodes[lru].key.take();
        self.detach(lru);
        if let Some(ref k) = key {
            self.key_to_idx.remove(k);
        }
        key
    }

    fn remove(&mut self, key: &K) {
        if let Some(&idx) = self.key_to_idx.get(key) {
            self.detach(idx);
            self.key_to_idx.remove(key);
        }
    }

    fn clear(&mut self) {
        self.key_to_idx.clear();
        self.nodes.truncate(1);
        self.head = 0;
        self.tail = 0;
    }

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

    fn detach(&mut self, idx: usize) {
        let prev = self.nodes[idx].prev;
        let next = self.nodes[idx].next;

        if prev != 0 {
            self.nodes[prev].next = next;
        } else {
            self.head = next;
        }

        if next != 0 {
            self.nodes[next].prev = prev;
        } else {
            self.tail = prev;
        }
    }

    fn attach_front(&mut self, idx: usize) {
        self.nodes[idx].prev = 0;
        self.nodes[idx].next = self.head;

        if self.head != 0 {
            self.nodes[self.head].prev = idx;
        } else {
            self.tail = idx;
        }

        self.head = idx;
    }
}

pub struct LruTtlCache<K, V> {
    map: DashMap<K, CacheEntry<V>>,
    lru: Mutex<LruTracker<K>>,
    capacity: usize,
    ttl: Duration,
}

impl<K: Eq + Hash + Clone, V: Clone> LruTtlCache<K, V> {
    pub fn new(capacity: usize, ttl: Duration) -> Self {
        Self {
            map: DashMap::with_capacity(capacity),
            lru: Mutex::new(LruTracker::new()),
            capacity,
            ttl,
        }
    }

    pub fn get(&self, key: &K) -> Option<V> {
        let entry = self.map.get(key)?;
        let expired = entry.expires_at <= Instant::now();
        let value = entry.value.clone();
        drop(entry);

        if expired {
            self.remove(key);
            return None;
        }

        if let Ok(mut lru) = self.lru.lock() {
            lru.touch(key);
        }

        Some(value)
    }

    pub fn insert(&self, key: K, value: V) {
        self.insert_with_ttl(key, value, self.ttl);
    }

    pub fn insert_with_ttl(&self, key: K, value: V, ttl: Duration) {
        let now = Instant::now();

        if self.map.contains_key(&key) {
            self.map.insert(
                key.clone(),
                CacheEntry {
                    value,
                    expires_at: now + ttl,
                },
            );
            if let Ok(mut lru) = self.lru.lock() {
                lru.touch(&key);
            }
            return;
        }

        if let Ok(mut lru) = self.lru.lock() {
            if lru.len() >= self.capacity
                && let Some(evicted_key) = lru.pop_back()
            {
                self.map.remove(&evicted_key);
            }

            self.map.insert(
                key.clone(),
                CacheEntry {
                    value,
                    expires_at: now + ttl,
                },
            );
            lru.push_front(key);
        }
    }

    pub fn remove(&self, key: &K) -> Option<V> {
        if let Ok(mut lru) = self.lru.lock() {
            lru.remove(key);
        }
        self.map.remove(key).map(|(_, entry)| entry.value)
    }

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

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

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

    pub fn clear(&self) {
        self.map.clear();
        if let Ok(mut lru) = self.lru.lock() {
            lru.clear();
        }
    }
}

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

    #[test]
    fn test_insert_and_get() {
        let cache = LruTtlCache::new(3, Duration::from_secs(60));
        cache.insert("a", 1);
        cache.insert("b", 2);
        cache.insert("c", 3);

        assert_eq!(cache.get(&"a"), Some(1));
        assert_eq!(cache.get(&"b"), Some(2));
        assert_eq!(cache.get(&"c"), Some(3));
    }

    #[test]
    fn test_lru_eviction() {
        let cache = LruTtlCache::new(2, Duration::from_secs(60));
        cache.insert("a", 1);
        cache.insert("b", 2);
        cache.get(&"a");
        cache.insert("c", 3);

        assert_eq!(cache.get(&"a"), Some(1));
        assert_eq!(cache.get(&"b"), None);
        assert_eq!(cache.get(&"c"), Some(3));
    }

    #[test]
    fn test_ttl_expiry() {
        let cache = LruTtlCache::new(3, Duration::from_millis(10));
        cache.insert("a", 1);
        std::thread::sleep(Duration::from_millis(20));

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

    #[test]
    fn test_update_existing() {
        let cache = LruTtlCache::new(3, Duration::from_secs(60));
        cache.insert("a", 1);
        cache.insert("a", 100);

        assert_eq!(cache.get(&"a"), Some(100));
        assert_eq!(cache.len(), 1);
    }

    #[test]
    fn test_remove() {
        let cache = LruTtlCache::new(3, Duration::from_secs(60));
        cache.insert("a", 1);
        cache.insert("b", 2);

        assert_eq!(cache.remove(&"a"), Some(1));
        assert_eq!(cache.get(&"a"), None);
        assert_eq!(cache.len(), 1);
    }

    #[test]
    fn test_concurrent_access() {
        let cache = std::sync::Arc::new(LruTtlCache::new(10, Duration::from_secs(60)));
        let c1 = cache.clone();
        let c2 = cache.clone();

        let t1 = std::thread::spawn(move || {
            for i in 0..100 {
                c1.insert(i, i * 2);
            }
        });

        let t2 = std::thread::spawn(move || {
            for i in 0..100 {
                let _ = c2.get(&i);
            }
        });

        t1.join().unwrap();
        t2.join().unwrap();

        assert!(cache.len() <= 10);
    }
}