using System;
using System.Collections.Generic;

namespace ln.collections
{
    public delegate bool CacheMayForget<K,V>(ExtendedCache<K,V> cache,V item);
    
    public class ExtendedCache<K,V>
    {
        public event CacheMayForget<K, V> OnCacheMayForget;

        private MappingBTree<K, ExtendedCacheItem<K, V>> cacheItems =
            new MappingBTree<K, ExtendedCacheItem<K, V>>((i) => i.Key);

        private BTreeValueSet<DateTime, ExtendedCacheItem<K, V>> cacheAges =
            new BTreeValueSet<DateTime, ExtendedCacheItem<K, V>>();

        private int _targetSize = 4096;
        public int TargetSize
        {
            get => _targetSize;
            set
            {
                _targetSize = value;
                if (Count > _targetSize) Invalidate();
            }
        }

        public int Count => cacheItems.Count;


        public ExtendedCache()
        {
        }

        public ExtendedCache(int targetSize) : this()
        {
            TargetSize = targetSize;
        }
        
        public void Clear()
        {
            cacheItems.Clear();
            cacheAges.Clear();
        }

        public void Invalidate()
        {
            if (Count <= _targetSize)
                return;

            foreach (ExtendedCache<K,V>.ExtendedCacheItem<K,V> eci in cacheAges.Values)
            {
                if (MayForget(eci.Value))
                    Invalidate(eci);
                
                if (Count <= _targetSize)
                    break;
            }
        }

        private bool MayForget(V item)
        {
            foreach (CacheMayForget<K,V> mayForget in OnCacheMayForget.GetInvocationList() ?? new CacheMayForget<K, V>[0])
            {
                if (!mayForget(this, item))
                    return false;
            }
            return true;
        }
        
        public V this[K key]
        {
            get
            {
                if (TryGet(key, out V value))
                    return value;
                throw new KeyNotFoundException();
            }
        }

        public bool TryGet(K key, out V value)
        {
            if (cacheItems.TryGet(key, out ExtendedCacheItem<K, V> eci))
            {
                value = eci.Value;
                return true;
            }

            value = default(V);
            return false;
        }

        private void UpdateCacheItem(ExtendedCacheItem<K, V> eci)
        {
            cacheAges.Remove(eci.Timestamp, eci);
            eci.Timestamp = DateTime.Now;
            cacheAges.Add(eci.Timestamp, eci);
        }
        
        public bool Ensure(K key, V value)
        {
            if (cacheItems.TryGet(key, out ExtendedCacheItem<K, V> eci))
            {
                if (!Object.ReferenceEquals(eci.Value, value))
                    return false;
                UpdateCacheItem(eci);
                return true;
            }
            else
            {
                eci = new ExtendedCacheItem<K, V>(key, value);
                cacheItems.Add(eci);
                cacheAges.Add(eci.Timestamp, eci);
                return true;
            }
        }

        public bool Invalidate(K key)
        {
            if (cacheItems.TryGet(key, out ExtendedCacheItem<K, V> eci))
            {
                Invalidate(eci);
                return true;
            }
            return false;
        }

        public void Invalidate(ExtendedCacheItem<K, V> eci)
        {
            cacheAges.Remove(eci.Timestamp, eci);
            cacheItems.RemoveKey(eci.Key);
        }
        
        public class ExtendedCacheItem<TK, TV>
        {
            public DateTime Timestamp;
            public TK Key;
            public TV Value;

            public ExtendedCacheItem(TK key, TV value)
            {
                Key = key;
                Value = value;
                Timestamp = DateTime.Now;
            }
        }
    }
}