using System;
using System.Collections.Generic;
using System.Text;

namespace ln.collections
{
    public class ObjectIdentityCache
    {
        Dictionary<object, object> objectsByIdentity = new Dictionary<object, object>();
        Dictionary<object, object> identitiesByObject = new Dictionary<object, object>();

        Func<object, object> IdGetter;

        public ObjectIdentityCache(Func<object,object> idGetter)
        {
            if (idGetter == null)
                throw new ArgumentNullException(nameof(idGetter));

            IdGetter = idGetter;
        }

        public object this[object identity] => objectsByIdentity[identity];

        public void Update(object o)
        {
            if (identitiesByObject.TryGetValue(o, out object knownObject) && (!object.Equals(o, knownObject)))
                throw new ArgumentException("another instance already known with this identity", nameof(o));

            if (!object.ReferenceEquals(null, knownObject))
            {
                object knownIdentity = identitiesByObject[knownObject];
                object objectIdentity = IdGetter(knownObject);

                if (!object.Equals(knownIdentity, objectIdentity))
                {
                    objectsByIdentity.Remove(knownIdentity);
                    objectsByIdentity.Add(knownIdentity, knownObject);
                    identitiesByObject[knownObject] = objectIdentity;
                }
            } else
            {
                object objectIdentity = IdGetter(o);

                objectsByIdentity.Add(objectIdentity, o);
                identitiesByObject.Add(o, objectIdentity);
            }
        }

        public void RemoveObject(object o)
        {
            if (identitiesByObject.TryGetValue(o, out object objectIdentity))
            {
                identitiesByObject.Remove(o);
                objectsByIdentity.Remove(objectIdentity);
            }
        }
        public void RemoveIdentity(object objectIdentity)
        {
            if (objectsByIdentity.TryGetValue(objectIdentity, out object o))
            {
                objectsByIdentity.Remove(objectIdentity);
                identitiesByObject.Remove(o);
            }
        }


    }
}