using System;
using System.Collections.Immutable;
using Optional;
using static Optional.OptionExtensions;
using static System.Collections.Immutable.ImmutableInterlocked;

/*
???? Should we have an explicit transaction class/ID?
???? Should we split things into threaded vs action
*/

namespace db;

public enum CommitResults
{
	Invalid,
	Perfect,
	Collisions,
}

public interface IID<TS>
{
	TS id { get; }
}

public class DB<TID, T> where T : IID<TID>
{
	object m_lock = new object();

	//Current snapshot of the DB.
	ImmutableDictionary<TID, T> m_objs = ImmutableDictionary<TID, T>.Empty;

	//List of committed Ids based on when they were committed.
	ImmutableList<TID> m_committed = ImmutableList<TID>.Empty;

	ImmutableDictionary<TID, T> Objects => m_objs;

	public DB()
	{
		LogGC.RegisterObjectId( m_lock );
	}

	// @@@@ TODO This returns an entity that can be changing.  It should be a lazy instantiated copy
	public Option<T> lookup( TID id )
	{
		if( m_objs.TryGetValue( id, out T obj ) )
		{
			return obj.Some();
		}
		else
		{
			// LOG
		}

		return obj.None();
	}

	public (Tx<TID, T>, Option<T>) checkout( TID id )
	{
		var tx = new Tx<TID, T>( m_committed.Count, m_activeTransaction, this );

		var v = lookup( id );

		v.Match( t => {
			tx.checkout( id );
		}, () => {
		} );

		return (tx, v);
	}

	public Tx<TID, T> checkout( TID id, out Option<T> tOut )
	{
		var (tx, v) = checkout(id);

		tOut = v;

		return tx;
	}

	public Tx<TID, T> checkout()
	{
		var tx = new Tx<TID, T>( m_committed.Count, m_activeTransaction, this );

		return tx;
	}

	public CommitResults commit( ref Tx<TID, T> co )
	{
		co = null;
		return commit_internal_single( co );
	}

	public ImmutableDictionary<TID, T> getSnapshot()
	{
		ImmutableDictionary<TID, T> res = m_objs;
		return res;
	}


	internal CommitResults commit_internal_single( Tx<TID, T> tx )
	{
		//var collision = false;

		//Check for previously committed things
		var start = tx.Start;

		var curCommitted = m_committed;

		foreach( var t in tx.Checkouts )
		{
			for( int i = start; i < curCommitted.Count; ++i )
			{
				if( !t.id.Equals( curCommitted[i] ) ) { }
				else
				{
					//collision = true;
					return CommitResults.Collisions;
				}
			}
		}

		// @@@@ LOCK
		lock( m_committed )
		{
			TID[] committed = new TID[tx.Checkouts.Count];

			for( var i = 0; i < tx.Checkouts.Count; ++i )
			{
				committed[i] = tx.Checkouts[i].id;
				m_objs = m_objs.Add( tx.Checkouts[i].id, tx.Checkouts[i] );
			}

			m_committed = m_committed.AddRange(committed);

			foreach( var v in tx.Adds )
			{
				m_objs = m_objs.Add( v.id, v );
			}

			return CommitResults.Perfect;
		}
	}



	Option<Tx<TID, T>> m_activeTransaction = Option.None<Tx<TID, T>>();

}

public enum TxStates
{
	Invalid,
	Running,
	Committed,
}


//This only works for a single thread
public class Tx<TID, T>: IDisposable where T : IID<TID>
{
	internal ImmutableList<T> Checkouts => m_checkouts;
	internal TxStates State => m_state;
	internal int Start => m_start;
	internal ImmutableList<T> Adds => m_adds;

	internal Tx( int start, DB<TID, T> db )
		:
		this(start, Option.None<Tx<TID, T>>(), db)
	{
	}

	internal Tx( int start, Option<Tx<TID, T>> parentTx, DB<TID, T> db )
	{
		m_start = start;
		m_parentTx = parentTx;
		m_childTx = m_childTx.Add(this);
		m_db = db;
		m_state = TxStates.Running;
	}

	public void Dispose()
	{
		// Dispose of unmanaged resources.
		Dispose( true );
		// Suppress finalization.
		GC.SuppressFinalize( this );
	}

	public void Dispose(bool isFromDispose )
	{
		if( isFromDispose )
		{
			m_db.commit_internal_single( this );
		}
	}

	public Option<T> checkout( TID id )
	{
		var v = m_db.lookup( id );

		v.MatchSome( t => { m_checkouts = m_checkouts.Add( t ); } );

		return v;
	}

	public void add( T obj )
	{
		m_adds = m_adds.Add(obj);
	}


	int m_start = -1;
	DB<TID, T> m_db;

	//Do we need these?  Do we need both?
	Option<Tx<TID, T>> m_parentTx;
	ImmutableList<Tx<TID, T>> m_childTx = ImmutableList<Tx<TID, T>>.Empty;

	TxStates m_state = TxStates.Invalid;
	ImmutableList<T> m_checkouts = ImmutableList<T>.Empty;

	// New objects created this pass
	ImmutableList<T> m_adds = ImmutableList<T>.Empty;
}