//
// TensorFlow.cs; Bindings to the TensorFlow C API for .NET
// 
// Authors:
//   Miguel de Icaza (miguel@microsoft.com)
//
using System;
using System.Numerics;
using System.Runtime.InteropServices;
using System.Text;
using size_t = System.UIntPtr;
using TF_Tensor = System.IntPtr;

namespace TensorFlow
{
	/// <summary>
	/// The Variable class holds the TFOutput nodes that are used to initialize, read and assign a value to a variable.   
	/// </summary>
	/// <remarks>
	/// A variable maintains state in the graph across calls to `run()`. You add a
	/// variable to the graph by constructing an instance of the class `Variable`.
	/// 
	/// The `Variable()` constructor requires an initial value for the variable,
	/// which can be a `Tensor` of any type and shape. The initial value defines the
	/// type and shape of the variable. After construction, the type and shape of
	/// the variable are fixed. The value can be changed using one of the assign
	/// methods.
	/// 
	/// When a variable is created a VarHandleOp is created which is returned as 
	/// the VariableOp property, an assign operation is created that can be accessed
	/// using the assignHandle and you can read the value of the variable using the
	/// ReadHandle.
	/// 
	/// When you launch the graph, variables have to be explicitly initialized before
	/// you can run Ops that use their value. You can initialize a variable by
	/// running its *initializer op*, restoring the variable from a save file, or
	/// simply running an `assign` Op that assigns a value to the variable. In fact,
	/// the variable *initializer op* is just an `assign` Op that assigns the
	/// variable's initial value to the variable itself.
	/// 
	/// There is an implicit conversion from the Variable into the VarHandleOp if
	/// used.
	/// </remarks>
	public class Variable
	{
		TFOutput variableHandle;
		TFOutput readHandle;
		TFOperation assignOp;

		/// <summary>
		/// Returns the ReadVariableOp that is used to fetch the value of the variable from the graph.
		/// </summary>
		/// <value>The read op.</value>
		public TFOutput Read => readHandle;

        /// <summary>
        /// Returns the ReadVariableOp that is used to fetch the value of the variable from the graph.
        /// </summary>
        /// <value>The read op.</value>
        public TFOutput ReadAfter(params TFOperation[] dependencies)
        {
            if(dependencies.Length > 0)
            {
                var graph = dependencies[0].graph;
                using (graph.WithDependencies(dependencies))
                {
                    return graph.ReadVariableOp(variableHandle, readHandle.OutputType);
                }
            }
            else
            {
                return readHandle;
            }
        }

        /// <summary>
        /// Returns the AssignVariableOp that is used to assign the initial value to the variable from the graph.
        /// </summary>
        /// <value>The assign op.</value>
        public TFOperation Assign => assignOp;

		/// <summary>
		/// Returns the VarHandleOp that was created using the shape of the initial value.
		/// </summary>
		/// <value>The variable op.</value>
		public TFOutput VariableOp => variableHandle;

		internal Variable (TFOutput variableHandle, TFOutput readHandle, TFOperation assignOp)
		{
			this.variableHandle = variableHandle;
			this.readHandle = readHandle;
			this.assignOp = assignOp;
		}

		/// <summary>
		/// Returns the VarHandleOp (the VariableOp property).
		/// </summary>
		/// <returns>The variable handle created for the variable.</returns>
		/// <param name="variable">Variable reference.</param>
		/// <remarks>
		/// This implicit operator exists to preserve the compatibility with code that
		/// created Variables and expected the result to be the VariableOp.
		/// </remarks>
		public static implicit operator TFOutput (Variable variable)
		{
			return variable.VariableOp;
		}
	}
}