// $Id$
//
//    File: nnann_function.h
// Created: Tue Mar 22 23:07:10 EDT 2011
// Creator: davidl (on Darwin Amelia.local 9.8.0 i386)
//

#ifndef _nnann_function_
#define _nnann_function_

/// The nnann_function represents an activation function. It really provides an interface
/// so different activation functions can be provided allowing the other nnann classes
/// to use them without having to know them all a priori. The interface mainly defines
/// two virtual methods. The "f(x)" method is the activation function itself while the
/// "dfdx(x)" method is the first derivative w.r.t. x. If the "dfdx(x)" method is not 
/// implemented in a base class, a numerical derivative function is provided.

class nnann_function{
	public:
		nnann_function(){};
		virtual ~nnann_function(){};

		virtual std::string GetName(void)=0;
		
		virtual double f(double x)=0; ///< Activation function

		/// Derivative of activation function.
		/// If subclass doesn't implement dfdx, then a simple numerical
		/// derivative is done. This may not be very accurate or efficient.
		virtual double dfdx(double x){ 
			double dx = 0.01;
			double dx_over_2 = dx/2.0;
			double df = f(x+dx_over_2) - f(x-dx_over_2);
			return df/dx;
		}
		
		/// Calculate error on input based on the input (x) and the target output.
		/// The default routine is just to (target-f(x))/dfdx(x).
		virtual double InputError(double x, double target){
			double df_dx = dfdx(x);
			if(df_dx==0.0)df_dx=1.0E-6;
			return (target-f(x))/df_dx;
		}
	
	protected:
	

};

#endif // _nnann_function_