#include "header.h"

//Compute the Gradient!

//There are some commented out sections; to compute a function differently, recomment out blocks.
// vector * vecin	The inputed vector, cloned into vector alterposi
// vector * df		The gradient vector, filled as this gradfunk evaluates.
// double result	The result of the initial function evaluation
// 			Usefull if using the central difference.
// double interval	The interval; how we farstep from x to calculate slope



void gradfunk(vector * vecin, vector * df, double result, double interval)
{
//vecin and df should have the same size.
	int n = vecin->size, i;

	//We need a slightly altered vector to see how f changes with x.
	//We shall call it alterposi if it is a positive increase.
//	vector alternegi = craftvec(n, vecin->elements);
	vector alterposi = craftvec(n, vecin->elements);
	vector thrashdata = craftvecEZ(n);//Holds the parts of the function during evaluation.  Not used for anything beyond that.

	for (i=0; i< n; i++)
	{
//		alternegi.elements[i] -= interval;
		alterposi.elements[i] += interval;//Edit the interval.
//		df->elements[i] = ( sumfunk(&alternegi,&thrashdata) - sumfunk(&alternegi,&thrashdata) ) / (2*interval);	//Calculate the central difference.
//		df->elements[i] = ( result - sumfunk(&alternegi,&thrashdata) ) / (interval);	//Calculate the backward difference.
		df->elements[i] = ( sumfunk(&alterposi,&thrashdata) - result ) / (interval);	//Calculate the forward difference.
		//alternegi.elements[i] = vecin->elements[i];
		alterposi.elements[i] = vecin->elements[i];//Restore the altered vec so we're ready for the next calculation
	}

	destroyvec(&thrashdata);	//Free the garbage data.
	destroyvec(&alterposi);	//Free the positively altered vector
//	destroyvec(alternegi);  //Free the negatively altered vector
	return;
}