/****************************************************************
* This file is part of the gpu acceleration library for gromacs.
* Author: V. Vishal
* Copyright (C) Pande Group, Stanford, 2006
*****************************************************************/

//Harmonic bonds kernel
//Input is a stream of i, j pairs
//parms is float2( b0, kA )
//Output is two streams of forces fi, fj
//Can be optimized as necessary
kernel void kbonds_harmonic(
		float xstrwidth, //atom stream width
		float2 atoms<>,
		float2 parms<>,
		float4 posq[][],
		out float3 fi<>,
		out float3 fj<>
		) {
	float2 ai, aj;
	float3 rij;
	float rinv;
	
	ai.y = floor( atoms.x / xstrwidth );
	ai.x = atoms.x - ai.y * xstrwidth;

	aj.y = floor( atoms.y / xstrwidth );
	aj.x = atoms.y - aj.y * xstrwidth;

	rij = posq[ai].xyz - posq[aj].xyz; //3
	rinv = rsqrt( dot(rij, rij) ); //6
	fi = -parms.y * ( 1.0f - parms.x * rinv ) * rij; //6
	fj = -fi; 

	//Total: 15 flops
}