module.exports = FrictionEquation;

var Equation = require('./Equation');
var Vec3 = require('../math/Vec3');
var Mat3 = require('../math/Mat3');

/**
 * Constrains the slipping in a contact along a tangent
 * @class FrictionEquation
 * @constructor
 * @author schteppe
 * @param {Body} bodyA
 * @param {Body} bodyB
 * @param {Number} slipForce should be +-F_friction = +-mu * F_normal = +-mu * m * g
 * @extends Equation
 */
function FrictionEquation(bodyA, bodyB, slipForce){
    Equation.call(this,bodyA, bodyB, -slipForce, slipForce);
    this.ri = new Vec3();
    this.rj = new Vec3();
    this.t = new Vec3(); // tangent
}

FrictionEquation.prototype = new Equation();
FrictionEquation.prototype.constructor = FrictionEquation;

var FrictionEquation_computeB_temp1 = new Vec3();
var FrictionEquation_computeB_temp2 = new Vec3();
FrictionEquation.prototype.computeB = function(h){
    var a = this.a,
        b = this.b,
        bi = this.bi,
        bj = this.bj,
        ri = this.ri,
        rj = this.rj,
        rixt = FrictionEquation_computeB_temp1,
        rjxt = FrictionEquation_computeB_temp2,
        t = this.t;

    // Caluclate cross products
    ri.cross(t,rixt);
    rj.cross(t,rjxt);

    // G = [-t -rixt t rjxt]
    // And remember, this is a pure velocity constraint, g is always zero!
    var GA = this.jacobianElementA,
        GB = this.jacobianElementB;
    t.negate(GA.spatial);
    rixt.negate(GA.rotational);
    GB.spatial.copy(t);
    GB.rotational.copy(rjxt);

    var GW = this.computeGW();
    var GiMf = this.computeGiMf();

    var B = - GW * b - h * GiMf;

    return B;
};