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+/*
+Bullet Continuous Collision Detection and Physics Library
+Copyright (c) 2003-2006 Erwin Coumans  http://continuousphysics.com/Bullet/
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the use of this software.
+Permission is granted to anyone to use this software for any purpose, 
+including commercial applications, and to alter it and redistribute it freely, 
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef JACOBIAN_ENTRY_H
+#define JACOBIAN_ENTRY_H
+
+#include "LinearMath/btVector3.h"
+#include "BulletDynamics/Dynamics/btRigidBody.h"
+
+
+//notes:
+// Another memory optimization would be to store m_1MinvJt in the remaining 3 w components
+// which makes the btJacobianEntry memory layout 16 bytes
+// if you only are interested in angular part, just feed massInvA and massInvB zero
+
+/// Jacobian entry is an abstraction that allows to describe constraints
+/// it can be used in combination with a constraint solver
+/// Can be used to relate the effect of an impulse to the constraint error
+ATTRIBUTE_ALIGNED16(class) btJacobianEntry
+{
+public:
+	btJacobianEntry() {};
+	//constraint between two different rigidbodies
+	btJacobianEntry(
+		const btMatrix3x3& world2A,
+		const btMatrix3x3& world2B,
+		const btVector3& rel_pos1,const btVector3& rel_pos2,
+		const btVector3& jointAxis,
+		const btVector3& inertiaInvA, 
+		const btScalar massInvA,
+		const btVector3& inertiaInvB,
+		const btScalar massInvB)
+		:m_linearJointAxis(jointAxis)
+	{
+		m_aJ = world2A*(rel_pos1.cross(m_linearJointAxis));
+		m_bJ = world2B*(rel_pos2.cross(-m_linearJointAxis));
+		m_0MinvJt	= inertiaInvA * m_aJ;
+		m_1MinvJt = inertiaInvB * m_bJ;
+		m_Adiag = massInvA + m_0MinvJt.dot(m_aJ) + massInvB + m_1MinvJt.dot(m_bJ);
+
+		btAssert(m_Adiag > btScalar(0.0));
+	}
+
+	//angular constraint between two different rigidbodies
+	btJacobianEntry(const btVector3& jointAxis,
+		const btMatrix3x3& world2A,
+		const btMatrix3x3& world2B,
+		const btVector3& inertiaInvA,
+		const btVector3& inertiaInvB)
+		:m_linearJointAxis(btVector3(btScalar(0.),btScalar(0.),btScalar(0.)))
+	{
+		m_aJ= world2A*jointAxis;
+		m_bJ = world2B*-jointAxis;
+		m_0MinvJt	= inertiaInvA * m_aJ;
+		m_1MinvJt = inertiaInvB * m_bJ;
+		m_Adiag =  m_0MinvJt.dot(m_aJ) + m_1MinvJt.dot(m_bJ);
+
+		btAssert(m_Adiag > btScalar(0.0));
+	}
+
+	//angular constraint between two different rigidbodies
+	btJacobianEntry(const btVector3& axisInA,
+		const btVector3& axisInB,
+		const btVector3& inertiaInvA,
+		const btVector3& inertiaInvB)
+		: m_linearJointAxis(btVector3(btScalar(0.),btScalar(0.),btScalar(0.)))
+		, m_aJ(axisInA)
+		, m_bJ(-axisInB)
+	{
+		m_0MinvJt	= inertiaInvA * m_aJ;
+		m_1MinvJt = inertiaInvB * m_bJ;
+		m_Adiag =  m_0MinvJt.dot(m_aJ) + m_1MinvJt.dot(m_bJ);
+
+		btAssert(m_Adiag > btScalar(0.0));
+	}
+
+	//constraint on one rigidbody
+	btJacobianEntry(
+		const btMatrix3x3& world2A,
+		const btVector3& rel_pos1,const btVector3& rel_pos2,
+		const btVector3& jointAxis,
+		const btVector3& inertiaInvA, 
+		const btScalar massInvA)
+		:m_linearJointAxis(jointAxis)
+	{
+		m_aJ= world2A*(rel_pos1.cross(jointAxis));
+		m_bJ = world2A*(rel_pos2.cross(-jointAxis));
+		m_0MinvJt	= inertiaInvA * m_aJ;
+		m_1MinvJt = btVector3(btScalar(0.),btScalar(0.),btScalar(0.));
+		m_Adiag = massInvA + m_0MinvJt.dot(m_aJ);
+
+		btAssert(m_Adiag > btScalar(0.0));
+	}
+
+	btScalar	getDiagonal() const { return m_Adiag; }
+
+	// for two constraints on the same rigidbody (for example vehicle friction)
+	btScalar	getNonDiagonal(const btJacobianEntry& jacB, const btScalar massInvA) const
+	{
+		const btJacobianEntry& jacA = *this;
+		btScalar lin = massInvA * jacA.m_linearJointAxis.dot(jacB.m_linearJointAxis);
+		btScalar ang = jacA.m_0MinvJt.dot(jacB.m_aJ);
+		return lin + ang;
+	}
+
+	
+
+	// for two constraints on sharing two same rigidbodies (for example two contact points between two rigidbodies)
+	btScalar	getNonDiagonal(const btJacobianEntry& jacB,const btScalar massInvA,const btScalar massInvB) const
+	{
+		const btJacobianEntry& jacA = *this;
+		btVector3 lin = jacA.m_linearJointAxis * jacB.m_linearJointAxis;
+		btVector3 ang0 = jacA.m_0MinvJt * jacB.m_aJ;
+		btVector3 ang1 = jacA.m_1MinvJt * jacB.m_bJ;
+		btVector3 lin0 = massInvA * lin ;
+		btVector3 lin1 = massInvB * lin;
+		btVector3 sum = ang0+ang1+lin0+lin1;
+		return sum[0]+sum[1]+sum[2];
+	}
+
+	btScalar getRelativeVelocity(const btVector3& linvelA,const btVector3& angvelA,const btVector3& linvelB,const btVector3& angvelB)
+	{
+		btVector3 linrel = linvelA - linvelB;
+		btVector3 angvela  = angvelA * m_aJ;
+		btVector3 angvelb  = angvelB * m_bJ;
+		linrel *= m_linearJointAxis;
+		angvela += angvelb;
+		angvela += linrel;
+		btScalar rel_vel2 = angvela[0]+angvela[1]+angvela[2];
+		return rel_vel2 + SIMD_EPSILON;
+	}
+//private:
+
+	btVector3	m_linearJointAxis;
+	btVector3	m_aJ;
+	btVector3	m_bJ;
+	btVector3	m_0MinvJt;
+	btVector3	m_1MinvJt;
+	//Optimization: can be stored in the w/last component of one of the vectors
+	btScalar	m_Adiag;
+
+};
+
+#endif //JACOBIAN_ENTRY_H