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+/*
+* Copyright (c) 2006-2007 Erin Catto http://www.gphysics.com
+*
+* 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 B2_GEAR_JOINT_H
+#define B2_GEAR_JOINT_H
+
+#include "b2Joint.h"
+
+class b2RevoluteJoint;
+class b2PrismaticJoint;
+
+/// Gear joint definition. This definition requires two existing
+/// revolute or prismatic joints (any combination will work).
+/// The provided joints must attach a dynamic body to a static body.
+struct b2GearJointDef : public b2JointDef
+{
+	b2GearJointDef()
+	{
+		type = e_gearJoint;
+		joint1 = NULL;
+		joint2 = NULL;
+		ratio = 1.0f;
+	}
+
+	/// The first revolute/prismatic joint attached to the gear joint.
+	b2Joint* joint1;
+
+	/// The second revolute/prismatic joint attached to the gear joint.
+	b2Joint* joint2;
+
+	/// The gear ratio.
+	/// @see b2GearJoint for explanation.
+	float32 ratio;
+};
+
+/// A gear joint is used to connect two joints together. Either joint
+/// can be a revolute or prismatic joint. You specify a gear ratio
+/// to bind the motions together:
+/// coordinate1 + ratio * coordinate2 = constant
+/// The ratio can be negative or positive. If one joint is a revolute joint
+/// and the other joint is a prismatic joint, then the ratio will have units
+/// of length or units of 1/length.
+/// @warning The revolute and prismatic joints must be attached to
+/// fixed bodies (which must be body1 on those joints).
+class b2GearJoint : public b2Joint
+{
+public:
+	b2Vec2 GetAnchor1() const;
+	b2Vec2 GetAnchor2() const;
+
+	b2Vec2 GetReactionForce() const;
+	float32 GetReactionTorque() const;
+
+	/// Get the gear ratio.
+	float32 GetRatio() const;
+
+	//--------------- Internals Below -------------------
+
+	b2GearJoint(const b2GearJointDef* data);
+
+	void InitVelocityConstraints(const b2TimeStep& step);
+	void SolveVelocityConstraints(const b2TimeStep& step);
+	bool SolvePositionConstraints();
+
+	b2Body* m_ground1;
+	b2Body* m_ground2;
+
+	// One of these is NULL.
+	b2RevoluteJoint* m_revolute1;
+	b2PrismaticJoint* m_prismatic1;
+
+	// One of these is NULL.
+	b2RevoluteJoint* m_revolute2;
+	b2PrismaticJoint* m_prismatic2;
+
+	b2Vec2 m_groundAnchor1;
+	b2Vec2 m_groundAnchor2;
+
+	b2Vec2 m_localAnchor1;
+	b2Vec2 m_localAnchor2;
+
+	b2Jacobian m_J;
+
+	float32 m_constant;
+	float32 m_ratio;
+
+	// Effective mass
+	float32 m_mass;
+
+	// Impulse for accumulation/warm starting.
+	float32 m_force;
+};
+
+#endif