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diff --git a/src/BulletSoftBody/btDeformableMassSpringForce.h b/src/BulletSoftBody/btDeformableMassSpringForce.h
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+++ b/src/BulletSoftBody/btDeformableMassSpringForce.h
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+/*
+ Bullet Continuous Collision Detection and Physics Library
+ Copyright (c) 2019 Google Inc. http://bulletphysics.org
+ 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 BT_MASS_SPRING_H
+#define BT_MASS_SPRING_H
+
+#include "btDeformableLagrangianForce.h"
+
+class btDeformableMassSpringForce : public btDeformableLagrangianForce
+{
+public:
+//    using TVStack = btDeformableLagrangianForce::TVStack;
+    typedef btAlignedObjectArray<btVector3> TVStack;
+    btDeformableMassSpringForce()
+    {
+    }
+    
+    virtual void addScaledImplicitForce(btScalar scale, TVStack& force)
+    {
+        addScaledDampingForce(scale, force);
+    }
+    
+    virtual void addScaledExplicitForce(btScalar scale, TVStack& force)
+    {
+        addScaledElasticForce(scale, force);
+    }
+    
+    virtual void addScaledDampingForce(btScalar scale, TVStack& force)
+    {
+        int numNodes = getNumNodes();
+        btAssert(numNodes <= force.size())
+        for (int i = 0; i < m_softBodies.size(); ++i)
+        {
+            const btSoftBody* psb = m_softBodies[i];
+            for (int j = 0; j < psb->m_links.size(); ++j)
+            {
+                const btSoftBody::Link& link = psb->m_links[j];
+                btSoftBody::Node* node1 = link.m_n[0];
+                btSoftBody::Node* node2 = link.m_n[1];
+                size_t id1 = node1->index;
+                size_t id2 = node2->index;
+                
+                // damping force
+                btVector3 v_diff = (node2->m_v - node1->m_v);
+                btScalar k_damp = psb->m_dampingCoefficient;
+                btVector3 scaled_force = scale * v_diff * k_damp;
+                force[id1] += scaled_force;
+                force[id2] -= scaled_force;
+            }
+        }
+    }
+    
+    virtual void addScaledElasticForce(btScalar scale, TVStack& force)
+    {
+        int numNodes = getNumNodes();
+        btAssert(numNodes <= force.size())
+        for (int i = 0; i < m_softBodies.size(); ++i)
+        {
+            const btSoftBody* psb = m_softBodies[i];
+            for (int j = 0; j < psb->m_links.size(); ++j)
+            {
+                const btSoftBody::Link& link = psb->m_links[j];
+                btSoftBody::Node* node1 = link.m_n[0];
+                btSoftBody::Node* node2 = link.m_n[1];
+                btScalar kLST = link.Feature::m_material->m_kLST;
+                btScalar r = link.m_rl;
+                size_t id1 = node1->index;
+                size_t id2 = node2->index;
+                
+                // elastic force
+                // explicit elastic force
+                btVector3 dir = (node2->m_q - node1->m_q);
+                btVector3 dir_normalized = dir.normalized();
+                btVector3 scaled_force = scale * kLST * (dir - dir_normalized * r);
+                force[id1] += scaled_force;
+                force[id2] -= scaled_force;
+            }
+        }
+    }
+    
+    virtual void addScaledForceDifferential(btScalar scale, const TVStack& dv, TVStack& df)
+    {
+        // implicit damping force differential
+        for (int i = 0; i < m_softBodies.size(); ++i)
+        {
+            const btSoftBody* psb = m_softBodies[i];
+            btScalar scaled_k_damp = psb->m_dampingCoefficient * scale;
+            for (int j = 0; j < psb->m_links.size(); ++j)
+            {
+                const btSoftBody::Link& link = psb->m_links[j];
+                btSoftBody::Node* node1 = link.m_n[0];
+                btSoftBody::Node* node2 = link.m_n[1];
+                size_t id1 = node1->index;
+                size_t id2 = node2->index;
+                btVector3 local_scaled_df = scaled_k_damp * (dv[id2] - dv[id1]);
+                df[id1] += local_scaled_df;
+                df[id2] -= local_scaled_df;
+            }
+        }
+    }
+    
+    virtual btDeformableLagrangianForceType getForceType()
+    {
+        return BT_MASSSPRING_FORCE;
+    }
+    
+};
+
+#endif /* btMassSpring_h */