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diff --git a/thirdparty/bullet/BulletSoftBody/btPreconditioner.h b/thirdparty/bullet/BulletSoftBody/btPreconditioner.h
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+++ b/thirdparty/bullet/BulletSoftBody/btPreconditioner.h
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+/*
+ Written by Xuchen Han <xuchenhan2015@u.northwestern.edu>
+ 
+ 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_PRECONDITIONER_H
+#define BT_PRECONDITIONER_H
+
+class Preconditioner
+{
+public:
+    typedef btAlignedObjectArray<btVector3> TVStack;
+    virtual void operator()(const TVStack& x, TVStack& b) = 0;
+    virtual void reinitialize(bool nodeUpdated) = 0;
+    virtual ~Preconditioner(){}
+};
+
+class DefaultPreconditioner : public Preconditioner
+{
+public:
+    virtual void operator()(const TVStack& x, TVStack& b)
+    {
+        btAssert(b.size() == x.size());
+        for (int i = 0; i < b.size(); ++i)
+            b[i] = x[i];
+    }
+    virtual void reinitialize(bool nodeUpdated)
+    {
+    }
+    
+    virtual ~DefaultPreconditioner(){}
+};
+
+class MassPreconditioner : public Preconditioner
+{
+    btAlignedObjectArray<btScalar> m_inv_mass;
+    const btAlignedObjectArray<btSoftBody *>& m_softBodies;
+public:
+    MassPreconditioner(const btAlignedObjectArray<btSoftBody *>& softBodies)
+    : m_softBodies(softBodies)
+    {
+    }
+    
+    virtual void reinitialize(bool nodeUpdated)
+    {
+        if (nodeUpdated)
+        {
+            m_inv_mass.clear();
+            for (int i = 0; i < m_softBodies.size(); ++i)
+            {
+                btSoftBody* psb = m_softBodies[i];
+                for (int j = 0; j < psb->m_nodes.size(); ++j)
+                    m_inv_mass.push_back(psb->m_nodes[j].m_im);
+            }
+        }
+    }
+    
+    virtual void operator()(const TVStack& x, TVStack& b)
+    {
+        btAssert(b.size() == x.size());
+        btAssert(m_inv_mass.size() == x.size());
+        for (int i = 0; i < b.size(); ++i)
+        {
+            b[i] = x[i] * m_inv_mass[i];
+        }
+    }
+};
+
+#endif /* BT_PRECONDITIONER_H */