MakeParticle and elasticity plugin refactoring.

- Removed the explicit dependency on a plugin name.
- Creation of particles from user-defined vertices (if a plugin is present).
- Add skin support for an irregular mesh
- Fixes #642 and #674.

PiperOrigin-RevId: 561313771
Change-Id: I450f91f0d3b578cc18d97ed7180a0a8944b7dd41

plugin/elasticity/elasticity.h

@@ -0,0 +1,50 @@
+// Copyright 2022 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_PLUGIN_ELASTICITY_ELASTICITY_H_
+#define MUJOCO_PLUGIN_ELASTICITY_ELASTICITY_H_
+
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include <mujoco/mujoco.h>
+
+namespace mujoco::plugin::elasticity {
+
+struct PairHash
+{
+    template <class T1, class T2>
+    std::size_t operator() (const std::pair<T1, T2>& pair) const {
+        return std::hash<T1>()(pair.first) ^ std::hash<T2>()(pair.second);
+    }
+};
+
+// copied from mjXUtil
+void String2Vector(const std::string& txt, std::vector<int>& vec);
+
+// reads numeric attributes
+bool CheckAttr(const char* name, const mjModel* m, int instance);
+
+// Cartesian distance between 3D vectors
+mjtNum SquaredDist3(const mjtNum pos1[3], const mjtNum pos2[3]);
+
+// updates square lengths of edges
+void UpdateSquaredLengths(std::vector<mjtNum>& len,
+                          const std::vector<std::pair<int, int> >& edges,
+                          const mjtNum* x);
+
+}  // namespace mujoco::plugin::elasticity
+
+#endif  // MUJOCO_PLUGIN_ELASTICITY_ELASTICITY_H_