Add an ArrayList container type.

PiperOrigin-RevId: 558887266
Change-Id: I201978bfa5e6cfa2f4e495feaecf61374baf7b7b

src/engine/engine_util_container.h

@@ -0,0 +1,64 @@
+// Copyright 2023 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_SRC_ENGINE_ENGINE_UTIL_CONTAINER_H_
+#define MUJOCO_SRC_ENGINE_ENGINE_UTIL_CONTAINER_H_
+
+#include <mujoco/mjdata.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+//---------------------------------- mjArrayList ---------------------------------------------------
+struct mjArrayList_ {
+  // pointer to mjData to allow for growth of the list
+  mjData* d;
+
+  // size of element
+  size_t element_size;
+
+  // maximum number of elements
+  size_t capacity;
+
+  // number of elements in this list
+  size_t size;
+
+  // pointer to the next segment of the array list, NULL if last
+  struct mjArrayList_* next_segment;
+
+  // buffer for data in this segment stored in d->arena
+  void* buffer;
+};
+typedef struct mjArrayList_ mjArrayList;
+
+// stack allocate and initialize new mjArrayList
+mjArrayList* mju_arrayListCreate(mjData* d, size_t element_size, size_t initial_capacity);
+
+// returns total number of elements in mjArrayList
+size_t mju_arrayListSize(mjArrayList* array_list);
+
+// copies an element into an mjArrayList
+void mju_arrayListAdd(mjArrayList* array_list, void* element);
+
+// returns a pointer to the element at the specified location of the arraylist
+// NULL returned if index is not in the mjArrayList
+void* mju_arrayListAt(mjArrayList* array_list, size_t index);
+
+#ifdef __cplusplus
+}
+#endif
+
+
+#endif  // MUJOCO_SRC_ENGINE_ENGINE_UTIL_CONTAINER_H_