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diff --git a/libdimension/internal/bvh.h b/libdimension/internal/bvh.h
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+/*************************************************************************
+ * Copyright (C) 2012-2014 Tavian Barnes <tavianator@tavianator.com>     *
+ *                                                                       *
+ * This file is part of The Dimension Library.                           *
+ *                                                                       *
+ * The Dimension Library is free software; you can redistribute it and/  *
+ * or modify it under the terms of the GNU Lesser General Public License *
+ * as published by the Free Software Foundation; either version 3 of the *
+ * License, or (at your option) any later version.                       *
+ *                                                                       *
+ * The Dimension Library is distributed in the hope that it will be      *
+ * useful, but WITHOUT ANY WARRANTY; without even the implied warranty   *
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  *
+ * Lesser General Public License for more details.                       *
+ *                                                                       *
+ * You should have received a copy of the GNU Lesser General Public      *
+ * License along with this program.  If not, see                         *
+ * <http://www.gnu.org/licenses/>.                                       *
+ *************************************************************************/
+
+/**
+ * @file.
+ * Bounding volume hierarchy.  This generic interface allows different data
+ * structures to be represented in the same way, thus sharing code for their
+ * traversal algorithm.
+ */
+
+#ifndef DMNSN_INTERNAL_BVH_H
+#define DMNSN_INTERNAL_BVH_H
+
+#include "internal.h"
+#include "dimension/math.h"
+#include "dimension/model.h"
+#include <stdbool.h>
+
+/// A bounding volume hierarchy.
+typedef struct dmnsn_bvh dmnsn_bvh;
+
+/// Available BVH implementations.
+typedef enum dmnsn_bvh_kind {
+  DMNSN_BVH_NONE,
+  DMNSN_BVH_PRTREE,
+} dmnsn_bvh_kind;
+
+/// Create a BVH.
+DMNSN_INTERNAL dmnsn_bvh *dmnsn_new_bvh(const dmnsn_array *objects,
+                                        dmnsn_bvh_kind kind);
+/// Delete a BVH.
+DMNSN_INTERNAL void dmnsn_delete_bvh(dmnsn_bvh *bvh);
+
+/// Find the closest ray-object intersection in the tree.
+DMNSN_INTERNAL bool dmnsn_bvh_intersection(const dmnsn_bvh *bvh, dmnsn_ray ray, dmnsn_intersection *intersection, bool reset);
+/// Determine whether a point is inside any object in the tree.
+DMNSN_INTERNAL bool dmnsn_bvh_inside(const dmnsn_bvh *bvh, dmnsn_vector point);
+/// Return the bounding box of the whole hierarchy.
+DMNSN_INTERNAL dmnsn_aabb dmnsn_bvh_aabb(const dmnsn_bvh *bvh);
+
+/// A non-flat BVH representation, used by BVH implementations.
+typedef struct dmnsn_bvh_node {
+  dmnsn_aabb aabb;                   ///< The bounding box of this node.
+  dmnsn_object *object;              ///< The object, for leaf nodes.
+  unsigned int nchildren;            ///< How many children this node has.
+  unsigned int max_children;         ///< Maximum number of children.
+  struct dmnsn_bvh_node *children[]; ///< Flexible array of children.
+} dmnsn_bvh_node;
+
+/// Create a BVH node.
+DMNSN_INTERNAL dmnsn_bvh_node *dmnsn_new_bvh_node(unsigned int max_children);
+
+/// Create a BVH leaf node.
+DMNSN_INTERNAL dmnsn_bvh_node *dmnsn_new_bvh_leaf_node(dmnsn_object *object);
+
+/// Delete a BVH node.
+DMNSN_INTERNAL void dmnsn_delete_bvh_node(dmnsn_bvh_node *node);
+
+/// Add a child to a BVH node.
+DMNSN_INTERNAL void dmnsn_bvh_node_add(dmnsn_bvh_node *parent, dmnsn_bvh_node *child);
+
+#endif // DMNSN_INTERNAL_BVH_H