UT_BVH.h

/*
 * Copyright (c) 2018 Side Effects Software Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 * COMMENTS:
 *      Bounding Volume Hierarchy (BVH) implementation.
 *      To call functions not implemented here, also include UT_BVHImpl.h
 */

#pragma once

#ifndef __HDK_UT_BVH_h__
#define __HDK_UT_BVH_h__

#include "UT_FixedVector.h"
#include "UT_SmallArray.h"
#include "SYS_Types.h"
#include <limits>
#include <memory>

template<typename T> class UT_Array;
class v4uf;
class v4uu;

namespace HDK_Sample {

namespace UT {

template<typename T,uint NAXES>
struct Box {
    T vals[NAXES][2];

    SYS_FORCE_INLINE Box() noexcept = default;
    SYS_FORCE_INLINE constexpr Box(const Box &other) noexcept = default;
    SYS_FORCE_INLINE constexpr Box(Box &&other) noexcept = default;
    SYS_FORCE_INLINE Box& operator=(const Box &other) noexcept = default;
    SYS_FORCE_INLINE Box& operator=(Box &&other) noexcept = default;

    template<typename S>
    SYS_FORCE_INLINE Box(const Box<S,NAXES>& other) noexcept {
        static_assert((std::is_pod<Box<T,NAXES>>::value) || !std::is_pod<T>::value,
            "UT::Box should be POD, for better performance in UT_Array, etc.");

        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = T(other.vals[axis][0]);
            vals[axis][1] = T(other.vals[axis][1]);
        }
    }
    template<typename S,bool INSTANTIATED>
    SYS_FORCE_INLINE Box(const UT_FixedVector<S,NAXES,INSTANTIATED>& pt) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = pt[axis];
            vals[axis][1] = pt[axis];
        }
    }
    template<typename S>
    SYS_FORCE_INLINE Box& operator=(const Box<S,NAXES>& other) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = T(other.vals[axis][0]);
            vals[axis][1] = T(other.vals[axis][1]);
        }
        return *this;
    }

    SYS_FORCE_INLINE const T* operator[](const size_t axis) const noexcept {
        UT_ASSERT_P(axis < NAXES);
        return vals[axis];
    }
    SYS_FORCE_INLINE T* operator[](const size_t axis) noexcept {
        UT_ASSERT_P(axis < NAXES);
        return vals[axis];
    }

    SYS_FORCE_INLINE void initBounds() noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = std::numeric_limits<T>::max();
            vals[axis][1] = -std::numeric_limits<T>::max();
        }
    }
    /// Copy the source box.
    /// NOTE: This is so that in templated code that may have a Box or a
    ///       UT_FixedVector, it can call initBounds and still work.
    SYS_FORCE_INLINE void initBounds(const Box<T,NAXES>& src) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = src.vals[axis][0];
            vals[axis][1] = src.vals[axis][1];
        }
    }
    /// Initialize with the union of the source boxes.
    /// NOTE: This is so that in templated code that may have Box's or a
    ///       UT_FixedVector's, it can call initBounds and still work.
    SYS_FORCE_INLINE void initBoundsUnordered(const Box<T,NAXES>& src0, const Box<T,NAXES>& src1) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = SYSmin(src0.vals[axis][0], src1.vals[axis][0]);
            vals[axis][1] = SYSmax(src0.vals[axis][1], src1.vals[axis][1]);
        }
    }
    SYS_FORCE_INLINE void combine(const Box<T,NAXES>& src) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            T& minv = vals[axis][0];
            T& maxv = vals[axis][1];
            const T curminv = src.vals[axis][0];
            const T curmaxv = src.vals[axis][1];
            minv = (minv < curminv) ? minv : curminv;
            maxv = (maxv > curmaxv) ? maxv : curmaxv;
        }
    }
    SYS_FORCE_INLINE void enlargeBounds(const Box<T,NAXES>& src) noexcept {
        combine(src);
    }

    template<typename S,bool INSTANTIATED>
    SYS_FORCE_INLINE
    void initBounds(const UT_FixedVector<S,NAXES,INSTANTIATED>& pt) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = pt[axis];
            vals[axis][1] = pt[axis];
        }
    }
    template<bool INSTANTIATED>
    SYS_FORCE_INLINE
    void initBounds(const UT_FixedVector<T,NAXES,INSTANTIATED>& min, const UT_FixedVector<T,NAXES,INSTANTIATED>& max) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = min[axis];
            vals[axis][1] = max[axis];
        }
    }
    template<bool INSTANTIATED>
    SYS_FORCE_INLINE
    void initBoundsUnordered(const UT_FixedVector<T,NAXES,INSTANTIATED>& p0, const UT_FixedVector<T,NAXES,INSTANTIATED>& p1) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = SYSmin(p0[axis], p1[axis]);
            vals[axis][1] = SYSmax(p0[axis], p1[axis]);
        }
    }
    template<bool INSTANTIATED>
    SYS_FORCE_INLINE
    void enlargeBounds(const UT_FixedVector<T,NAXES,INSTANTIATED>& pt) noexcept {
        for (uint axis = 0; axis < NAXES; ++axis) {
            vals[axis][0] = SYSmin(vals[axis][0], pt[axis]);
            vals[axis][1] = SYSmax(vals[axis][1], pt[axis]);
        }
    }

    SYS_FORCE_INLINE
    UT_FixedVector<T,NAXES> getMin() const noexcept {
        UT_FixedVector<T,NAXES> v;
        for (uint axis = 0; axis < NAXES; ++axis) {
            v[axis] = vals[axis][0];
        }
        return v;
    }

    SYS_FORCE_INLINE
    UT_FixedVector<T,NAXES> getMax() const noexcept {
        UT_FixedVector<T,NAXES> v;
        for (uint axis = 0; axis < NAXES; ++axis) {
            v[axis] = vals[axis][1];
        }
        return v;
    }

    T diameter2() const noexcept {
        T diff = (vals[0][1]-vals[0][0]);
        T sum = diff*diff;
        for (uint axis = 1; axis < NAXES; ++axis) {
            diff = (vals[axis][1]-vals[axis][0]);
            sum += diff*diff;
        }
        return sum;
    }
    T volume() const noexcept {
        T product = (vals[0][1]-vals[0][0]);
        for (uint axis = 1; axis < NAXES; ++axis) {
            product *= (vals[axis][1]-vals[axis][0]);
        }
        return product;
    }
    T half_surface_area() const noexcept {
        if (NAXES==1) {
            // NOTE: Although this should technically be 1,
            //       that doesn't make any sense as a heuristic,
            //       so we fall back to the "volume" of this box.
            return (vals[0][1]-vals[0][0]);
        }
        if (NAXES==2) {
            const T d0 = (vals[0][1]-vals[0][0]);
            const T d1 = (vals[1][1]-vals[1][0]);
            return d0 + d1;
        }
        if (NAXES==3) {
            const T d0 = (vals[0][1]-vals[0][0]);
            const T d1 = (vals[1][1]-vals[1][0]);
            const T d2 = (vals[2][1]-vals[2][0]);
            return d0*d1 + d1*d2 + d2*d0;
        }
        if (NAXES==4) {
            const T d0 = (vals[0][1]-vals[0][0]);
            const T d1 = (vals[1][1]-vals[1][0]);
            const T d2 = (vals[2][1]-vals[2][0]);
            const T d3 = (vals[3][1]-vals[3][0]);
            // This is just d0d1d2 + d1d2d3 + d2d3d0 + d3d0d1 refactored.
            const T d0d1 = d0*d1;
            const T d2d3 = d2*d3;
            return d0d1*(d2+d3) + d2d3*(d0+d1);
        }

        T sum = 0;
        for (uint skipped_axis = 0; skipped_axis < NAXES; ++skipped_axis) {
            T product = 1;
            for (uint axis = 0; axis < NAXES; ++axis) {
                if (axis != skipped_axis) {
                    product *= (vals[axis][1]-vals[axis][0]);
                }
            }
            sum += product;
        }
        return sum;
    }
    T axis_sum() const noexcept {
        T sum = (vals[0][1]-vals[0][0]);
        for (uint axis = 1; axis < NAXES; ++axis) {
            sum += (vals[axis][1]-vals[axis][0]);
        }
        return sum;
    }
    template<bool INSTANTIATED0,bool INSTANTIATED1>
    SYS_FORCE_INLINE void intersect(
        T &box_tmin,
        T &box_tmax,
        const UT_FixedVector<uint,NAXES,INSTANTIATED0> &signs,
        const UT_FixedVector<T,NAXES,INSTANTIATED1> &origin,
        const UT_FixedVector<T,NAXES,INSTANTIATED1> &inverse_direction
    ) const noexcept {
        for (int axis = 0; axis < NAXES; ++axis)
        {
            uint sign = signs[axis];
            T t1 = (vals[axis][sign]   - origin[axis]) * inverse_direction[axis];
            T t2 = (vals[axis][sign^1] - origin[axis]) * inverse_direction[axis];
            box_tmin = SYSmax(t1, box_tmin);
            box_tmax = SYSmin(t2, box_tmax);
        }
    }
    SYS_FORCE_INLINE void intersect(const Box& other, Box& dest) const noexcept {
        for (int axis = 0; axis < NAXES; ++axis)
        {
            dest.vals[axis][0] = SYSmax(vals[axis][0], other.vals[axis][0]);
            dest.vals[axis][1] = SYSmin(vals[axis][1], other.vals[axis][1]);
        }
    }
    template<bool INSTANTIATED>
    SYS_FORCE_INLINE T minDistance2(
        const UT_FixedVector<T,NAXES,INSTANTIATED> &p
    ) const noexcept {
        T diff = SYSmax(SYSmax(vals[0][0]-p[0], p[0]-vals[0][1]), T(0.0f));
        T d2 = diff*diff;
        for (int axis = 1; axis < NAXES; ++axis)
        {
            diff = SYSmax(SYSmax(vals[axis][0]-p[axis], p[axis]-vals[axis][1]), T(0.0f));
            d2 += diff*diff;
        }
        return d2;
    }
    template<bool INSTANTIATED>
    SYS_FORCE_INLINE T maxDistance2(
        const UT_FixedVector<T,NAXES,INSTANTIATED> &p
    ) const noexcept {
        T diff = SYSmax(p[0]-vals[0][0], vals[0][1]-p[0]);
        T d2 = diff*diff;
        for (int axis = 1; axis < NAXES; ++axis)
        {
            diff = SYSmax(p[axis]-vals[axis][0], vals[axis][1]-p[axis]);
            d2 += diff*diff;
        }
        return d2;
    }
};

/// Used by BVH::init to specify the heuristic to use for choosing between different box splits.
/// I tried putting this inside the BVH class, but I had difficulty getting it to compile.
enum class BVH_Heuristic {
    /// Tries to minimize the sum of axis lengths of the boxes.
    /// This is useful for applications where the probability of a box being applicable to a
    /// query is proportional to the "length", e.g. the probability of a random infinite plane
    /// intersecting the box.
    BOX_PERIMETER,

    /// Tries to minimize the "surface area" of the boxes.
    /// In 3D, uses the surface area; in 2D, uses the perimeter; in 1D, uses the axis length.
    /// This is what most applications, e.g. ray tracing, should use, particularly when the
    /// probability of a box being applicable to a query is proportional to the surface "area",
    /// e.g. the probability of a random ray hitting the box.
    ///
    /// NOTE: USE THIS ONE IF YOU ARE UNSURE!
    BOX_AREA,

    /// Tries to minimize the "volume" of the boxes.
    /// Uses the product of all axis lengths as a heuristic, (volume in 3D, area in 2D, length in 1D).
    /// This is useful for applications where the probability of a box being applicable to a
    /// query is proportional to the "volume", e.g. the probability of a random point being inside the box.
    BOX_VOLUME,

    /// Tries to minimize the "radii" of the boxes (i.e. the distance from the centre to a corner).
    /// This is useful for applications where the probability of a box being applicable to a
    /// query is proportional to the distance to the box centre, e.g. the probability of a random
    /// infinite plane being within the "radius" of the centre.
    BOX_RADIUS,

    /// Tries to minimize the squared "radii" of the boxes (i.e. the squared distance from the centre to a corner).
    /// This is useful for applications where the probability of a box being applicable to a
    /// query is proportional to the squared distance to the box centre, e.g. the probability of a random
    /// ray passing within the "radius" of the centre.
    BOX_RADIUS2,

    /// Tries to minimize the cubed "radii" of the boxes (i.e. the cubed distance from the centre to a corner).
    /// This is useful for applications where the probability of a box being applicable to a
    /// query is proportional to the cubed distance to the box centre, e.g. the probability of a random
    /// point being within the "radius" of the centre.
    BOX_RADIUS3,

    /// Tries to minimize the depth of the tree by primarily splitting at the median of the max axis.
    /// It may fall back to minimizing the area, but the tree depth should be unaffected.
    ///
    /// FIXME: This is not fully implemented yet.
    MEDIAN_MAX_AXIS
};

template<uint N>
class BVH {
public:
    using INT_TYPE = uint;
    struct Node {
        INT_TYPE child[N];

        static constexpr INT_TYPE theN = N;
        static constexpr INT_TYPE EMPTY = INT_TYPE(-1);
        static constexpr INT_TYPE INTERNAL_BIT = (INT_TYPE(1)<<(sizeof(INT_TYPE)*8 - 1));
        SYS_FORCE_INLINE static INT_TYPE markInternal(INT_TYPE internal_node_num) noexcept {
            return internal_node_num | INTERNAL_BIT;
        }
        SYS_FORCE_INLINE static bool isInternal(INT_TYPE node_int) noexcept {
            return (node_int & INTERNAL_BIT) != 0;
        }
        SYS_FORCE_INLINE static INT_TYPE getInternalNum(INT_TYPE node_int) noexcept {
            return node_int & ~INTERNAL_BIT;
        }
    };
private:
    struct FreeDeleter {
        SYS_FORCE_INLINE void operator()(Node* p) const {
            if (p) {
                // The pointer was allocated with malloc by UT_Array,
                // so it must be freed with free.
                free(p);
            }
        }
    };

    std::unique_ptr<Node[],FreeDeleter> myRoot;
    INT_TYPE myNumNodes;
public:
    SYS_FORCE_INLINE BVH() noexcept : myRoot(nullptr), myNumNodes(0) {}

    template<BVH_Heuristic H,typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE=INT_TYPE>
    void init(const BOX_TYPE* boxes, const INT_TYPE nboxes, SRC_INT_TYPE* indices=nullptr, bool reorder_indices=false, INT_TYPE max_items_per_leaf=1) noexcept;

    template<BVH_Heuristic H,typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE=INT_TYPE>
    void init(Box<T,NAXES> axes_minmax, const BOX_TYPE* boxes, INT_TYPE nboxes, SRC_INT_TYPE* indices=nullptr, bool reorder_indices=false, INT_TYPE max_items_per_leaf=1) noexcept;

    SYS_FORCE_INLINE
    INT_TYPE getNumNodes() const noexcept
    {
        return myNumNodes;
    }
    SYS_FORCE_INLINE
    const Node *getNodes() const noexcept
    {
        return myRoot.get();
    }

    SYS_FORCE_INLINE
    void clear() noexcept {
        myRoot.reset();
        myNumNodes = 0;
    }

    /// For each node, this effectively does:
    /// LOCAL_DATA local_data[MAX_ORDER];
    /// bool descend = functors.pre(nodei, parent_data);
    /// if (!descend)
    ///     return;
    /// for each child {
    ///     if (isitem(child))
    ///         functors.item(getitemi(child), nodei, local_data[child]);
    ///     else if (isnode(child))
    ///         recurse(getnodei(child), local_data);
    /// }
    /// functors.post(nodei, parent_nodei, data_for_parent, num_children, local_data);
    template<typename LOCAL_DATA,typename FUNCTORS>
    void traverse(
        FUNCTORS &functors,
        LOCAL_DATA *data_for_parent=nullptr) const noexcept;

    /// This acts like the traverse function, except if the number of nodes in two subtrees
    /// of a node contain at least parallel_threshold nodes, they may be executed in parallel.
    /// If parallel_threshold is 0, even item_functor may be executed on items in parallel.
    /// NOTE: Make sure that your functors don't depend on the order that they're executed in,
    ///       e.g. don't add values from sibling nodes together except in post functor,
    ///       else they might have nondeterministic roundoff or miss some values entirely.
    template<typename LOCAL_DATA,typename FUNCTORS>
    void traverseParallel(
        INT_TYPE parallel_threshold,
        FUNCTORS &functors,
        LOCAL_DATA *data_for_parent=nullptr) const noexcept;

    /// For each node, this effectively does:
    /// LOCAL_DATA local_data[MAX_ORDER];
    /// uint descend = functors.pre(nodei, parent_data);
    /// if (!descend)
    ///     return;
    /// for each child {
    ///     if (!(descend & (1<<child)))
    ///         continue;
    ///     if (isitem(child))
    ///         functors.item(getitemi(child), nodei, local_data[child]);
    ///     else if (isnode(child))
    ///         recurse(getnodei(child), local_data);
    /// }
    /// functors.post(nodei, parent_nodei, data_for_parent, num_children, local_data);
    template<typename LOCAL_DATA,typename FUNCTORS>
    void traverseVector(
        FUNCTORS &functors,
        LOCAL_DATA *data_for_parent=nullptr) const noexcept;

    /// Prints a text representation of the tree to stdout.
    void debugDump() const;

    template<typename SRC_INT_TYPE>
    static void createTrivialIndices(SRC_INT_TYPE* indices, const INT_TYPE n) noexcept;

private:
    template<typename LOCAL_DATA,typename FUNCTORS>
    void traverseHelper(
        INT_TYPE nodei,
        INT_TYPE parent_nodei,
        FUNCTORS &functors,
        LOCAL_DATA *data_for_parent=nullptr) const noexcept;

    template<typename LOCAL_DATA,typename FUNCTORS>
    void traverseParallelHelper(
        INT_TYPE nodei,
        INT_TYPE parent_nodei,
        INT_TYPE parallel_threshold,
        INT_TYPE next_node_id,
        FUNCTORS &functors,
        LOCAL_DATA *data_for_parent=nullptr) const noexcept;

    template<typename LOCAL_DATA,typename FUNCTORS>
    void traverseVectorHelper(
        INT_TYPE nodei,
        INT_TYPE parent_nodei,
        FUNCTORS &functors,
        LOCAL_DATA *data_for_parent=nullptr) const noexcept;

    template<typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void computeFullBoundingBox(Box<T,NAXES>& axes_minmax, const BOX_TYPE* boxes, const INT_TYPE nboxes, SRC_INT_TYPE* indices) noexcept;

    template<BVH_Heuristic H,typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void initNode(UT_Array<Node>& nodes, Node &node, const Box<T,NAXES>& axes_minmax, const BOX_TYPE* boxes, SRC_INT_TYPE* indices, const INT_TYPE nboxes) noexcept;

    template<BVH_Heuristic H,typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void initNodeReorder(UT_Array<Node>& nodes, Node &node, const Box<T,NAXES>& axes_minmax, const BOX_TYPE* boxes, SRC_INT_TYPE* indices, const INT_TYPE nboxes, const INT_TYPE indices_offset, const INT_TYPE max_items_per_leaf) noexcept;

    template<BVH_Heuristic H,typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void multiSplit(const Box<T,NAXES>& axes_minmax, const BOX_TYPE* boxes, SRC_INT_TYPE* indices, INT_TYPE nboxes, SRC_INT_TYPE* sub_indices[N+1], Box<T,NAXES> sub_boxes[N]) noexcept;

    template<BVH_Heuristic H,typename T,uint NAXES,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void split(const Box<T,NAXES>& axes_minmax, const BOX_TYPE* boxes, SRC_INT_TYPE* indices, INT_TYPE nboxes, SRC_INT_TYPE*& split_indices, Box<T,NAXES>* split_boxes) noexcept;

    template<INT_TYPE PARALLEL_THRESHOLD, typename SRC_INT_TYPE>
    static void adjustParallelChildNodes(INT_TYPE nparallel, UT_Array<Node>& nodes, Node& node, UT_Array<Node>* parallel_nodes, SRC_INT_TYPE* sub_indices) noexcept;

    template<typename T,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void nthElement(const BOX_TYPE* boxes, SRC_INT_TYPE* indices, const SRC_INT_TYPE* indices_end, const uint axis, SRC_INT_TYPE*const nth) noexcept;

    template<typename T,typename BOX_TYPE,typename SRC_INT_TYPE>
    static void partitionByCentre(const BOX_TYPE* boxes, SRC_INT_TYPE*const indices, const SRC_INT_TYPE*const indices_end, const uint axis, const T pivotx2, SRC_INT_TYPE*& ppivot_start, SRC_INT_TYPE*& ppivot_end) noexcept;

    /// An overestimate of the number of nodes needed.
    /// At worst, we could have only 2 children in every leaf, and
    /// then above that, we have a geometric series with r=1/N and a=(sub_nboxes/2)/N
    /// The true worst case might be a little worst than this, but
    /// it's probably fairly unlikely.
    SYS_FORCE_INLINE static INT_TYPE nodeEstimate(const INT_TYPE nboxes) noexcept {
        return nboxes/2 + nboxes/(2*(N-1));
    }

    template<BVH_Heuristic H,typename T, uint NAXES>
    SYS_FORCE_INLINE static T unweightedHeuristic(const Box<T, NAXES>& box) noexcept {
        if (H == BVH_Heuristic::BOX_PERIMETER) {
            return box.axis_sum();
        }
        if (H == BVH_Heuristic::BOX_AREA) {
            return box.half_surface_area();
        }
        if (H == BVH_Heuristic::BOX_VOLUME) {
            return box.volume();
        }
        if (H == BVH_Heuristic::BOX_RADIUS) {
            T diameter2 = box.diameter2();
            return SYSsqrt(diameter2);
        }
        if (H == BVH_Heuristic::BOX_RADIUS2) {
            return box.diameter2();
        }
        if (H == BVH_Heuristic::BOX_RADIUS3) {
            T diameter2 = box.diameter2();
            return diameter2*SYSsqrt(diameter2);
        }
        UT_ASSERT_MSG(0, "BVH_Heuristic::MEDIAN_MAX_AXIS should be handled separately by caller!");
        return T(1);
    }

    /// 16 equal-length spans (15 evenly-spaced splits) should be enough for a decent heuristic
    static constexpr INT_TYPE NSPANS = 16;
    static constexpr INT_TYPE NSPLITS = NSPANS-1;

    /// At least 1/16 of all boxes must be on each side, else we could end up with a very deep tree
    static constexpr INT_TYPE MIN_FRACTION = 16;
};

} // UT namespace

template<uint N>
using UT_BVH = UT::BVH<N>;

} // End HDK_Sample namespace
#endif