hierarchy_tree-inl.h

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#ifndef HPP_FCL_HIERARCHY_TREE_INL_H
#define HPP_FCL_HIERARCHY_TREE_INL_H

#include "hpp/fcl/broadphase/detail/hierarchy_tree.h"

namespace hpp {
namespace fcl {

namespace detail {

//==============================================================================
template <typename BV>
HierarchyTree<BV>::HierarchyTree(int bu_threshold_, int topdown_level_) {
  root_node = nullptr;
  n_leaves = 0;
  free_node = nullptr;
  max_lookahead_level = -1;
  opath = 0;
  bu_threshold = bu_threshold_;
  topdown_level = topdown_level_;
}

//==============================================================================
template <typename BV>
HierarchyTree<BV>::~HierarchyTree() {
  clear();
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::init(std::vector<Node*>& leaves, int level) {
  switch (level) {
    case 0:
      init_0(leaves);
      break;
    case 1:
      init_1(leaves);
      break;
    case 2:
      init_2(leaves);
      break;
    case 3:
      init_3(leaves);
      break;
    default:
      init_0(leaves);
  }
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::insert(const BV& bv,
                                                            void* data) {
  Node* leaf = createNode(nullptr, bv, data);
  insertLeaf(root_node, leaf);
  ++n_leaves;
  return leaf;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::remove(Node* leaf) {
  removeLeaf(leaf);
  deleteNode(leaf);
  --n_leaves;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::clear() {
  if (root_node) recurseDeleteNode(root_node);
  n_leaves = 0;
  delete free_node;
  free_node = nullptr;
  max_lookahead_level = -1;
  opath = 0;
}

//==============================================================================
template <typename BV>
bool HierarchyTree<BV>::empty() const {
  return (nullptr == root_node);
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::update(Node* leaf, int lookahead_level) {
  // TODO(DamrongGuoy): Since we update a leaf node by removing and
  //  inserting the same leaf node, it is likely to change the structure of
  //  the tree even if no object's pose has changed. In the future,
  //  find a way to preserve the structure of the tree to solve this issue:
  //  https://github.com/flexible-collision-library/fcl/issues/368

  // First we remove the leaf node pointed by `leaf` variable from the tree.
  // The `sub_root` variable is the root of the subtree containing nodes
  // whose BVs were adjusted due to node removal.
  typename HierarchyTree<BV>::Node* sub_root = removeLeaf(leaf);
  if (sub_root) {
    if (lookahead_level > 0) {
      // For positive `lookahead_level`, we move the `sub_root` variable up.
      for (int i = 0; (i < lookahead_level) && sub_root->parent; ++i)
        sub_root = sub_root->parent;
    } else
      // By default, lookahead_level = -1, and we reset the `sub_root` variable
      // to the root of the entire tree.
      sub_root = root_node;
  }
  // Then we insert the node pointed by `leaf` variable back into the
  // subtree rooted at `sub_root` variable.
  insertLeaf(sub_root, leaf);
}

//==============================================================================
template <typename BV>
bool HierarchyTree<BV>::update(Node* leaf, const BV& bv) {
  if (leaf->bv.contain(bv)) return false;
  update_(leaf, bv);
  return true;
}

//==============================================================================
template <typename S, typename BV>
struct UpdateImpl {
  static bool run(const HierarchyTree<BV>& tree,
                  typename HierarchyTree<BV>::Node* leaf, const BV& bv,
                  const Vec3f& /*vel*/, FCL_REAL /*margin*/) {
    if (leaf->bv.contain(bv)) return false;
    tree.update_(leaf, bv);
    return true;
  }

  static bool run(const HierarchyTree<BV>& tree,
                  typename HierarchyTree<BV>::Node* leaf, const BV& bv,
                  const Vec3f& /*vel*/) {
    if (leaf->bv.contain(bv)) return false;
    tree.update_(leaf, bv);
    return true;
  }
};

//==============================================================================
template <typename BV>
bool HierarchyTree<BV>::update(Node* leaf, const BV& bv, const Vec3f& vel,
                               FCL_REAL margin) {
  return UpdateImpl<FCL_REAL, BV>::run(*this, leaf, bv, vel, margin);
}

//==============================================================================
template <typename BV>
bool HierarchyTree<BV>::update(Node* leaf, const BV& bv, const Vec3f& vel) {
  return UpdateImpl<FCL_REAL, BV>::run(*this, leaf, bv, vel);
}

//==============================================================================
template <typename BV>
size_t HierarchyTree<BV>::getMaxHeight() const {
  if (!root_node) return 0;
  return getMaxHeight(root_node);
}

//==============================================================================
template <typename BV>
size_t HierarchyTree<BV>::getMaxDepth() const {
  if (!root_node) return 0;

  size_t max_depth;
  getMaxDepth(root_node, 0, max_depth);
  return max_depth;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::balanceBottomup() {
  if (root_node) {
    std::vector<Node*> leaves;
    leaves.reserve(n_leaves);
    fetchLeaves(root_node, leaves);
    bottomup(leaves.begin(), leaves.end());
    root_node = leaves[0];
  }
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::balanceTopdown() {
  if (root_node) {
    std::vector<Node*> leaves;
    leaves.reserve(n_leaves);
    fetchLeaves(root_node, leaves);
    root_node = topdown(leaves.begin(), leaves.end());
  }
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::balanceIncremental(int iterations) {
  if (iterations < 0) iterations = (int)n_leaves;
  if (root_node && (iterations > 0)) {
    for (int i = 0; i < iterations; ++i) {
      Node* node = root_node;
      unsigned int bit = 0;
      while (!node->isLeaf()) {
        node = sort(node, root_node)->children[(opath >> bit) & 1];
        bit = (bit + 1) & (sizeof(unsigned int) * 8 - 1);
      }
      update(node);
      ++opath;
    }
  }
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::refit() {
  if (root_node) recurseRefit(root_node);
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::extractLeaves(const Node* root,
                                      std::vector<Node*>& leaves) const {
  if (!root->isLeaf()) {
    extractLeaves(root->children[0], leaves);
    extractLeaves(root->children[1], leaves);
  } else
    leaves.push_back(root);
}

//==============================================================================
template <typename BV>
size_t HierarchyTree<BV>::size() const {
  return n_leaves;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::getRoot() const {
  return root_node;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node*& HierarchyTree<BV>::getRoot() {
  return root_node;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::print(Node* root, int depth) {
  for (int i = 0; i < depth; ++i) std::cout << " ";
  std::cout << " (" << root->bv.min_[0] << ", " << root->bv.min_[1] << ", "
            << root->bv.min_[2] << "; " << root->bv.max_[0] << ", "
            << root->bv.max_[1] << ", " << root->bv.max_[2] << ")" << std::endl;
  if (root->isLeaf()) {
  } else {
    print(root->children[0], depth + 1);
    print(root->children[1], depth + 1);
  }
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::bottomup(const NodeVecIterator lbeg,
                                 const NodeVecIterator lend) {
  NodeVecIterator lcur_end = lend;
  while (lbeg < lcur_end - 1) {
    NodeVecIterator min_it1, min_it2;
    FCL_REAL min_size = (std::numeric_limits<FCL_REAL>::max)();
    for (NodeVecIterator it1 = lbeg; it1 < lcur_end; ++it1) {
      for (NodeVecIterator it2 = it1 + 1; it2 < lcur_end; ++it2) {
        FCL_REAL cur_size = ((*it1)->bv + (*it2)->bv).size();
        if (cur_size < min_size) {
          min_size = cur_size;
          min_it1 = it1;
          min_it2 = it2;
        }
      }
    }

    Node* n[2] = {*min_it1, *min_it2};
    Node* p = createNode(nullptr, n[0]->bv, n[1]->bv, nullptr);
    p->children[0] = n[0];
    p->children[1] = n[1];
    n[0]->parent = p;
    n[1]->parent = p;
    *min_it1 = p;
    Node* tmp = *min_it2;
    lcur_end--;
    *min_it2 = *lcur_end;
    *lcur_end = tmp;
  }
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::topdown(
    const NodeVecIterator lbeg, const NodeVecIterator lend) {
  switch (topdown_level) {
    case 0:
      return topdown_0(lbeg, lend);
      break;
    case 1:
      return topdown_1(lbeg, lend);
      break;
    default:
      return topdown_0(lbeg, lend);
  }
}

//==============================================================================
template <typename BV>
size_t HierarchyTree<BV>::getMaxHeight(Node* node) const {
  if (!node->isLeaf()) {
    size_t height1 = getMaxHeight(node->children[0]);
    size_t height2 = getMaxHeight(node->children[1]);
    return std::max(height1, height2) + 1;
  } else
    return 0;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::getMaxDepth(Node* node, size_t depth,
                                    size_t& max_depth) const {
  if (!node->isLeaf()) {
    getMaxDepth(node->children[0], depth + 1, max_depth);
    getMaxDepth(node->children[1], depth + 1, max_depth);
  } else
    max_depth = std::max(max_depth, depth);
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::topdown_0(
    const NodeVecIterator lbeg, const NodeVecIterator lend) {
  long num_leaves = lend - lbeg;
  if (num_leaves > 1) {
    if (num_leaves > bu_threshold) {
      BV vol = (*lbeg)->bv;
      for (NodeVecIterator it = lbeg + 1; it < lend; ++it) vol += (*it)->bv;

      int best_axis = 0;
      FCL_REAL extent[3] = {vol.width(), vol.height(), vol.depth()};
      if (extent[1] > extent[0]) best_axis = 1;
      if (extent[2] > extent[best_axis]) best_axis = 2;

      // compute median
      NodeVecIterator lcenter = lbeg + num_leaves / 2;
      std::nth_element(lbeg, lcenter, lend,
                       std::bind(&nodeBaseLess<BV>, std::placeholders::_1,
                                 std::placeholders::_2, std::ref(best_axis)));

      Node* node = createNode(nullptr, vol, nullptr);
      node->children[0] = topdown_0(lbeg, lcenter);
      node->children[1] = topdown_0(lcenter, lend);
      node->children[0]->parent = node;
      node->children[1]->parent = node;
      return node;
    } else {
      bottomup(lbeg, lend);
      return *lbeg;
    }
  }
  return *lbeg;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::topdown_1(
    const NodeVecIterator lbeg, const NodeVecIterator lend) {
  long num_leaves = lend - lbeg;
  if (num_leaves > 1) {
    if (num_leaves > bu_threshold) {
      Vec3f split_p = (*lbeg)->bv.center();
      BV vol = (*lbeg)->bv;
      NodeVecIterator it;
      for (it = lbeg + 1; it < lend; ++it) {
        split_p += (*it)->bv.center();
        vol += (*it)->bv;
      }
      split_p /= static_cast<FCL_REAL>(num_leaves);
      int best_axis = -1;
      long bestmidp = num_leaves;
      int splitcount[3][2] = {{0, 0}, {0, 0}, {0, 0}};
      for (it = lbeg; it < lend; ++it) {
        Vec3f x = (*it)->bv.center() - split_p;
        for (int j = 0; j < 3; ++j) ++splitcount[j][x[j] > 0 ? 1 : 0];
      }

      for (int i = 0; i < 3; ++i) {
        if ((splitcount[i][0] > 0) && (splitcount[i][1] > 0)) {
          long midp = std::abs(splitcount[i][0] - splitcount[i][1]);
          if (midp < bestmidp) {
            best_axis = i;
            bestmidp = midp;
          }
        }
      }

      if (best_axis < 0) best_axis = 0;

      FCL_REAL split_value = split_p[best_axis];
      NodeVecIterator lcenter = lbeg;
      for (it = lbeg; it < lend; ++it) {
        if ((*it)->bv.center()[best_axis] < split_value) {
          Node* temp = *it;
          *it = *lcenter;
          *lcenter = temp;
          ++lcenter;
        }
      }

      Node* node = createNode(nullptr, vol, nullptr);
      node->children[0] = topdown_1(lbeg, lcenter);
      node->children[1] = topdown_1(lcenter, lend);
      node->children[0]->parent = node;
      node->children[1]->parent = node;
      return node;
    } else {
      bottomup(lbeg, lend);
      return *lbeg;
    }
  }
  return *lbeg;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::init_0(std::vector<Node*>& leaves) {
  clear();
  root_node = topdown(leaves.begin(), leaves.end());
  n_leaves = leaves.size();
  max_lookahead_level = -1;
  opath = 0;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::init_1(std::vector<Node*>& leaves) {
  clear();

  BV bound_bv;
  if (leaves.size() > 0) bound_bv = leaves[0]->bv;
  for (size_t i = 1; i < leaves.size(); ++i) bound_bv += leaves[i]->bv;

  morton_functor<FCL_REAL, uint32_t> coder(bound_bv);
  for (size_t i = 0; i < leaves.size(); ++i)
    leaves[i]->code = coder(leaves[i]->bv.center());

  std::sort(leaves.begin(), leaves.end(), SortByMorton());

  root_node = mortonRecurse_0(leaves.begin(), leaves.end(),
                              (1 << (coder.bits() - 1)), coder.bits() - 1);

  refit();
  n_leaves = leaves.size();
  max_lookahead_level = -1;
  opath = 0;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::init_2(std::vector<Node*>& leaves) {
  clear();

  BV bound_bv;
  if (leaves.size() > 0) bound_bv = leaves[0]->bv;
  for (size_t i = 1; i < leaves.size(); ++i) bound_bv += leaves[i]->bv;

  morton_functor<FCL_REAL, uint32_t> coder(bound_bv);
  for (size_t i = 0; i < leaves.size(); ++i)
    leaves[i]->code = coder(leaves[i]->bv.center());

  std::sort(leaves.begin(), leaves.end(), SortByMorton());

  root_node = mortonRecurse_1(leaves.begin(), leaves.end(),
                              (1 << (coder.bits() - 1)), coder.bits() - 1);

  refit();
  n_leaves = leaves.size();
  max_lookahead_level = -1;
  opath = 0;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::init_3(std::vector<Node*>& leaves) {
  clear();

  BV bound_bv;
  if (leaves.size() > 0) bound_bv = leaves[0]->bv;
  for (size_t i = 1; i < leaves.size(); ++i) bound_bv += leaves[i]->bv;

  morton_functor<FCL_REAL, uint32_t> coder(bound_bv);
  for (size_t i = 0; i < leaves.size(); ++i)
    leaves[i]->code = coder(leaves[i]->bv.center());

  std::sort(leaves.begin(), leaves.end(), SortByMorton());

  root_node = mortonRecurse_2(leaves.begin(), leaves.end());

  refit();
  n_leaves = leaves.size();
  max_lookahead_level = -1;
  opath = 0;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::mortonRecurse_0(
    const NodeVecIterator lbeg, const NodeVecIterator lend,
    const uint32_t& split, int bits) {
  long num_leaves = lend - lbeg;
  if (num_leaves > 1) {
    if (bits > 0) {
      Node dummy;
      dummy.code = split;
      NodeVecIterator lcenter =
          std::lower_bound(lbeg, lend, &dummy, SortByMorton());

      if (lcenter == lbeg) {
        uint32_t split2 = split | (1 << (bits - 1));
        return mortonRecurse_0(lbeg, lend, split2, bits - 1);
      } else if (lcenter == lend) {
        uint32_t split1 = (split & (~(1 << bits))) | (1 << (bits - 1));
        return mortonRecurse_0(lbeg, lend, split1, bits - 1);
      } else {
        uint32_t split1 = (split & (~(1 << bits))) | (1 << (bits - 1));
        uint32_t split2 = split | (1 << (bits - 1));

        Node* child1 = mortonRecurse_0(lbeg, lcenter, split1, bits - 1);
        Node* child2 = mortonRecurse_0(lcenter, lend, split2, bits - 1);
        Node* node = createNode(nullptr, nullptr);
        node->children[0] = child1;
        node->children[1] = child2;
        child1->parent = node;
        child2->parent = node;
        return node;
      }
    } else {
      Node* node = topdown(lbeg, lend);
      return node;
    }
  } else
    return *lbeg;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::mortonRecurse_1(
    const NodeVecIterator lbeg, const NodeVecIterator lend,
    const uint32_t& split, int bits) {
  long num_leaves = lend - lbeg;
  if (num_leaves > 1) {
    if (bits > 0) {
      Node dummy;
      dummy.code = split;
      NodeVecIterator lcenter =
          std::lower_bound(lbeg, lend, &dummy, SortByMorton());

      if (lcenter == lbeg) {
        uint32_t split2 = split | (1 << (bits - 1));
        return mortonRecurse_1(lbeg, lend, split2, bits - 1);
      } else if (lcenter == lend) {
        uint32_t split1 = (split & (~(1 << bits))) | (1 << (bits - 1));
        return mortonRecurse_1(lbeg, lend, split1, bits - 1);
      } else {
        uint32_t split1 = (split & (~(1 << bits))) | (1 << (bits - 1));
        uint32_t split2 = split | (1 << (bits - 1));

        Node* child1 = mortonRecurse_1(lbeg, lcenter, split1, bits - 1);
        Node* child2 = mortonRecurse_1(lcenter, lend, split2, bits - 1);
        Node* node = createNode(nullptr, nullptr);
        node->children[0] = child1;
        node->children[1] = child2;
        child1->parent = node;
        child2->parent = node;
        return node;
      }
    } else {
      Node* child1 = mortonRecurse_1(lbeg, lbeg + num_leaves / 2, 0, bits - 1);
      Node* child2 = mortonRecurse_1(lbeg + num_leaves / 2, lend, 0, bits - 1);
      Node* node = createNode(nullptr, nullptr);
      node->children[0] = child1;
      node->children[1] = child2;
      child1->parent = node;
      child2->parent = node;
      return node;
    }
  } else
    return *lbeg;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::mortonRecurse_2(
    const NodeVecIterator lbeg, const NodeVecIterator lend) {
  long num_leaves = lend - lbeg;
  if (num_leaves > 1) {
    Node* child1 = mortonRecurse_2(lbeg, lbeg + num_leaves / 2);
    Node* child2 = mortonRecurse_2(lbeg + num_leaves / 2, lend);
    Node* node = createNode(nullptr, nullptr);
    node->children[0] = child1;
    node->children[1] = child2;
    child1->parent = node;
    child2->parent = node;
    return node;
  } else
    return *lbeg;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::update_(Node* leaf, const BV& bv) {
  Node* root = removeLeaf(leaf);
  if (root) {
    if (max_lookahead_level >= 0) {
      for (int i = 0; (i < max_lookahead_level) && root->parent; ++i)
        root = root->parent;
    } else
      root = root_node;
  }

  leaf->bv = bv;
  insertLeaf(root, leaf);
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::sort(Node* n, Node*& r) {
  Node* p = n->parent;
  if (p > n) {
    size_t i = indexOf(n);
    size_t j = 1 - i;
    Node* s = p->children[j];
    Node* q = p->parent;
    if (q)
      q->children[indexOf(p)] = n;
    else
      r = n;
    s->parent = n;
    p->parent = n;
    n->parent = q;
    p->children[0] = n->children[0];
    p->children[1] = n->children[1];
    n->children[0]->parent = p;
    n->children[1]->parent = p;
    n->children[i] = p;
    n->children[j] = s;
    std::swap(p->bv, n->bv);
    return p;
  }
  return n;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::insertLeaf(Node* const sub_root, Node* const leaf)
// Attempts to insert `leaf` into a subtree rooted at `sub_root`.
// 1. If the whole tree is empty, then `leaf` simply becomes the tree.
// 2. Otherwise, a leaf node called `sibling` of the subtree rooted at
//    `sub_root` is selected (the selection criteria is a black box for this
//    algorithm), and the `sibling` leaf is replaced by an internal node with
//    two children, `sibling` and `leaf`. The bounding volumes are updated as
//    necessary.
{
  if (!root_node) {
    // If the entire tree is empty, the node pointed by `leaf` variable will
    // become the root of the tree.
    root_node = leaf;
    leaf->parent = nullptr;
    return;
  }
  // Traverse the tree from the given `sub_root` down to an existing leaf
  // node that we call `sibling`. The `sibling` node will eventually become
  // the sibling of the given `leaf` node.
  Node* sibling = sub_root;
  while (!sibling->isLeaf()) {
    sibling = sibling->children[select(*leaf, *(sibling->children[0]),
                                       *(sibling->children[1]))];
  }
  Node* prev = sibling->parent;
  // Create a new `node` that later will become the new parent of both the
  // `sibling` and the given `leaf`.
  Node* node = createNode(prev, leaf->bv, sibling->bv, nullptr);
  if (prev)
  // If the parent `prev` of the `sibling` is an interior node, we will
  // replace the `sibling` with the subtree {node {`sibling`, leaf}} like
  // this:
  //        Before                After
  //
  //          ╱                     ╱
  //        prev                  prev
  //        ╱  ╲        ─>        ╱  ╲
  //  sibling  ...             node  ...
  //                           ╱  ╲
  //                     sibling  leaf
  {
    prev->children[indexOf(sibling)] = node;
    node->children[0] = sibling;
    sibling->parent = node;
    node->children[1] = leaf;
    leaf->parent = node;
    // Now that we've inserted `leaf` some of the existing bounding
    // volumes might not fully enclose their children. Walk up the tree
    // looking for parents that don't already enclose their children
    // and create a new tight-fitting bounding volume for those.
    do {
      if (!prev->bv.contain(node->bv))
        prev->bv = prev->children[0]->bv + prev->children[1]->bv;
      else
        break;
      node = prev;
    } while (nullptr != (prev = node->parent));
  } else
  // If the `sibling` has no parent, i.e., the tree is a singleton,
  // we will replace it with the 3-node tree {node {`sibling`, `leaf`}} like
  // this:
  //
  //        node
  //        ╱  ╲
  //  sibling  leaf
  {
    node->children[0] = sibling;
    sibling->parent = node;
    node->children[1] = leaf;
    leaf->parent = node;
    root_node = node;
  }

  // Note that the above algorithm always adds the new `leaf` node as the right
  // child, i.e., children[1].  Calling removeLeaf(l) followed by calling
  // this function insertLeaf(l) where l is a left child will result in
  // switching l and its sibling even if no object's pose has changed.
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::removeLeaf(
    Node* const leaf) {
  // Deletes `leaf` by replacing the subtree consisting of `leaf`, its sibling,
  // and its parent with just its sibling. It then "tightens" the ancestor
  // bounding volumes. Returns a pointer to the parent of the highest node whose
  // BV changed due to the removal.
  if (leaf == root_node) {
    // If the `leaf` node is the only node in the tree, the tree becomes empty.
    root_node = nullptr;
    return nullptr;
  }
  Node* parent = leaf->parent;
  Node* prev = parent->parent;
  Node* sibling = parent->children[1 - indexOf(leaf)];
  if (prev) {
    // If the parent node is not the root node, the sibling node will
    // replace the parent node like this:
    //
    //            Before              After
    //             ...                 ...
    //             ╱                   ╱
    //           prev                prev
    //          ╱   ╲               ╱   ╲
    //     parent   ...    ─>  sibling  ...
    //      ╱  ╲                 ╱╲
    //  leaf  sibling           ...
    //           ╱╲
    //          ...
    //
    // Step 1: replace the subtree {parent {leaf, sibling {...}}} with
    // {sibling {...}}.
    prev->children[indexOf(parent)] = sibling;
    sibling->parent = prev;
    deleteNode(parent);
    // Step 2: tighten up the BVs of the ancestor nodes.
    while (prev) {
      BV new_bv = prev->children[0]->bv + prev->children[1]->bv;
      if (!(new_bv == prev->bv)) {
        prev->bv = new_bv;
        prev = prev->parent;
      } else
        break;
    }

    return prev ? prev : root_node;
  } else {
    // If the parent node is the root node, the sibling node will become the
    // root of the whole tree like this:
    //
    //     Before                   After
    //
    //     parent
    //      ╱  ╲
    //  leaf  sibling     ─>       sibling
    //           ╱╲                  ╱╲
    //          ...                 ...
    root_node = sibling;
    sibling->parent = nullptr;
    deleteNode(parent);
    return root_node;
  }
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::fetchLeaves(Node* root, std::vector<Node*>& leaves,
                                    int depth) {
  if ((!root->isLeaf()) && depth) {
    fetchLeaves(root->children[0], leaves, depth - 1);
    fetchLeaves(root->children[1], leaves, depth - 1);
    deleteNode(root);
  } else {
    leaves.push_back(root);
  }
}

//==============================================================================
template <typename BV>
size_t HierarchyTree<BV>::indexOf(Node* node) {
  // node cannot be nullptr
  return (node->parent->children[1] == node);
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::createNode(Node* parent,
                                                                const BV& bv,
                                                                void* data) {
  Node* node = createNode(parent, data);
  node->bv = bv;
  return node;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::createNode(Node* parent,
                                                                const BV& bv1,
                                                                const BV& bv2,
                                                                void* data) {
  Node* node = createNode(parent, data);
  node->bv = bv1 + bv2;
  return node;
}

//==============================================================================
template <typename BV>
typename HierarchyTree<BV>::Node* HierarchyTree<BV>::createNode(Node* parent,
                                                                void* data) {
  Node* node = nullptr;
  if (free_node) {
    node = free_node;
    free_node = nullptr;
  } else
    node = new Node();
  node->parent = parent;
  node->data = data;
  node->children[1] = 0;
  return node;
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::deleteNode(Node* node) {
  if (free_node != node) {
    delete free_node;
    free_node = node;
  }
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::recurseDeleteNode(Node* node) {
  if (!node->isLeaf()) {
    recurseDeleteNode(node->children[0]);
    recurseDeleteNode(node->children[1]);
  }

  if (node == root_node) root_node = nullptr;
  deleteNode(node);
}

//==============================================================================
template <typename BV>
void HierarchyTree<BV>::recurseRefit(Node* node) {
  if (!node->isLeaf()) {
    recurseRefit(node->children[0]);
    recurseRefit(node->children[1]);
    node->bv = node->children[0]->bv + node->children[1]->bv;
  } else
    return;
}

//==============================================================================
template <typename BV>
bool nodeBaseLess(NodeBase<BV>* a, NodeBase<BV>* b, int d) {
  if (a->bv.center()[d] < b->bv.center()[d]) return true;
  return false;
}

//==============================================================================
template <typename S, typename BV>
struct SelectImpl {
  static std::size_t run(const NodeBase<BV>& /*query*/,
                         const NodeBase<BV>& /*node1*/,
                         const NodeBase<BV>& /*node2*/) {
    return 0;
  }

  static std::size_t run(const BV& /*query*/, const NodeBase<BV>& /*node1*/,
                         const NodeBase<BV>& /*node2*/) {
    return 0;
  }
};

//==============================================================================
template <typename BV>
size_t select(const NodeBase<BV>& query, const NodeBase<BV>& node1,
              const NodeBase<BV>& node2) {
  return SelectImpl<FCL_REAL, BV>::run(query, node1, node2);
}

//==============================================================================
template <typename BV>
size_t select(const BV& query, const NodeBase<BV>& node1,
              const NodeBase<BV>& node2) {
  return SelectImpl<FCL_REAL, BV>::run(query, node1, node2);
}

//==============================================================================
template <typename S>
struct SelectImpl<S, AABB> {
  static std::size_t run(const NodeBase<AABB>& node,
                         const NodeBase<AABB>& node1,
                         const NodeBase<AABB>& node2) {
    const AABB& bv = node.bv;
    const AABB& bv1 = node1.bv;
    const AABB& bv2 = node2.bv;
    Vec3f v = bv.min_ + bv.max_;
    Vec3f v1 = v - (bv1.min_ + bv1.max_);
    Vec3f v2 = v - (bv2.min_ + bv2.max_);
    FCL_REAL d1 = fabs(v1[0]) + fabs(v1[1]) + fabs(v1[2]);
    FCL_REAL d2 = fabs(v2[0]) + fabs(v2[1]) + fabs(v2[2]);
    return (d1 < d2) ? 0 : 1;
  }

  static std::size_t run(const AABB& query, const NodeBase<AABB>& node1,
                         const NodeBase<AABB>& node2) {
    const AABB& bv = query;
    const AABB& bv1 = node1.bv;
    const AABB& bv2 = node2.bv;
    Vec3f v = bv.min_ + bv.max_;
    Vec3f v1 = v - (bv1.min_ + bv1.max_);
    Vec3f v2 = v - (bv2.min_ + bv2.max_);
    FCL_REAL d1 = fabs(v1[0]) + fabs(v1[1]) + fabs(v1[2]);
    FCL_REAL d2 = fabs(v2[0]) + fabs(v2[1]) + fabs(v2[2]);
    return (d1 < d2) ? 0 : 1;
  }
};

}  // namespace detail
}  // namespace fcl
}  // namespace hpp

#endif