dynamic_tree.c

// SPDX-FileCopyrightText: 2025 Erin Catto
// SPDX-License-Identifier: MIT

#include "aabb.h"
#include "core.h"
#include "joint.h"
#include "simd.h"

#include "box3d/collision.h"
#include "box3d/constants.h"
#include "box3d/math_functions.h"

#include <float.h>
#include <stdio.h>
#include <string.h>

#define B3_TREE_STACK_SIZE 1024

static b3TreeNode b3_defaultTreeNode = {
	.aabb = { { 0.0f, 0.0f, 0.0f }, { 0.0f, 0.0f, 0.0f } },
	.categoryBits = B3_DEFAULT_CATEGORY_BITS,
	.children =
		{
			.child1 = B3_NULL_INDEX,
			.child2 = B3_NULL_INDEX,
		},
	.parent = B3_NULL_INDEX,
	.height = 0,
	.flags = b3_allocatedNode,
};

static inline bool b3IsLeaf( const b3TreeNode* node )
{
	return node->flags & b3_leafNode;
}

static inline bool b3IsAllocated( const b3TreeNode* node )
{
	return node->flags & b3_allocatedNode;
}

static inline uint16_t b3MaxUInt16( uint16_t a, uint16_t b )
{
	return a > b ? a : b;
}

b3DynamicTree b3DynamicTree_Create( int proxyCapacity )
{
	int capacity = b3MaxInt( proxyCapacity, 16 );

	b3DynamicTree tree;

	// memset needed for deterministic serialization
	memset( &tree, 0, sizeof( b3DynamicTree ) );

	tree.version = B3_DYNAMIC_TREE_VERSION;
	tree.root = B3_NULL_INDEX;

	// maximum node count for a full binary tree is 2 * leafCount - 1
	tree.nodeCapacity = 2 * capacity - 1;
	tree.nodeCount = 0;

	tree.nodes = (b3TreeNode*)b3Alloc( tree.nodeCapacity * sizeof( b3TreeNode ) );

	memset( tree.nodes, 0, tree.nodeCapacity * sizeof( b3TreeNode ) );

	// Build a linked list for the free list.
	// todo use a bump allocator until the capacity is consumed (see b3PoolAllocator)
	for ( int i = 0; i < tree.nodeCapacity - 1; ++i )
	{
		tree.nodes[i].next = i + 1;
	}

	tree.nodes[tree.nodeCapacity - 1].next = B3_NULL_INDEX;
	tree.freeList = 0;

	tree.proxyCount = 0;

	tree.leafIndices = NULL;
	tree.leafBoxes = NULL;
	tree.leafCenters = NULL;
	tree.binIndices = NULL;
	tree.rebuildCapacity = 0;

	return tree;
}

void b3DynamicTree_Destroy( b3DynamicTree* tree )
{
	b3Free( tree->nodes, tree->nodeCapacity * sizeof( b3TreeNode ) );
	b3Free( tree->leafIndices, tree->rebuildCapacity * sizeof( int ) );
	b3Free( tree->leafBoxes, tree->rebuildCapacity * sizeof( b3AABB ) );
	b3Free( tree->leafCenters, tree->rebuildCapacity * sizeof( b3Vec3 ) );
	b3Free( tree->binIndices, tree->rebuildCapacity * sizeof( int ) );

	memset( tree, 0, sizeof( b3DynamicTree ) );
}

// Allocate a node from the pool. Grow the pool if necessary.
static int b3AllocateNode( b3DynamicTree* tree )
{
	// Expand the node pool as needed.
	if ( tree->freeList == B3_NULL_INDEX )
	{
		B3_ASSERT( tree->nodeCount == tree->nodeCapacity );

		// The free list is empty. Rebuild a bigger pool.
		b3TreeNode* oldNodes = tree->nodes;
		int oldCapacity = tree->nodeCapacity;
		tree->nodeCapacity += oldCapacity >> 1;
		tree->nodes = (b3TreeNode*)b3Alloc( tree->nodeCapacity * sizeof( b3TreeNode ) );
		B3_ASSERT( oldNodes != NULL );
		memcpy( tree->nodes, oldNodes, tree->nodeCount * sizeof( b3TreeNode ) );
		memset( tree->nodes + tree->nodeCount, 0, ( tree->nodeCapacity - tree->nodeCount ) * sizeof( b3TreeNode ) );
		b3Free( oldNodes, oldCapacity * sizeof( b3TreeNode ) );

		// Build a linked list for the free list. The parent pointer becomes the "next" pointer.
		// todo avoid building freelist?
		for ( int i = tree->nodeCount; i < tree->nodeCapacity - 1; ++i )
		{
			tree->nodes[i].next = i + 1;
		}

		tree->nodes[tree->nodeCapacity - 1].next = B3_NULL_INDEX;
		tree->freeList = tree->nodeCount;
	}

	// Peel a node off the free list.
	int nodeIndex = tree->freeList;
	b3TreeNode* node = tree->nodes + nodeIndex;
	tree->freeList = node->next;
	*node = b3_defaultTreeNode;
	++tree->nodeCount;
	return nodeIndex;
}

// Return a node to the pool.
static void b3FreeNode( b3DynamicTree* tree, int nodeId )
{
	B3_ASSERT( 0 <= nodeId && nodeId < tree->nodeCapacity );
	B3_ASSERT( 0 < tree->nodeCount );
	tree->nodes[nodeId].next = tree->freeList;
	tree->nodes[nodeId].flags = 0;
	tree->freeList = nodeId;
	--tree->nodeCount;
}

// Greedy algorithm for sibling selection using the SAH
// We have three nodes A-(B,C) and want to add a leaf D, there are three choices.
// 1: make a new parent for A and D : E-(A-(B,C), D)
// 2: associate D with B
//   a: B is a leaf : A-(E-(B,D), C)
//   b: B is an internal node: A-(B{D},C)
// 3: associate D with C
//   a: C is a leaf : A-(B, E-(C,D))
//   b: C is an internal node: A-(B, C{D})
// All of these have a clear cost except when B or C is an internal node. Hence we need to be greedy.

// The cost for cases 1, 2a, and 3a can be computed using the sibling cost formula.
// cost of sibling H = area(union(H, D)) + increased area of ancestors

// Suppose B (or C) is an internal node, then the lowest cost would be one of two cases:
// case1: D becomes a sibling of B
// case2: D becomes a descendant of B along with a new internal node of area(D).
static int b3FindBestSibling( const b3DynamicTree* tree, b3AABB boxD )
{
	b3Vec3 centerD = b3AABB_Center( boxD );
	float areaD = b3Perimeter( boxD );

	const b3TreeNode* nodes = tree->nodes;
	int rootIndex = tree->root;

	b3AABB rootBox = nodes[rootIndex].aabb;

	// Area of current node
	float areaBase = b3Perimeter( rootBox );

	// Area of inflated node
	float directCost = b3Perimeter( b3AABB_Union( rootBox, boxD ) );
	float inheritedCost = 0.0f;

	int bestSibling = rootIndex;
	float bestCost = directCost;

	// Descend the tree from root, following a single greedy path.
	int index = rootIndex;
	while ( b3IsLeaf( nodes + index ) == false )
	{
		int child1 = nodes[index].children.child1;
		int child2 = nodes[index].children.child2;

		// Cost of creating a new parent for this node and the new leaf
		float cost = directCost + inheritedCost;

		// Sometimes there are multiple identical costs within tolerance.
		// This breaks the ties using the centroid distance.
		if ( cost < bestCost )
		{
			bestSibling = index;
			bestCost = cost;
		}

		// Inheritance cost seen by children
		inheritedCost += directCost - areaBase;

		bool leaf1 = b3IsLeaf( nodes + child1 );
		bool leaf2 = b3IsLeaf( nodes + child2 );

		// Cost of descending into child 1
		float lowerCost1 = FLT_MAX;
		b3AABB box1 = nodes[child1].aabb;
		float directCost1 = b3Perimeter( b3AABB_Union( box1, boxD ) );
		float area1 = 0.0f;
		if ( leaf1 )
		{
			// Child 1 is a leaf
			// Cost of creating new node and increasing area of node P
			float cost1 = directCost1 + inheritedCost;

			// Need this here due to while condition above
			if ( cost1 < bestCost )
			{
				bestSibling = child1;
				bestCost = cost1;
			}
		}
		else
		{
			// Child 1 is an internal node
			area1 = b3Perimeter( box1 );

			// Lower bound cost of inserting under child 1.
			lowerCost1 = inheritedCost + directCost1 + b3MinFloat( areaD - area1, 0.0f );
		}

		// Cost of descending into child 2
		float lowerCost2 = FLT_MAX;
		b3AABB box2 = nodes[child2].aabb;
		float directCost2 = b3Perimeter( b3AABB_Union( box2, boxD ) );
		float area2 = 0.0f;
		if ( leaf2 )
		{
			// Child 2 is a leaf
			// Cost of creating new node and increasing area of node P
			float cost2 = directCost2 + inheritedCost;

			// Need this here due to while condition above
			if ( cost2 < bestCost )
			{
				bestSibling = child2;
				bestCost = cost2;
			}
		}
		else
		{
			// Child 2 is an internal node
			area2 = b3Perimeter( box2 );

			// Lower bound cost of inserting under child 2. This is not the cost
			// of child 2, it is the best we can hope for under child 2.
			lowerCost2 = inheritedCost + directCost2 + b3MinFloat( areaD - area2, 0.0f );
		}

		if ( leaf1 && leaf2 )
		{
			break;
		}

		// Can the cost possibly be decreased?
		if ( bestCost <= lowerCost1 && bestCost <= lowerCost2 )
		{
			break;
		}

		if ( lowerCost1 == lowerCost2 && leaf1 == false )
		{
			B3_ASSERT( lowerCost1 < FLT_MAX );
			B3_ASSERT( lowerCost2 < FLT_MAX );

			// No clear choice based on lower bound surface area. This can happen when both
			// children fully contain D. Fall back to node distance.
			b3Vec3 d1 = b3Sub( b3AABB_Center( box1 ), centerD );
			b3Vec3 d2 = b3Sub( b3AABB_Center( box2 ), centerD );
			lowerCost1 = b3LengthSquared( d1 );
			lowerCost2 = b3LengthSquared( d2 );
		}

		// Descend
		if ( lowerCost1 < lowerCost2 && leaf1 == false )
		{
			index = child1;
			areaBase = area1;
			directCost = directCost1;
		}
		else
		{
			index = child2;
			areaBase = area2;
			directCost = directCost2;
		}

		B3_ASSERT( b3IsLeaf( nodes + index ) == false );
	}

	return bestSibling;
}

enum b3RotateType
{
	b3_rotateNone,
	b3_rotateBF,
	b3_rotateBG,
	b3_rotateCD,
	b3_rotateCE
};

// Perform a left or right rotation if node A is imbalanced.
// Returns the new root index.
static void b3RotateNodes( b3DynamicTree* tree, int iA )
{
	B3_ASSERT( iA != B3_NULL_INDEX );

	b3TreeNode* nodes = tree->nodes;

	b3TreeNode* A = nodes + iA;
	if ( b3IsLeaf( A ) == true )
	{
		return;
	}

	int iB = A->children.child1;
	int iC = A->children.child2;
	B3_ASSERT( 0 <= iB && iB < tree->nodeCapacity );
	B3_ASSERT( 0 <= iC && iC < tree->nodeCapacity );

	b3TreeNode* B = nodes + iB;
	b3TreeNode* C = nodes + iC;

	bool isLeafB = b3IsLeaf( B );
	bool isLeafC = b3IsLeaf( C );

	if ( isLeafB == true && isLeafC == false )
	{
		int iF = C->children.child1;
		int iG = C->children.child2;
		b3TreeNode* F = nodes + iF;
		b3TreeNode* G = nodes + iG;
		B3_ASSERT( 0 <= iF && iF < tree->nodeCapacity );
		B3_ASSERT( 0 <= iG && iG < tree->nodeCapacity );

		// Base cost
		float costBase = b3Perimeter( C->aabb );

		// Cost of swapping B and F
		b3AABB aabbBG = b3AABB_Union( B->aabb, G->aabb );
		float costBF = b3Perimeter( aabbBG );

		// Cost of swapping B and G
		b3AABB aabbBF = b3AABB_Union( B->aabb, F->aabb );
		float costBG = b3Perimeter( aabbBF );

		if ( costBase < costBF && costBase < costBG )
		{
			// Rotation does not improve cost
			return;
		}

		if ( costBF < costBG )
		{
			// Swap B and F
			A->children.child1 = iF;
			C->children.child1 = iB;

			B->parent = iC;
			F->parent = iA;

			C->aabb = aabbBG;

			C->height = 1 + b3MaxUInt16( B->height, G->height );
			A->height = 1 + b3MaxUInt16( C->height, F->height );
			C->categoryBits = B->categoryBits | G->categoryBits;
			A->categoryBits = C->categoryBits | F->categoryBits;
			C->flags |= ( B->flags | G->flags ) & b3_enlargedNode;
			A->flags |= ( C->flags | F->flags ) & b3_enlargedNode;
		}
		else
		{
			// Swap B and G
			A->children.child1 = iG;
			C->children.child2 = iB;

			B->parent = iC;
			G->parent = iA;

			C->aabb = aabbBF;

			C->height = 1 + b3MaxUInt16( B->height, F->height );
			A->height = 1 + b3MaxUInt16( C->height, G->height );
			C->categoryBits = B->categoryBits | F->categoryBits;
			A->categoryBits = C->categoryBits | G->categoryBits;
			C->flags |= ( B->flags | F->flags ) & b3_enlargedNode;
			A->flags |= ( C->flags | G->flags ) & b3_enlargedNode;
		}
	}
	else if ( isLeafC == true && isLeafB == false )
	{
		// C is a leaf and B is internal

		int iD = B->children.child1;
		int iE = B->children.child2;
		b3TreeNode* D = nodes + iD;
		b3TreeNode* E = nodes + iE;
		B3_ASSERT( 0 <= iD && iD < tree->nodeCapacity );
		B3_ASSERT( 0 <= iE && iE < tree->nodeCapacity );

		// Base cost
		float costBase = b3Perimeter( B->aabb );

		// Cost of swapping C and D
		b3AABB aabbCE = b3AABB_Union( C->aabb, E->aabb );
		float costCD = b3Perimeter( aabbCE );

		// Cost of swapping C and E
		b3AABB aabbCD = b3AABB_Union( C->aabb, D->aabb );
		float costCE = b3Perimeter( aabbCD );

		if ( costBase < costCD && costBase < costCE )
		{
			// Rotation does not improve cost
			return;
		}

		if ( costCD < costCE )
		{
			// Swap C and D
			A->children.child2 = iD;
			B->children.child1 = iC;

			C->parent = iB;
			D->parent = iA;

			B->aabb = aabbCE;

			B->height = 1 + b3MaxUInt16( C->height, E->height );
			A->height = 1 + b3MaxUInt16( B->height, D->height );
			B->categoryBits = C->categoryBits | E->categoryBits;
			A->categoryBits = B->categoryBits | D->categoryBits;
			B->flags |= ( C->flags | E->flags ) & b3_enlargedNode;
			A->flags |= ( B->flags | D->flags ) & b3_enlargedNode;
		}
		else
		{
			// Swap C and E
			A->children.child2 = iE;
			B->children.child2 = iC;

			C->parent = iB;
			E->parent = iA;

			B->aabb = aabbCD;

			B->height = 1 + b3MaxUInt16( C->height, D->height );
			A->height = 1 + b3MaxUInt16( B->height, E->height );
			B->categoryBits = C->categoryBits | D->categoryBits;
			A->categoryBits = B->categoryBits | E->categoryBits;
			B->flags |= ( C->flags | D->flags ) & b3_enlargedNode;
			A->flags |= ( B->flags | E->flags ) & b3_enlargedNode;
		}
	}
	else if ( isLeafB == false && isLeafC == false )
	{
		// All grand children exist so there are many options for rotation
		int iD = B->children.child1;
		int iE = B->children.child2;
		int iF = C->children.child1;
		int iG = C->children.child2;

		B3_ASSERT( 0 <= iD && iD < tree->nodeCapacity );
		B3_ASSERT( 0 <= iE && iE < tree->nodeCapacity );
		B3_ASSERT( 0 <= iF && iF < tree->nodeCapacity );
		B3_ASSERT( 0 <= iG && iG < tree->nodeCapacity );

		b3TreeNode* D = nodes + iD;
		b3TreeNode* E = nodes + iE;
		b3TreeNode* F = nodes + iF;
		b3TreeNode* G = nodes + iG;

		// Base cost
		float areaB = b3Perimeter( B->aabb );
		float areaC = b3Perimeter( C->aabb );
		float costBase = areaB + areaC;
		enum b3RotateType bestRotation = b3_rotateNone;
		float bestCost = costBase;

		// Cost of swapping B and F
		b3AABB aabbBG = b3AABB_Union( B->aabb, G->aabb );
		float costBF = areaB + b3Perimeter( aabbBG );
		if ( costBF < bestCost )
		{
			bestRotation = b3_rotateBF;
			bestCost = costBF;
		}

		// Cost of swapping B and G
		b3AABB aabbBF = b3AABB_Union( B->aabb, F->aabb );
		float costBG = areaB + b3Perimeter( aabbBF );
		if ( costBG < bestCost )
		{
			bestRotation = b3_rotateBG;
			bestCost = costBG;
		}

		// Cost of swapping C and D
		b3AABB aabbCE = b3AABB_Union( C->aabb, E->aabb );
		float costCD = areaC + b3Perimeter( aabbCE );
		if ( costCD < bestCost )
		{
			bestRotation = b3_rotateCD;
			bestCost = costCD;
		}

		// Cost of swapping C and E
		b3AABB aabbCD = b3AABB_Union( C->aabb, D->aabb );
		float costCE = areaC + b3Perimeter( aabbCD );
		if ( costCE < bestCost )
		{
			bestRotation = b3_rotateCE;
			// bestCost = costCE;
		}

		switch ( bestRotation )
		{
			case b3_rotateNone:
				break;

			case b3_rotateBF:
				A->children.child1 = iF;
				C->children.child1 = iB;

				B->parent = iC;
				F->parent = iA;

				C->aabb = aabbBG;

				C->height = 1 + b3MaxUInt16( B->height, G->height );
				A->height = 1 + b3MaxUInt16( C->height, F->height );
				C->categoryBits = B->categoryBits | G->categoryBits;
				A->categoryBits = C->categoryBits | F->categoryBits;
				C->flags |= ( B->flags | G->flags ) & b3_enlargedNode;
				A->flags |= ( C->flags | F->flags ) & b3_enlargedNode;
				break;

			case b3_rotateBG:
				A->children.child1 = iG;
				C->children.child2 = iB;

				B->parent = iC;
				G->parent = iA;

				C->aabb = aabbBF;

				C->height = 1 + b3MaxUInt16( B->height, F->height );
				A->height = 1 + b3MaxUInt16( C->height, G->height );
				C->categoryBits = B->categoryBits | F->categoryBits;
				A->categoryBits = C->categoryBits | G->categoryBits;
				C->flags |= ( B->flags | F->flags ) & b3_enlargedNode;
				A->flags |= ( C->flags | G->flags ) & b3_enlargedNode;
				break;

			case b3_rotateCD:
				A->children.child2 = iD;
				B->children.child1 = iC;

				C->parent = iB;
				D->parent = iA;

				B->aabb = aabbCE;

				B->height = 1 + b3MaxUInt16( C->height, E->height );
				A->height = 1 + b3MaxUInt16( B->height, D->height );
				B->categoryBits = C->categoryBits | E->categoryBits;
				A->categoryBits = B->categoryBits | D->categoryBits;
				B->flags |= ( C->flags | E->flags ) & b3_enlargedNode;
				A->flags |= ( B->flags | D->flags ) & b3_enlargedNode;
				break;

			case b3_rotateCE:
				A->children.child2 = iE;
				B->children.child2 = iC;

				C->parent = iB;
				E->parent = iA;

				B->aabb = aabbCD;

				B->height = 1 + b3MaxUInt16( C->height, D->height );
				A->height = 1 + b3MaxUInt16( B->height, E->height );
				B->categoryBits = C->categoryBits | D->categoryBits;
				A->categoryBits = B->categoryBits | E->categoryBits;
				B->flags |= ( C->flags | D->flags ) & b3_enlargedNode;
				A->flags |= ( B->flags | E->flags ) & b3_enlargedNode;
				break;

			default:
				B3_ASSERT( false );
				break;
		}
	}
}

// It would be nicer if the root had zero height but maintaining this would drastically increase
// insertion cost because whole sub-trees would need the height to be updated.
static void b3InsertLeaf( b3DynamicTree* tree, int leaf, bool shouldRotate )
{
	if ( tree->root == B3_NULL_INDEX )
	{
		tree->root = leaf;
		tree->nodes[tree->root].parent = B3_NULL_INDEX;
		return;
	}

	// Stage 1: find the best sibling for this node
	b3AABB leafAABB = tree->nodes[leaf].aabb;
	int sibling = b3FindBestSibling( tree, leafAABB );

	// Stage 2: create a new parent for the leaf and sibling
	int oldParent = tree->nodes[sibling].parent;
	int newParent = b3AllocateNode( tree );

	// warning: node pointer can change after allocation
	b3TreeNode* nodes = tree->nodes;
	nodes[newParent].parent = oldParent;
	nodes[newParent].userData = UINT64_MAX;
	nodes[newParent].aabb = b3AABB_Union( leafAABB, nodes[sibling].aabb );
	nodes[newParent].categoryBits = nodes[leaf].categoryBits | nodes[sibling].categoryBits;
	nodes[newParent].height = nodes[sibling].height + 1;

	if ( oldParent != B3_NULL_INDEX )
	{
		// The sibling was not the root.
		if ( nodes[oldParent].children.child1 == sibling )
		{
			nodes[oldParent].children.child1 = newParent;
		}
		else
		{
			nodes[oldParent].children.child2 = newParent;
		}

		nodes[newParent].children.child1 = sibling;
		nodes[newParent].children.child2 = leaf;
		nodes[sibling].parent = newParent;
		nodes[leaf].parent = newParent;
	}
	else
	{
		// The sibling was the root.
		nodes[newParent].children.child1 = sibling;
		nodes[newParent].children.child2 = leaf;
		nodes[sibling].parent = newParent;
		nodes[leaf].parent = newParent;
		tree->root = newParent;
	}

	// Stage 3: walk back up the tree fixing heights and AABBs
	int index = nodes[leaf].parent;
	while ( index != B3_NULL_INDEX )
	{
		int child1 = nodes[index].children.child1;
		int child2 = nodes[index].children.child2;

		B3_ASSERT( child1 != B3_NULL_INDEX );
		B3_ASSERT( child2 != B3_NULL_INDEX );

		nodes[index].aabb = b3AABB_Union( nodes[child1].aabb, nodes[child2].aabb );
		nodes[index].categoryBits = nodes[child1].categoryBits | nodes[child2].categoryBits;
		nodes[index].height = 1 + b3MaxUInt16( nodes[child1].height, nodes[child2].height );
		nodes[index].flags |= ( nodes[child1].flags | nodes[child2].flags ) & b3_enlargedNode;

		if ( shouldRotate )
		{
			b3RotateNodes( tree, index );
		}

		index = nodes[index].parent;
	}
}

static void b3RemoveLeaf( b3DynamicTree* tree, int leaf )
{
	if ( leaf == tree->root )
	{
		tree->root = B3_NULL_INDEX;
		return;
	}

	b3TreeNode* nodes = tree->nodes;

	int parent = nodes[leaf].parent;
	int grandParent = nodes[parent].parent;
	int sibling;
	if ( nodes[parent].children.child1 == leaf )
	{
		sibling = nodes[parent].children.child2;
	}
	else
	{
		sibling = nodes[parent].children.child1;
	}

	if ( grandParent != B3_NULL_INDEX )
	{
		// Destroy parent and connect sibling to grandParent.
		if ( nodes[grandParent].children.child1 == parent )
		{
			nodes[grandParent].children.child1 = sibling;
		}
		else
		{
			nodes[grandParent].children.child2 = sibling;
		}
		nodes[sibling].parent = grandParent;
		b3FreeNode( tree, parent );

		// Adjust ancestor bounds.
		int index = grandParent;
		while ( index != B3_NULL_INDEX )
		{
			b3TreeNode* node = nodes + index;
			b3TreeNode* child1 = nodes + node->children.child1;
			b3TreeNode* child2 = nodes + node->children.child2;

			// Fast union using SSE
			//__m128 aabb1 = _mm_load_ps(&child1->aabb.lowerBound.x);
			//__m128 aabb2 = _mm_load_ps(&child2->aabb.lowerBound.x);
			//__m128 lower = _mm_min_ps(aabb1, aabb2);
			//__m128 upper = _mm_max_ps(aabb1, aabb2);
			//__m128 aabb = _mm_shuffle_ps(lower, upper, _MM_SHUFFLE(3, 2, 1, 0));
			//_mm_store_ps(&node->aabb.lowerBound.x, aabb);

			node->aabb = b3AABB_Union( child1->aabb, child2->aabb );
			node->categoryBits = child1->categoryBits | child2->categoryBits;
			node->height = 1 + b3MaxUInt16( child1->height, child2->height );

			index = node->parent;
		}
	}
	else
	{
		tree->root = sibling;
		tree->nodes[sibling].parent = B3_NULL_INDEX;
		b3FreeNode( tree, parent );
	}
}

// Create a proxy in the tree as a leaf node. We return the index of the node instead of a pointer so that we can grow
// the node pool.
int b3DynamicTree_CreateProxy( b3DynamicTree* tree, b3AABB aabb, uint64_t categoryBits, uint64_t userData )
{
	B3_ASSERT( b3IsValidAABB( aabb ) );

	int proxyId = b3AllocateNode( tree );
	b3TreeNode* node = tree->nodes + proxyId;

	node->aabb = aabb;
	node->userData = userData;
	node->categoryBits = categoryBits;
	node->height = 0;
	node->flags = b3_allocatedNode | b3_leafNode;

	bool shouldRotate = true;
	b3InsertLeaf( tree, proxyId, shouldRotate );

	tree->proxyCount += 1;

	return proxyId;
}

void b3DynamicTree_DestroyProxy( b3DynamicTree* tree, int proxyId )
{
	B3_ASSERT( 0 <= proxyId && proxyId < tree->nodeCapacity );
	B3_ASSERT( b3IsLeaf( tree->nodes + proxyId ) );

	b3RemoveLeaf( tree, proxyId );
	b3FreeNode( tree, proxyId );

	B3_ASSERT( tree->proxyCount > 0 );
	tree->proxyCount -= 1;
}

int b3DynamicTree_GetProxyCount( const b3DynamicTree* tree )
{
	return tree->proxyCount;
}

void b3DynamicTree_MoveProxy( b3DynamicTree* tree, int proxyId, b3AABB aabb )
{
	B3_ASSERT( b3IsValidAABB( aabb ) );
	B3_ASSERT( 0 <= proxyId && proxyId < tree->nodeCapacity );
	B3_ASSERT( b3IsLeaf( tree->nodes + proxyId ) );

	b3RemoveLeaf( tree, proxyId );

	tree->nodes[proxyId].aabb = aabb;

	bool shouldRotate = false;
	b3InsertLeaf( tree, proxyId, shouldRotate );
}

void b3DynamicTree_EnlargeProxy( b3DynamicTree* tree, int proxyId, b3AABB aabb )
{
	b3TreeNode* nodes = tree->nodes;
	B3_VALIDATE( b3IsValidAABB( aabb ) );
	B3_ASSERT( 0 <= proxyId && proxyId < tree->nodeCapacity );
	B3_VALIDATE( b3IsLeaf( tree->nodes + proxyId ) );

	// Caller must ensure this
	B3_VALIDATE( b3AABB_Contains( nodes[proxyId].aabb, aabb ) == false );

	b3TreeNode* node = nodes + proxyId;
	node->aabb = aabb;

	int parentIndex = node->parent;
	while ( parentIndex != B3_NULL_INDEX )
	{
		node = nodes + parentIndex;
		bool changed = b3EnlargeAABB( &node->aabb, aabb );

		// todo not sure why this node is marked as enlarged even if it didn't change
		node->flags |= b3_enlargedNode;

		parentIndex = node->parent;

		if ( changed == false )
		{
			break;
		}
	}

	while ( parentIndex != B3_NULL_INDEX )
	{
		node = nodes + parentIndex;
		if ( node->flags & b3_enlargedNode )
		{
			// early out because this ancestor was previously ascended and marked as enlarged
			break;
		}

		node->flags |= b3_enlargedNode;
		parentIndex = node->parent;
	}
}

void b3DynamicTree_SetCategoryBits( b3DynamicTree* tree, int proxyId, uint64_t categoryBits )
{
	b3TreeNode* nodes = tree->nodes;

	B3_ASSERT( b3IsLeaf( nodes + proxyId ) );

	nodes[proxyId].categoryBits = categoryBits;

	// Fix up category bits in ancestor internal nodes
	int nodeIndex = nodes[proxyId].parent;
	while ( nodeIndex != B3_NULL_INDEX )
	{
		b3TreeNode* node = nodes + nodeIndex;
		int child1 = node->children.child1;
		B3_ASSERT( child1 != B3_NULL_INDEX );
		int child2 = node->children.child2;
		B3_ASSERT( child2 != B3_NULL_INDEX );
		node->categoryBits = nodes[child1].categoryBits | nodes[child2].categoryBits;

		nodeIndex = node->parent;
	}
}

uint64_t b3DynamicTree_GetCategoryBits( b3DynamicTree* tree, int proxyId )
{
	B3_ASSERT( 0 <= proxyId && proxyId < tree->nodeCapacity );
	return tree->nodes[proxyId].categoryBits;
}

int b3DynamicTree_GetHeight( const b3DynamicTree* tree )
{
	if ( tree->root == B3_NULL_INDEX )
	{
		return 0;
	}

	return tree->nodes[tree->root].height;
}

float b3DynamicTree_GetAreaRatio( const b3DynamicTree* tree )
{
	if ( tree->root == B3_NULL_INDEX )
	{
		return 0.0f;
	}

	const b3TreeNode* root = tree->nodes + tree->root;
	float rootArea = b3Perimeter( root->aabb );

	float totalArea = 0.0f;
	for ( int i = 0; i < tree->nodeCapacity; ++i )
	{
		const b3TreeNode* node = tree->nodes + i;
		if ( b3IsAllocated( node ) == false || b3IsLeaf( node ) || i == tree->root )
		{
			continue;
		}

		totalArea += b3Perimeter( node->aabb );
	}

	return totalArea / rootArea;
}

b3AABB b3DynamicTree_GetRootBounds( const b3DynamicTree* tree )
{
	if ( tree->root != B3_NULL_INDEX )
	{
		return tree->nodes[tree->root].aabb;
	}

	b3AABB empty = { b3Vec3_zero, b3Vec3_zero };
	return empty;
}

#if B3_ENABLE_VALIDATION
// Compute the height of a sub-tree.
static int b3ComputeHeightRecurse( const b3DynamicTree* tree, int nodeId )
{
	B3_ASSERT( 0 <= nodeId && nodeId < tree->nodeCapacity );
	b3TreeNode* node = tree->nodes + nodeId;

	if ( b3IsLeaf( node ) )
	{
		return 0;
	}

	int height1 = b3ComputeHeightRecurse( tree, node->children.child1 );
	int height2 = b3ComputeHeightRecurse( tree, node->children.child2 );
	return 1 + b3MaxInt( height1, height2 );
}

static int b3ComputeHeight( const b3DynamicTree* tree )
{
	int height = b3ComputeHeightRecurse( tree, tree->root );
	return height;
}

static void b3ValidateStructure( const b3DynamicTree* tree, int index )
{
	if ( index == B3_NULL_INDEX )
	{
		return;
	}

	if ( index == tree->root )
	{
		B3_ASSERT( tree->nodes[index].parent == B3_NULL_INDEX );
	}

	const b3TreeNode* node = tree->nodes + index;

	B3_ASSERT( node->flags == 0 || ( node->flags & b3_allocatedNode ) != 0 );

	if ( b3IsLeaf( node ) )
	{
		B3_ASSERT( node->height == 0 );
		return;
	}

	int child1 = node->children.child1;
	int child2 = node->children.child2;

	B3_ASSERT( 0 <= child1 && child1 < tree->nodeCapacity );
	B3_ASSERT( 0 <= child2 && child2 < tree->nodeCapacity );

	B3_ASSERT( tree->nodes[child1].parent == index );
	B3_ASSERT( tree->nodes[child2].parent == index );

	if ( ( tree->nodes[child1].flags | tree->nodes[child2].flags ) & b3_enlargedNode )
	{
		B3_ASSERT( node->flags & b3_enlargedNode );
	}

	b3ValidateStructure( tree, child1 );
	b3ValidateStructure( tree, child2 );
}

static void b3ValidateMetrics( const b3DynamicTree* tree, int index )
{
	if ( index == B3_NULL_INDEX )
	{
		return;
	}

	const b3TreeNode* node = tree->nodes + index;

	B3_VALIDATE( b3IsValidAABB( node->aabb ) );

	if ( b3IsLeaf( node ) )
	{
		B3_ASSERT( node->height == 0 );
		return;
	}

	int child1 = node->children.child1;
	int child2 = node->children.child2;

	B3_ASSERT( 0 <= child1 && child1 < tree->nodeCapacity );
	B3_ASSERT( 0 <= child2 && child2 < tree->nodeCapacity );

	int height1 = tree->nodes[child1].height;
	int height2 = tree->nodes[child2].height;
	int height = 1 + b3MaxInt( height1, height2 );
	B3_ASSERT( node->height == height );

	// b3AABB aabb = b3AABB_Union(tree->nodes[child1].aabb, tree->nodes[child2].aabb);

	B3_ASSERT( b3AABB_Contains( node->aabb, tree->nodes[child1].aabb ) );
	B3_ASSERT( b3AABB_Contains( node->aabb, tree->nodes[child2].aabb ) );

	// B3_ASSERT(aabb.lowerBound.x == node->aabb.lowerBound.x);
	// B3_ASSERT(aabb.lowerBound.y == node->aabb.lowerBound.y);
	// B3_ASSERT(aabb.upperBound.x == node->aabb.upperBound.x);
	// B3_ASSERT(aabb.upperBound.y == node->aabb.upperBound.y);

	uint64_t categoryBits = tree->nodes[child1].categoryBits | tree->nodes[child2].categoryBits;
	B3_ASSERT( node->categoryBits == categoryBits );

	b3ValidateMetrics( tree, child1 );
	b3ValidateMetrics( tree, child2 );
}
#endif

void b3DynamicTree_Validate( const b3DynamicTree* tree )
{
#if B3_ENABLE_VALIDATION
	if ( tree->root == B3_NULL_INDEX )
	{
		return;
	}

	b3ValidateStructure( tree, tree->root );
	b3ValidateMetrics( tree, tree->root );

	int freeCount = 0;
	int freeIndex = tree->freeList;
	while ( freeIndex != B3_NULL_INDEX )
	{
		B3_ASSERT( 0 <= freeIndex && freeIndex < tree->nodeCapacity );
		freeIndex = tree->nodes[freeIndex].next;
		++freeCount;
	}

	int height = b3DynamicTree_GetHeight( tree );
	int computedHeight = b3ComputeHeight( tree );
	B3_ASSERT( height == computedHeight );

	B3_ASSERT( tree->nodeCount + freeCount == tree->nodeCapacity );
#else
	B3_UNUSED( tree );
#endif
}

void b3DynamicTree_ValidateNoEnlarged( const b3DynamicTree* tree )
{
#if B3_ENABLE_VALIDATION == 1
	int capacity = tree->nodeCapacity;
	const b3TreeNode* nodes = tree->nodes;
	for ( int i = 0; i < capacity; ++i )
	{
		const b3TreeNode* node = nodes + i;
		if ( node->flags & b3_allocatedNode )
		{
			B3_ASSERT( ( node->flags & b3_enlargedNode ) == 0 );
		}
	}
#else
	B3_UNUSED( tree );
#endif
}

int b3DynamicTree_GetByteCount( const b3DynamicTree* tree )
{
	size_t size = sizeof( b3DynamicTree ) + sizeof( b3TreeNode ) * tree->nodeCapacity +
				  tree->rebuildCapacity * ( sizeof( int ) + sizeof( b3AABB ) + sizeof( b3Vec3 ) + sizeof( int ) );

	return (int)size;
}

b3TreeStats b3DynamicTree_Query( const b3DynamicTree* tree, b3AABB aabb, uint64_t maskBits, bool requireAllBits,
								 b3TreeQueryCallbackFcn* callback, void* context )
{
	b3TreeStats result = { 0 };

	if ( tree->nodeCount == 0 )
	{
		return result;
	}

	int stack[B3_TREE_STACK_SIZE];
	int stackCount = 0;
	stack[stackCount++] = tree->root;

	while ( stackCount > 0 )
	{
		int nodeId = stack[--stackCount];
		if ( nodeId == B3_NULL_INDEX )
		{
			// todo huh?
			B3_ASSERT( false );
			continue;
		}

		const b3TreeNode* node = tree->nodes + nodeId;
		result.nodeVisits += 1;

		// Assuming branch prediction deals with requireAllBits well
		uint64_t bitMatch = requireAllBits ? ( node->categoryBits & maskBits ) == maskBits : ( node->categoryBits & maskBits );

		if ( bitMatch && b3AABB_Overlaps( node->aabb, aabb ) )
		{
			if ( b3IsLeaf( node ) )
			{
				// callback to user code with proxy id
				bool proceed = callback( nodeId, node->userData, context );
				result.leafVisits += 1;

				if ( proceed == false )
				{
					return result;
				}
			}
			else
			{
				B3_ASSERT( stackCount < B3_TREE_STACK_SIZE - 1 );
				if ( stackCount < B3_TREE_STACK_SIZE - 1 )
				{
					stack[stackCount++] = node->children.child1;
					stack[stackCount++] = node->children.child2;
				}
			}
		}
	}

	return result;
}

B3_FORCE_INLINE float b3DistanceToNodeSqr( b3Vec3 point, const b3TreeNode* node )
{
	b3Vec3 r = b3Sub( point, b3Clamp( point, node->aabb.lowerBound, node->aabb.upperBound ) );
	return b3Dot( r, r );
}

struct b3QueryClosestItem
{
	int nodeIndex;
	float distanceToNodeSqr;
};

b3TreeStats b3DynamicTree_QueryClosest( const b3DynamicTree* tree, b3Vec3 point, uint64_t maskBits, bool requireAllBits,
										b3TreeQueryClosestCallbackFcn* callback, void* context, float* minDistanceSqr )
{
	b3TreeStats result = { 0 };

	if ( tree->nodeCount == 0 )
	{
		return result;
	}

	float minSqr = *minDistanceSqr;
	struct b3QueryClosestItem stack[B3_TREE_STACK_SIZE];
	int stackCount = 0;

	float rootDistanceSqr = b3DistanceToNodeSqr( point, tree->nodes + tree->root );
	stack[stackCount++] = (struct b3QueryClosestItem){
		.nodeIndex = tree->root,
		.distanceToNodeSqr = rootDistanceSqr,
	};

	while ( stackCount > 0 )
	{
		struct b3QueryClosestItem item = stack[--stackCount];
		const b3TreeNode* node = tree->nodes + item.nodeIndex;
		result.nodeVisits += 1;

		uint64_t bitMatch = requireAllBits ? ( node->categoryBits & maskBits ) == maskBits : ( node->categoryBits & maskBits );

		if ( bitMatch )
		{
			if ( item.distanceToNodeSqr < minSqr )
			{
				if ( b3IsLeaf( node ) )
				{
					// callback to user code with minimum distance squared so far and proxy id
					float dd = callback( minSqr, item.nodeIndex, node->userData, context );

					if ( dd < minSqr )
					{
						minSqr = dd;
					}

					result.leafVisits += 1;
				}
				else
				{
					B3_ASSERT( stackCount < B3_TREE_STACK_SIZE - 1 );
					if ( stackCount < B3_TREE_STACK_SIZE - 1 )
					{
						int child1 = node->children.child1;
						int child2 = node->children.child2;

						// Store the distance to node in the stack instead of recomputing after pop
						struct b3QueryClosestItem item1 = {
							.nodeIndex = child1,
							.distanceToNodeSqr = b3DistanceToNodeSqr( point, tree->nodes + child1 ),
						};

						struct b3QueryClosestItem item2 = {
							.nodeIndex = child2,
							.distanceToNodeSqr = b3DistanceToNodeSqr( point, tree->nodes + child2 ),
						};

						// Ensure we iterate the closest child first as we pop off the stack
						if ( item2.distanceToNodeSqr < item1.distanceToNodeSqr )
						{
							stack[stackCount++] = item1;
							stack[stackCount++] = item2;
						}
						else
						{
							stack[stackCount++] = item2;
							stack[stackCount++] = item1;
						}
					}
				}
			}
		}
	}

	*minDistanceSqr = minSqr;

	return result;
}

b3TreeStats b3DynamicTree_RayCast( const b3DynamicTree* tree, const b3RayCastInput* input, uint64_t maskBits, bool requireAllBits,
								   b3TreeRayCastCallbackFcn* callback, void* context )
{
	b3TreeStats result = { 0 };

	if ( tree->nodeCount == 0 )
	{
		return result;
	}

	b3Vec3 p1 = input->origin;
	b3Vec3 d = input->translation;

	b3V32 pv1 = b3LoadV( &p1.x );
	b3V32 dv = b3LoadV( &d.x );

	float maxFraction = input->maxFraction;

	b3Vec3 p2 = b3MulAdd( p1, maxFraction, d );

	// Build a bounding box for the segment.
	b3AABB segmentAABB = { b3Min( p1, p2 ), b3Max( p1, p2 ) };

	int stack[B3_TREE_STACK_SIZE];
	int stackCount = 0;
	stack[stackCount++] = tree->root;

	const b3TreeNode* nodes = tree->nodes;

	b3RayCastInput subInput = *input;

	while ( stackCount > 0 )
	{
		int nodeId = stack[--stackCount];
		if ( nodeId == B3_NULL_INDEX )
		{
			// todo is this possible?
			B3_ASSERT( false );
			continue;
		}

		const b3TreeNode* node = nodes + nodeId;
		result.nodeVisits += 1;

		b3AABB nodeAABB = node->aabb;

		// todo look at disassembly
		uint64_t bitMatch = requireAllBits ? ( node->categoryBits & maskBits ) == maskBits : ( node->categoryBits & maskBits );

		if ( bitMatch == 0 || b3AABB_Overlaps( nodeAABB, segmentAABB ) == false )
		{
			continue;
		}

		b3V32 lower = b3LoadV( &nodeAABB.lowerBound.x );
		b3V32 upper = b3LoadV( &nodeAABB.upperBound.x );

		bool edgeOverlap = b3TestBoundsRayOverlap( lower, upper, pv1, dv );
		if ( edgeOverlap == false )
		{
			continue;
		}

		if ( b3IsLeaf( node ) )
		{
			subInput.maxFraction = maxFraction;

			float value = callback( &subInput, nodeId, node->userData, context );
			result.leafVisits += 1;

			// The user may return -1 to indicate this shape should be skipped

			if ( value == 0.0f )
			{
				// The client has terminated the ray cast.
				return result;
			}

			if ( 0.0f < value && value <= maxFraction )
			{
				// Update segment bounding box.
				maxFraction = value;
				p2 = b3MulAdd( p1, maxFraction, d );
				segmentAABB.lowerBound = b3Min( p1, p2 );
				segmentAABB.upperBound = b3Max( p1, p2 );
			}
		}
		else
		{
			B3_ASSERT( stackCount < B3_TREE_STACK_SIZE - 1 );
			if ( stackCount < B3_TREE_STACK_SIZE - 1 )
			{
				b3Vec3 c1 = b3AABB_Center( nodes[node->children.child1].aabb );
				b3Vec3 c2 = b3AABB_Center( nodes[node->children.child2].aabb );
				if ( b3DistanceSquared( c1, p1 ) < b3DistanceSquared( c2, p1 ) )
				{
					stack[stackCount++] = node->children.child2;
					stack[stackCount++] = node->children.child1;
				}
				else
				{
					stack[stackCount++] = node->children.child1;
					stack[stackCount++] = node->children.child2;
				}
			}
		}
	}

	return result;
}

b3TreeStats b3DynamicTree_BoxCast( const b3DynamicTree* tree, const b3BoxCastInput* input, uint64_t maskBits, bool requireAllBits,
								   b3TreeBoxCastCallbackFcn* callback, void* context )
{
	b3TreeStats stats = { 0 };

	if ( tree->nodeCount == 0 )
	{
		return stats;
	}

	// The caller folds the shape radius and the world origin into the box
	b3AABB originAABB = input->box;

	b3Vec3 p1 = b3AABB_Center( originAABB );
	b3Vec3 extension = b3AABB_Extents( originAABB );

	b3Vec3 d = input->translation;

	b3V32 pv1 = b3LoadV( &p1.x );
	b3V32 dv = b3LoadV( &d.x );
	b3V32 ev = b3LoadV( &extension.x );

	float maxFraction = input->maxFraction;

	// Build total box for the cast
	b3Vec3 t = b3MulSV( maxFraction, input->translation );
	b3AABB totalAABB = {
		b3Min( originAABB.lowerBound, b3Add( originAABB.lowerBound, t ) ),
		b3Max( originAABB.upperBound, b3Add( originAABB.upperBound, t ) ),
	};

	b3BoxCastInput subInput = *input;
	const b3TreeNode* nodes = tree->nodes;

	int stack[B3_TREE_STACK_SIZE];
	int stackCount = 0;
	stack[stackCount++] = tree->root;

	while ( stackCount > 0 )
	{
		int nodeId = stack[--stackCount];
		if ( nodeId == B3_NULL_INDEX )
		{
			B3_ASSERT( false );
			continue;
		}

		const b3TreeNode* node = nodes + nodeId;
		stats.nodeVisits += 1;

		uint64_t bitMatch = requireAllBits ? ( node->categoryBits & maskBits ) == maskBits : ( node->categoryBits & maskBits );

		if ( bitMatch == 0 || b3AABB_Overlaps( node->aabb, totalAABB ) == false )
		{
			continue;
		}

		// radius extension is added to the node in this case
		b3V32 lower = b3SubV( b3LoadV( &node->aabb.lowerBound.x ), ev );
		b3V32 upper = b3AddV( b3LoadV( &node->aabb.upperBound.x ), ev );
		bool edgeOverlap = b3TestBoundsRayOverlap( lower, upper, pv1, dv );
		if ( edgeOverlap == false )
		{
			continue;
		}

		if ( b3IsLeaf( node ) )
		{
			subInput.maxFraction = maxFraction;

			float value = callback( &subInput, nodeId, node->userData, context );
			stats.leafVisits += 1;

			if ( value == 0.0f )
			{
				// The client has terminated the cast.
				return stats;
			}

			if ( 0.0f < value && value < maxFraction )
			{
				maxFraction = value;
				t = b3MulSV( maxFraction, input->translation );
				totalAABB.lowerBound = b3Min( originAABB.lowerBound, b3Add( originAABB.lowerBound, t ) );
				totalAABB.upperBound = b3Max( originAABB.upperBound, b3Add( originAABB.upperBound, t ) );
			}
		}
		else
		{
			B3_ASSERT( stackCount < B3_TREE_STACK_SIZE - 1 );
			if ( stackCount < B3_TREE_STACK_SIZE - 1 )
			{
				b3Vec3 c1 = b3AABB_Center( nodes[node->children.child1].aabb );
				b3Vec3 c2 = b3AABB_Center( nodes[node->children.child2].aabb );
				if ( b3DistanceSquared( c1, p1 ) < b3DistanceSquared( c2, p1 ) )
				{
					stack[stackCount++] = node->children.child2;
					stack[stackCount++] = node->children.child1;
				}
				else
				{
					stack[stackCount++] = node->children.child1;
					stack[stackCount++] = node->children.child2;
				}
			}
		}
	}

	return stats;
}

// Median split == 0, Surface area heuristic == 1
#define B3_TREE_HEURISTIC 0

#if B3_TREE_HEURISTIC == 0

// Median split heuristic
static int b3PartitionMid( int* indices, b3Vec3* centers, int count )
{
	// Handle trivial case
	if ( count <= 2 )
	{
		return count / 2;
	}

	b3Vec3 lowerBound = centers[0];
	b3Vec3 upperBound = centers[0];

	for ( int i = 1; i < count; ++i )
	{
		lowerBound = b3Min( lowerBound, centers[i] );
		upperBound = b3Max( upperBound, centers[i] );
	}

	b3Vec3 d = b3Sub( upperBound, lowerBound );
	b3Vec3 c = b3MulSV( 0.5f, b3Add( lowerBound, upperBound ) );

	// Partition longest axis using the Hoare partition scheme
	// https://en.wikipedia.org/wiki/Quicksort
	// https://nicholasvadivelu.com/2021/01/11/array-partition/
	int i1 = 0, i2 = count;
	if ( d.x >= d.y && d.x >= d.z )
	{
		float pivot = c.x;

		while ( i1 < i2 )
		{
			while ( i1 < i2 && centers[i1].x < pivot )
			{
				i1 += 1;
			};

			while ( i1 < i2 && centers[i2 - 1].x >= pivot )
			{
				i2 -= 1;
			};

			if ( i1 < i2 )
			{
				// Swap indices
				{
					int temp = indices[i1];
					indices[i1] = indices[i2 - 1];
					indices[i2 - 1] = temp;
				}

				// Swap centers
				{
					b3Vec3 temp = centers[i1];
					centers[i1] = centers[i2 - 1];
					centers[i2 - 1] = temp;
				}

				i1 += 1;
				i2 -= 1;
			}
		}
	}
	else if ( d.y >= d.z )
	{
		float pivot = c.y;

		while ( i1 < i2 )
		{
			while ( i1 < i2 && centers[i1].y < pivot )
			{
				i1 += 1;
			};

			while ( i1 < i2 && centers[i2 - 1].y >= pivot )
			{
				i2 -= 1;
			};

			if ( i1 < i2 )
			{
				// Swap indices
				{
					int temp = indices[i1];
					indices[i1] = indices[i2 - 1];
					indices[i2 - 1] = temp;
				}

				// Swap centers
				{
					b3Vec3 temp = centers[i1];
					centers[i1] = centers[i2 - 1];
					centers[i2 - 1] = temp;
				}

				i1 += 1;
				i2 -= 1;
			}
		}
	}
	else
	{
		float pivot = c.z;

		while ( i1 < i2 )
		{
			while ( i1 < i2 && centers[i1].z < pivot )
			{
				i1 += 1;
			};

			while ( i1 < i2 && centers[i2 - 1].z >= pivot )
			{
				i2 -= 1;
			};

			if ( i1 < i2 )
			{
				// Swap indices
				{
					int temp = indices[i1];
					indices[i1] = indices[i2 - 1];
					indices[i2 - 1] = temp;
				}

				// Swap centers
				{
					b3Vec3 temp = centers[i1];
					centers[i1] = centers[i2 - 1];
					centers[i2 - 1] = temp;
				}

				i1 += 1;
				i2 -= 1;
			}
		}
	}
	B3_ASSERT( i1 == i2 );

	if ( i1 > 0 && i1 < count )
	{
		return i1;
	}

	return count / 2;
}

#else

#define B3_BIN_COUNT 8

typedef struct b3TreeBin
{
	b3AABB aabb;
	int count;
} b3TreeBin;

typedef struct b3TreePlane
{
	b3AABB leftAABB;
	b3AABB rightAABB;
	int leftCount;
	int rightCount;
} b3TreePlane;

// "On Fast Construction of SAH-based Bounding Volume Hierarchies" by Ingo Wald
// Returns the left child count
static int b3PartitionSAH( int* indices, int* binIndices, b3AABB* boxes, int count )
{
	B3_ASSERT( count > 0 );

	b3TreeBin bins[B3_BIN_COUNT];
	b3TreePlane planes[B3_BIN_COUNT - 1];

	b3Vec3 center = b3AABB_Center( boxes[0] );
	b3AABB centroidAABB;
	centroidAABB.lowerBound = center;
	centroidAABB.upperBound = center;

	for ( int i = 1; i < count; ++i )
	{
		center = b3AABB_Center( boxes[i] );
		centroidAABB.lowerBound = b3Min( centroidAABB.lowerBound, center );
		centroidAABB.upperBound = b3Max( centroidAABB.upperBound, center );
	}

	b3Vec3 d = b3Sub( centroidAABB.upperBound, centroidAABB.lowerBound );

	// Find longest axis
	int axisIndex;
	float invD;
	if ( d.x > d.y )
	{
		axisIndex = 0;
		invD = d.x;
	}
	else
	{
		axisIndex = 1;
		invD = d.y;
	}

	invD = invD > 0.0f ? 1.0f / invD : 0.0f;

	// Initialize bin bounds and count
	for ( int i = 0; i < B3_BIN_COUNT; ++i )
	{
		bins[i].aabb.lowerBound = (b3Vec3){ FLT_MAX, FLT_MAX };
		bins[i].aabb.upperBound = (b3Vec3){ -FLT_MAX, -FLT_MAX };
		bins[i].count = 0;
	}

	// Assign boxes to bins and compute bin boxes
	// TODO_ERIN optimize
	float binCount = B3_BIN_COUNT;
	float lowerBoundArray[2] = { centroidAABB.lowerBound.x, centroidAABB.lowerBound.y };
	float minC = lowerBoundArray[axisIndex];
	for ( int i = 0; i < count; ++i )
	{
		b3Vec3 c = b3AABB_Center( boxes[i] );
		float cArray[2] = { c.x, c.y };
		int binIndex = (int)( binCount * ( cArray[axisIndex] - minC ) * invD );
		binIndex = b3ClampInt( binIndex, 0, B3_BIN_COUNT - 1 );
		binIndices[i] = binIndex;
		bins[binIndex].count += 1;
		bins[binIndex].aabb = b3AABB_Union( bins[binIndex].aabb, boxes[i] );
	}

	int planeCount = B3_BIN_COUNT - 1;

	// Prepare all the left planes, candidates for left child
	planes[0].leftCount = bins[0].count;
	planes[0].leftAABB = bins[0].aabb;
	for ( int i = 1; i < planeCount; ++i )
	{
		planes[i].leftCount = planes[i - 1].leftCount + bins[i].count;
		planes[i].leftAABB = b3AABB_Union( planes[i - 1].leftAABB, bins[i].aabb );
	}

	// Prepare all the right planes, candidates for right child
	planes[planeCount - 1].rightCount = bins[planeCount].count;
	planes[planeCount - 1].rightAABB = bins[planeCount].aabb;
	for ( int i = planeCount - 2; i >= 0; --i )
	{
		planes[i].rightCount = planes[i + 1].rightCount + bins[i + 1].count;
		planes[i].rightAABB = b3AABB_Union( planes[i + 1].rightAABB, bins[i + 1].aabb );
	}

	// Find best split to minimize SAH
	float minCost = FLT_MAX;
	int bestPlane = 0;
	for ( int i = 0; i < planeCount; ++i )
	{
		float leftArea = b3Perimeter( planes[i].leftAABB );
		float rightArea = b3Perimeter( planes[i].rightAABB );
		int leftCount = planes[i].leftCount;
		int rightCount = planes[i].rightCount;

		float cost = leftCount * leftArea + rightCount * rightArea;
		if ( cost < minCost )
		{
			bestPlane = i;
			minCost = cost;
		}
	}

	// Partition node indices and boxes using the Hoare partition scheme
	// https://en.wikipedia.org/wiki/Quicksort
	// https://nicholasvadivelu.com/2021/01/11/array-partition/
	int i1 = 0, i2 = count;
	while ( i1 < i2 )
	{
		while ( i1 < i2 && binIndices[i1] < bestPlane )
		{
			i1 += 1;
		};

		while ( i1 < i2 && binIndices[i2 - 1] >= bestPlane )
		{
			i2 -= 1;
		};

		if ( i1 < i2 )
		{
			// Swap indices
			{
				int temp = indices[i1];
				indices[i1] = indices[i2 - 1];
				indices[i2 - 1] = temp;
			}

			// Swap boxes
			{
				b3AABB temp = boxes[i1];
				boxes[i1] = boxes[i2 - 1];
				boxes[i2 - 1] = temp;
			}

			i1 += 1;
			i2 -= 1;
		}
	}
	B3_ASSERT( i1 == i2 );

	if ( i1 > 0 && i1 < count )
	{
		return i1;
	}
	else
	{
		return count / 2;
	}
}

#endif

// Temporary data used to track the rebuild of a tree node
struct b3RebuildItem
{
	int nodeIndex;
	int childCount;

	// Leaf indices
	int startIndex;
	int splitIndex;
	int endIndex;
};

// Returns root node index
static int b3BuildTree( b3DynamicTree* tree, int leafCount )
{
	b3TreeNode* nodes = tree->nodes;
	int* leafIndices = tree->leafIndices;

	if ( leafCount == 1 )
	{
		nodes[leafIndices[0]].parent = B3_NULL_INDEX;
		return leafIndices[0];
	}

#if B3_TREE_HEURISTIC == 0
	b3Vec3* leafCenters = tree->leafCenters;
#else
	b3AABB* leafBoxes = tree->leafBoxes;
	int* binIndices = tree->binIndices;
#endif

	// todo large stack item
	struct b3RebuildItem stack[B3_TREE_STACK_SIZE];
	int top = 0;

	stack[0].nodeIndex = b3AllocateNode( tree );
	stack[0].childCount = -1;
	stack[0].startIndex = 0;
	stack[0].endIndex = leafCount;
#if B3_TREE_HEURISTIC == 0
	stack[0].splitIndex = b3PartitionMid( leafIndices, leafCenters, leafCount );
#else
	stack[0].splitIndex = b3PartitionSAH( leafIndices, binIndices, leafBoxes, leafCount );
#endif

	while ( true )
	{
		struct b3RebuildItem* item = stack + top;

		item->childCount += 1;

		if ( item->childCount == 2 )
		{
			// This internal node has both children established

			if ( top == 0 )
			{
				// all done
				break;
			}

			struct b3RebuildItem* parentItem = stack + ( top - 1 );
			b3TreeNode* parentNode = nodes + parentItem->nodeIndex;

			if ( parentItem->childCount == 0 )
			{
				B3_ASSERT( parentNode->children.child1 == B3_NULL_INDEX );
				parentNode->children.child1 = item->nodeIndex;
			}
			else
			{
				B3_ASSERT( parentItem->childCount == 1 );
				B3_ASSERT( parentNode->children.child2 == B3_NULL_INDEX );
				parentNode->children.child2 = item->nodeIndex;
			}

			b3TreeNode* node = nodes + item->nodeIndex;

			B3_ASSERT( node->parent == B3_NULL_INDEX );
			node->parent = parentItem->nodeIndex;

			B3_ASSERT( node->children.child1 != B3_NULL_INDEX );
			B3_ASSERT( node->children.child2 != B3_NULL_INDEX );
			b3TreeNode* child1 = nodes + node->children.child1;
			b3TreeNode* child2 = nodes + node->children.child2;

			node->aabb = b3AABB_Union( child1->aabb, child2->aabb );
			node->height = 1 + b3MaxUInt16( child1->height, child2->height );
			node->categoryBits = child1->categoryBits | child2->categoryBits;

			// Pop stack
			top -= 1;
		}
		else
		{
			int startIndex, endIndex;
			if ( item->childCount == 0 )
			{
				startIndex = item->startIndex;
				endIndex = item->splitIndex;
			}
			else
			{
				B3_ASSERT( item->childCount == 1 );
				startIndex = item->splitIndex;
				endIndex = item->endIndex;
			}

			int count = endIndex - startIndex;

			if ( count == 1 )
			{
				int childIndex = leafIndices[startIndex];
				b3TreeNode* node = nodes + item->nodeIndex;

				if ( item->childCount == 0 )
				{
					B3_ASSERT( node->children.child1 == B3_NULL_INDEX );
					node->children.child1 = childIndex;
				}
				else
				{
					B3_ASSERT( item->childCount == 1 );
					B3_ASSERT( node->children.child2 == B3_NULL_INDEX );
					node->children.child2 = childIndex;
				}

				b3TreeNode* childNode = nodes + childIndex;
				B3_ASSERT( childNode->parent == B3_NULL_INDEX );
				childNode->parent = item->nodeIndex;
			}
			else
			{
				B3_ASSERT( count > 0 );
				B3_ASSERT( top < B3_TREE_STACK_SIZE );

				top += 1;
				struct b3RebuildItem* newItem = stack + top;
				newItem->nodeIndex = b3AllocateNode( tree );
				newItem->childCount = -1;
				newItem->startIndex = startIndex;
				newItem->endIndex = endIndex;
#if B3_TREE_HEURISTIC == 0
				newItem->splitIndex = b3PartitionMid( leafIndices + startIndex, leafCenters + startIndex, count );
#else
				newItem->splitIndex =
					b3PartitionSAH( leafIndices + startIndex, binIndices + startIndex, leafBoxes + startIndex, count );
#endif
				newItem->splitIndex += startIndex;
			}
		}
	}

	b3TreeNode* rootNode = nodes + stack[0].nodeIndex;
	B3_ASSERT( rootNode->parent == B3_NULL_INDEX );
	B3_ASSERT( rootNode->children.child1 != B3_NULL_INDEX );
	B3_ASSERT( rootNode->children.child2 != B3_NULL_INDEX );

	b3TreeNode* child1 = nodes + rootNode->children.child1;
	b3TreeNode* child2 = nodes + rootNode->children.child2;

	rootNode->aabb = b3AABB_Union( child1->aabb, child2->aabb );
	rootNode->height = 1 + b3MaxUInt16( child1->height, child2->height );
	rootNode->categoryBits = child1->categoryBits | child2->categoryBits;

	return stack[0].nodeIndex;
}

// Not safe to access tree during this operation because it may grow
int b3DynamicTree_Rebuild( b3DynamicTree* tree, bool fullBuild )
{
	int proxyCount = tree->proxyCount;
	if ( proxyCount == 0 )
	{
		return 0;
	}

	// Ensure capacity for rebuild space
	if ( proxyCount > tree->rebuildCapacity )
	{
		int newCapacity = proxyCount + proxyCount / 2;

		b3Free( tree->leafIndices, tree->rebuildCapacity * sizeof( int ) );
		tree->leafIndices = (int*)b3Alloc( newCapacity * sizeof( int ) );

#if B3_TREE_HEURISTIC == 0
		b3Free( tree->leafCenters, tree->rebuildCapacity * sizeof( b3Vec3 ) );
		tree->leafCenters = (b3Vec3*)b3Alloc( newCapacity * sizeof( b3Vec3 ) );
#else
		b3Free( tree->leafBoxes, tree->rebuildCapacity * sizeof( b3AABB ) );
		tree->leafBoxes = (b3AABB*)b3Alloc( newCapacity * sizeof( b3AABB ) );
		b3Free( tree->binIndices, tree->rebuildCapacity * sizeof( int ) );
		tree->binIndices = (int*)b3Alloc( newCapacity * sizeof( int ) );
#endif
		tree->rebuildCapacity = newCapacity;
	}

	int leafCount = 0;
	int stack[B3_TREE_STACK_SIZE];
	int stackCount = 0;

	int nodeIndex = tree->root;
	b3TreeNode* nodes = tree->nodes;
	b3TreeNode* node = nodes + nodeIndex;

	// These are the nodes that get sorted to rebuild the tree.
	// I'm using indices because the node pool may grow during the build.
	int* leafIndices = tree->leafIndices;

#if B3_TREE_HEURISTIC == 0
	b3Vec3* leafCenters = tree->leafCenters;
#else
	b3AABB* leafBoxes = tree->leafBoxes;
#endif

	// Gather all proxy nodes that have grown and all internal nodes that haven't grown. Both are
	// considered leaves in the tree rebuild.
	// Free all internal nodes that have grown.
	// todo use a node growth metric instead of simply enlarged to reduce rebuild size and frequency
	// this should be weighed against B3_MAX_AABB_MARGIN
	while ( true )
	{
		if ( b3IsLeaf( node ) == true || ( ( node->flags & b3_enlargedNode ) == 0 && fullBuild == false ) )
		{
			leafIndices[leafCount] = nodeIndex;
#if B3_TREE_HEURISTIC == 0
			leafCenters[leafCount] = b3AABB_Center( node->aabb );
#else
			leafBoxes[leafCount] = node->aabb;
#endif
			leafCount += 1;

			// Detach
			node->parent = B3_NULL_INDEX;
		}
		else
		{
			int doomedNodeIndex = nodeIndex;

			// Handle children
			nodeIndex = node->children.child1;

			B3_ASSERT( stackCount < B3_TREE_STACK_SIZE );
			if ( stackCount < B3_TREE_STACK_SIZE )
			{
				stack[stackCount++] = node->children.child2;
			}

			node = nodes + nodeIndex;

			// Remove doomed node
			b3FreeNode( tree, doomedNodeIndex );

			continue;
		}

		if ( stackCount == 0 )
		{
			break;
		}

		nodeIndex = stack[--stackCount];
		node = nodes + nodeIndex;
	}

#if B3_ENABLE_VALIDATION == 1
	int capacity = tree->nodeCapacity;
	for ( int i = 0; i < capacity; ++i )
	{
		if ( nodes[i].flags & b3_allocatedNode )
		{
			B3_ASSERT( ( nodes[i].flags & b3_enlargedNode ) == 0 );
		}
	}
#endif

	B3_ASSERT( leafCount <= proxyCount );

	tree->root = b3BuildTree( tree, leafCount );

	b3DynamicTree_Validate( tree );

	return leafCount;
}

static FILE* b3OpenTreeFile( const char* fileName, const char* mode )
{
	FILE* file = NULL;

#if defined( _MSC_VER )
	errno_t e = fopen_s( &file, fileName, mode );
	if ( e != 0 )
	{
		return NULL;
	}
#else
	file = fopen( fileName, mode );
	if ( file == NULL )
	{
		return NULL;
	}
#endif

	return file;
}

void b3DynamicTree_Save( const b3DynamicTree* tree, const char* fileName )
{
	FILE* file = b3OpenTreeFile( fileName, "wb" );
	if ( file == NULL )
	{
		return;
	}

	// Copy to allow setting some fields to zero
	b3DynamicTree temp = *tree;

	// Zero pointers and temp data
	temp.nodes = NULL;
	temp.leafIndices = NULL;
	temp.leafBoxes = NULL;
	temp.leafCenters = NULL;
	temp.binIndices = NULL;
	temp.rebuildCapacity = 0;

	// Write tree struct
	fwrite( &temp, sizeof( b3DynamicTree ), 1, file );

	// Write the node array, this includes the free list
	if ( tree->nodeCapacity > 0 && tree->nodes != NULL )
	{
		fwrite( tree->nodes, sizeof( b3TreeNode ), tree->nodeCapacity, file );
	}

	fclose( file );
}

b3DynamicTree b3DynamicTree_Load( const char* fileName, float scale )
{
	b3DynamicTree tree = { 0 };

	FILE* file = b3OpenTreeFile( fileName, "rb" );
	if ( file == NULL )
	{
		return tree;
	}

	int readCount = (int)fread( &tree, sizeof( b3DynamicTree ), 1, file );
	if ( readCount != 1 )
	{
		fclose( file );
		return tree;
	}

	if ( tree.version != B3_DYNAMIC_TREE_VERSION )
	{
		fclose( file );
		memset( &tree, 0, sizeof( b3DynamicTree ) );
		return tree;
	}

	if ( tree.nodeCapacity > 0 )
	{
		tree.nodes = (b3TreeNode*)b3Alloc( tree.nodeCapacity * sizeof( b3TreeNode ) );
		readCount = (int)fread( tree.nodes, sizeof( b3TreeNode ), tree.nodeCapacity, file );
		if ( readCount != tree.nodeCapacity )
		{
			b3Free( tree.nodes, tree.nodeCapacity * sizeof( b3TreeNode ) );
			fclose( file );
			memset( &tree, 0, sizeof( b3DynamicTree ) );
			return tree;
		}

		for ( int i = 0; i < tree.nodeCapacity; ++i )
		{
			b3TreeNode* node = tree.nodes + i;
			node->aabb.lowerBound = b3MulSV( scale, node->aabb.lowerBound );
			node->aabb.upperBound = b3MulSV( scale, node->aabb.upperBound );
		}
	}
	else
	{
		tree.nodes = NULL;
	}

	// Zero temp data fields
	tree.leafIndices = NULL;
	tree.leafBoxes = NULL;
	tree.leafCenters = NULL;
	tree.binIndices = NULL;
	tree.rebuildCapacity = 0;

	fclose( file );

	return tree;
}