void b2DynamicTree::RebuildBottomUp()

in Box2D/Collision/b2DynamicTree.cpp [697:771]
63 lines of code
8 McCabe index (conditional complexity)

void b2DynamicTree::RebuildBottomUp()
{
	int32* nodes = (int32*)b2Alloc(m_nodeCount * sizeof(int32));
	int32 count = 0;

	// Build array of leaves. Free the rest.
	for (int32 i = 0; i < m_nodeCapacity; ++i)
	{
		if (m_nodes[i].height < 0)
		{
			// free node in pool
			continue;
		}

		if (m_nodes[i].IsLeaf())
		{
			m_nodes[i].parent = b2_nullNode;
			nodes[count] = i;
			++count;
		}
		else
		{
			FreeNode(i);
		}
	}

	while (count > 1)
	{
		float32 minCost = b2_maxFloat;
		int32 iMin = -1, jMin = -1;
		for (int32 i = 0; i < count; ++i)
		{
			b2AABB aabbi = m_nodes[nodes[i]].aabb;

			for (int32 j = i + 1; j < count; ++j)
			{
				b2AABB aabbj = m_nodes[nodes[j]].aabb;
				b2AABB b;
				b.Combine(aabbi, aabbj);
				float32 cost = b.GetPerimeter();
				if (cost < minCost)
				{
					iMin = i;
					jMin = j;
					minCost = cost;
				}
			}
		}

		int32 index1 = nodes[iMin];
		int32 index2 = nodes[jMin];
		b2TreeNode* child1 = m_nodes + index1;
		b2TreeNode* child2 = m_nodes + index2;

		int32 parentIndex = AllocateNode();
		b2TreeNode* parent = m_nodes + parentIndex;
		parent->child1 = index1;
		parent->child2 = index2;
		parent->height = 1 + b2Max(child1->height, child2->height);
		parent->aabb.Combine(child1->aabb, child2->aabb);
		parent->parent = b2_nullNode;

		child1->parent = parentIndex;
		child2->parent = parentIndex;

		nodes[jMin] = nodes[count-1];
		nodes[iMin] = parentIndex;
		--count;
	}

	m_root = nodes[0];
	b2Free(nodes);

	Validate();
}