// This code written in 2010 by Forest Hale (lordhavoc ghdigital com), and placed into public domain.

#include <stdlib.h>
#include <memory.h>
#include "bih.h"

static int BIH_BuildNode(bih_t *bih, int numchildren, int *leaflist)
{
	int i;
	int j;
	int longestaxis;
	int axis;
	int nodenum;
	int front;
	int back;
	bih_node_t *node;
	bih_leaf_t *child;
	float splitdist;
	float d;
	float mins[3];
	float maxs[3];
	float size[3];
	if (numchildren < 2)
		return -1-leaflist[0];
	// if we run out of nodes it's the caller's fault, but don't crash
	if (bih->numnodes == bih->maxnodes)
	{
		if (!bih->error)
			bih->error = BIHERROR_OUT_OF_NODES;
		return -1-leaflist[0];
	}
	nodenum = bih->numnodes++;
	node = bih->nodes + nodenum;
	// calculate bounds of children
	child = bih->leafs + leaflist[0];
	mins[0] = child->mins[0];
	mins[1] = child->mins[1];
	mins[2] = child->mins[2];
	maxs[0] = child->maxs[0];
	maxs[1] = child->maxs[1];
	maxs[2] = child->maxs[2];
	for (i = 1;i < numchildren;i++)
	{
		child = bih->leafs + leaflist[i];
		if (mins[0] > child->mins[0]) mins[0] = child->mins[0];
		if (mins[1] > child->mins[1]) mins[1] = child->mins[1];
		if (mins[2] > child->mins[2]) mins[2] = child->mins[2];
		if (maxs[0] < child->maxs[0]) maxs[0] = child->maxs[0];
		if (maxs[1] < child->maxs[1]) maxs[1] = child->maxs[1];
		if (maxs[2] < child->maxs[2]) maxs[2] = child->maxs[2];
	}
	size[0] = maxs[0] - mins[0];
	size[1] = maxs[1] - mins[1];
	size[2] = maxs[2] - mins[2];
	// store bounds for node
	node->mins[0] = mins[0];
	node->mins[1] = mins[1];
	node->mins[2] = mins[2];
	node->maxs[0] = maxs[0];
	node->maxs[1] = maxs[1];
	node->maxs[2] = maxs[2];
	// pick longest axis
	longestaxis = 0;
	if (size[0] < size[1]) longestaxis = 1;
	if (size[longestaxis] < size[2]) longestaxis = 2;
	// iterate possible split axis choices:
	// longest (best raytracing performance)
	// x (longest had identical children, try X axis)
	// y (longest and X axis had identical children, try Y axis)
	// z (longest and X and Y axes had identical children, try Z axis)
	// arbitrary (all children have same bounds, divide the list)
	for (j = 0;j < 3;j++)
	{
		// if longest axis fails, we try each one until something works
		// (this also can fail, see below)
		switch(j)
		{
		default:
		case 0: axis = longestaxis;break;
		case 1: axis = (longestaxis + 1) % 3;break;
		case 2: axis = (longestaxis + 2) % 3;break;
		}
		// sort children into front and back lists
		node->type = BIH_SPLITX + axis;
		splitdist = (node->mins[axis] + node->maxs[axis]) * 0.5f;
		front = 0;
		back = 0;
		for (i = 0;i < numchildren;i++)
		{
			child = bih->leafs + leaflist[i];
			d = (child->mins[axis] + child->maxs[axis]) * 0.5f;
			if (d < splitdist)
				bih->leafsortscratch[back++] = leaflist[i];
			else
				leaflist[front++] = leaflist[i];
		}
		// now copy the back ones into the space made in the leaflist for them
		if (back)
			memcpy(leaflist + front, bih->leafsortscratch, back*sizeof(leaflist[0]));
		// if both sides have some children, it's good enough for us.
		if (front && back)
			break;
	}
	if (j == 3)
	{
		// the almost impossible case happened; all children have identical
		// bounds, so just divide them arbitrarily into two lists.
		node->type = BIH_SPLITX;
		back = numchildren >> 1;
		front = numchildren - back;
	}

	// we now have front and back children divided in leaflist...
	node->children[0] = BIH_BuildNode(bih, front, leaflist);
	node->children[1] = BIH_BuildNode(bih, back, leaflist + front);
	return nodenum;
}

int BIH_Build(bih_t *bih, int numleafs, bih_leaf_t *leafs, int maxnodes, bih_node_t *nodes, int *temp_leafsort, int *temp_leafsortscratch)
{
	int i;

	memset(bih, 0, sizeof(*bih));
	bih->numleafs = numleafs;
	bih->leafs = leafs;
	bih->leafsort = temp_leafsort;
	bih->leafsortscratch = temp_leafsortscratch;
	bih->numnodes = 0;
	bih->maxnodes = maxnodes;
	bih->nodes = nodes;

	// clear things we intend to rebuild
	memset(bih->nodes, 0, sizeof(bih->nodes[0]) * bih->maxnodes);
	for (i = 0;i < bih->numleafs;i++)
		bih->leafsort[i] = i;

	BIH_BuildNode(bih, bih->numleafs, bih->leafsort);
	return bih->error;
}