From 27dba8a90d52ec734ad2a496dda4197a8c5094f6 Mon Sep 17 00:00:00 2001
From: Quaternions <krakow20@gmail.com>
Date: Mon, 19 Aug 2024 13:33:18 -0700
Subject: [PATCH] bvh: handling for median clumps

---
 src/bvh.rs | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/bvh.rs b/src/bvh.rs
index b76c239..c95e2e4 100644
--- a/src/bvh.rs
+++ b/src/bvh.rs
@@ -137,19 +137,27 @@ fn generate_bvh_node<T>(boxen:Vec<(T,Aabb)>,force:bool)->BvhNode<T>{
 		let median_x=sort_x[h].1;
 		let median_y=sort_y[h].1;
 		let median_z=sort_z[h].1;
+		//locate a run of values equal to the median
 		//partition point gives the first index for which the predicate evaluates to false
-		let first_index_gt_median_x=sort_x.partition_point(|tup|!(median_x<tup.1));
-		let first_index_gt_median_y=sort_y.partition_point(|tup|!(median_y<tup.1));
-		let first_index_gt_median_z=sort_z.partition_point(|tup|!(median_z<tup.1));
+		let first_index_eq_median_x=sort_x.partition_point(|&(_,x)|x<median_x);
+		let first_index_eq_median_y=sort_y.partition_point(|&(_,y)|y<median_y);
+		let first_index_eq_median_z=sort_z.partition_point(|&(_,z)|z<median_z);
+		let first_index_gt_median_x=sort_x.partition_point(|&(_,x)|x<=median_x);
+		let first_index_gt_median_y=sort_y.partition_point(|&(_,y)|y<=median_y);
+		let first_index_gt_median_z=sort_z.partition_point(|&(_,z)|z<=median_z);
+		//pick which side median value copies go into such that both sides are as balanced as possible based on distance from n/2
+		let partition_point_x=if n.abs_diff(2*first_index_eq_median_x)<n.abs_diff(2*first_index_gt_median_x){first_index_eq_median_x}else{first_index_gt_median_x};
+		let partition_point_y=if n.abs_diff(2*first_index_eq_median_y)<n.abs_diff(2*first_index_gt_median_y){first_index_eq_median_y}else{first_index_gt_median_y};
+		let partition_point_z=if n.abs_diff(2*first_index_eq_median_z)<n.abs_diff(2*first_index_gt_median_z){first_index_eq_median_z}else{first_index_gt_median_z};
 		//this ids which octant the boxen is put in
 		let mut octant=vec![0;n];
-		for &(i,_) in &sort_x[first_index_gt_median_x..]{
+		for &(i,_) in &sort_x[partition_point_x..]{
 			octant[i]+=1<<0;
 		}
-		for &(i,_) in &sort_y[first_index_gt_median_y..]{
+		for &(i,_) in &sort_y[partition_point_y..]{
 			octant[i]+=1<<1;
 		}
-		for &(i,_) in &sort_z[first_index_gt_median_z..]{
+		for &(i,_) in &sort_z[partition_point_z..]{
 			octant[i]+=1<<2;
 		}
 		//generate lists for unique octant values