From 6b6f77f8af4ad9a1161985a6c9b09ac7eec9e10f Mon Sep 17 00:00:00 2001
From: "Dylan K. Taylor" <odigiman@gmail.com>
Date: Sat, 5 Sep 2020 00:16:07 +0100
Subject: [PATCH] Chunk: improved heightmap calculation performance
 recalculateHeightMapColumn is stateless, so it can't make any assumptions
 about which subchunks to check for blocks. However, in most the average case
 (6 allocated subchunks), this causes 2500+ useless
 SubChunk->getHighestBlockAt() calls (10 per column). Since we're calculating
 in bulk, we can figure out which subchunks are empty one time and ignore them
 for all 256 columns. In the average case, this produced a 50-60% performance
 improvement for heightmap calculation (~1.1 ms -> 0.5 ms). In extreme cases
 where the height is extremely varied, this produces no observable performance
 benefit, but for most cases with flattish terrain, it's an improvement. It
 can likely be further improved, but further performance improvements are
 outside the scope of this commit and will likely result in more complexity
 increases.

---
 src/world/format/Chunk.php | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/world/format/Chunk.php b/src/world/format/Chunk.php
index d6ea71e83..cf11ad540 100644
--- a/src/world/format/Chunk.php
+++ b/src/world/format/Chunk.php
@@ -266,9 +266,38 @@ class Chunk{
 	 * @phpstan-param \SplFixedArray<bool> $lightDiffusers
 	 */
 	public function recalculateHeightMap(\SplFixedArray $lightFilters, \SplFixedArray $lightDiffusers) : void{
+		$maxSubChunkY = $this->subChunks->count() - 1;
+		for(; $maxSubChunkY >= 0; $maxSubChunkY--){
+			if(!$this->getSubChunk($maxSubChunkY)->isEmptyFast()){
+				break;
+			}
+		}
+		if($maxSubChunkY === -1){ //whole column is definitely empty
+			$this->setHeightMapArray(array_fill(0, 256, 0));
+			return;
+		}
+
 		for($z = 0; $z < 16; ++$z){
 			for($x = 0; $x < 16; ++$x){
-				$this->recalculateHeightMapColumn($x, $z, $lightFilters, $lightDiffusers);
+				$y = null;
+				for($subChunkY = $maxSubChunkY; $subChunkY >= 0; $subChunkY--){
+					$subHighestBlockY = $this->getSubChunk($subChunkY)->getHighestBlockAt($x, $z);
+					if($subHighestBlockY !== -1){
+						$y = ($subChunkY * 16) + $subHighestBlockY;
+						break;
+					}
+				}
+
+				if($y === null){ //no blocks in the column
+					$this->setHeightMap($x, $z, 0);
+				}else{
+					for(; $y >= 0; --$y){
+						if($lightFilters[$state = $this->getFullBlock($x, $y, $z)] > 1 or $lightDiffusers[$state]){
+							$this->setHeightMap($x, $z, $y + 1);
+							break;
+						}
+					}
+				}
 			}
 		}
 	}