From 0f6c467566ab4765df340f6c9f4b11f084aca1f1 Mon Sep 17 00:00:00 2001
From: Matthias Schiffer <mschiffer@universe-factory.net>
Date: Wed, 17 Nov 2021 13:16:00 +0100
Subject: [PATCH] World: Chunk: replace division/modulo with shift/mask for
 height values

Preparation for negative y coordinates.
---
 src/World/Chunk.cpp | 14 +++++++-------
 src/World/Chunk.hpp | 15 ++++++++++-----
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/src/World/Chunk.cpp b/src/World/Chunk.cpp
index e99af79..3845c9b 100644
--- a/src/World/Chunk.cpp
+++ b/src/World/Chunk.cpp
@@ -53,7 +53,7 @@ uint8_t Chunk::getBiome(block_idx_t x, y_idx_t y, block_idx_t z) const {
 		throw std::invalid_argument("corrupt chunk data");
 
 	if (biomeInts)
-		return biomeInts->getValue((y/BGROUP)*BSIZE*BSIZE + (z/BGROUP)*BSIZE + (x/BGROUP));
+		return biomeInts->getValue((y>>BSHIFT)*BSIZE*BSIZE + (z>>BSHIFT)*BSIZE + (x>>BSHIFT));
 	else if (biomeIntsPre115)
 		return biomeIntsPre115->getValue(z*SIZE + x);
 	else if (biomeBytes)
@@ -67,14 +67,14 @@ Block Chunk::getBlock(block_idx_t x, Chunk::Height height, block_idx_t z) const
 
 	block.depth = height.depth;
 
-	section_idx_t Y = height.y / SIZE;
-	block_idx_t y = height.y % SIZE;
+	section_idx_t Y = height.y >> HSHIFT;
+	block_idx_t y = height.y & HMASK;
 
 	if (Y < sections.size() && sections[Y])
 		block.type = sections[Y]->getBlockStateAt(x, y, z);
 
-	section_idx_t Yt = (height.y + 1) / SIZE;
-	block_idx_t yt = (height.y + 1) % SIZE;
+	section_idx_t Yt = (height.y + 1) >> HSHIFT;
+	block_idx_t yt = (height.y + 1) & HMASK;
 
 	if (Yt < sections.size() && sections[Yt])
 		block.blockLight = sections[Yt]->getBlockLightAt(x, yt, z);
@@ -97,7 +97,7 @@ bool Chunk::getHeight(
 		return false;
 
 	if (height->y == 0)
-		height->y = SIZE*section->getY() + y;
+		height->y = (section->getY() << HSHIFT) + y;
 
 	if (!(flags & WITH_DEPTH))
 		return true;
@@ -105,7 +105,7 @@ bool Chunk::getHeight(
 	if (type->flags & BLOCK_WATER)
 		return false;
 
-	height->depth = SIZE*section->getY() + y;
+	height->depth = (section->getY() << HSHIFT) + y;
 
 	return true;
 }
diff --git a/src/World/Chunk.hpp b/src/World/Chunk.hpp
index 26d10d1..817b29f 100644
--- a/src/World/Chunk.hpp
+++ b/src/World/Chunk.hpp
@@ -29,13 +29,18 @@ public:
 	// Maximum Y value
 	static const y_idx_t MAXY = 256;
 
+	// Shift to get from height to section index
+	static const unsigned HSHIFT = 4;
+	// Mask to get from height to y index inside section
+	static const block_idx_t HMASK = 0xf;
+
 	// Since Minecraft 1.15, biome information is stored for
 	// 4x4x4 block groups
-	static const uint32_t BGROUP = 4;
+	static const unsigned BSHIFT = 2;
 	// Number of biome values in a chunk in x/z dimensions
-	static const uint32_t BSIZE = SIZE / BGROUP;
+	static const uint32_t BSIZE = SIZE >> BSHIFT;
 	// Number of biome values in a chunk in y dimension
-	static const uint32_t BMAXY = MAXY / BGROUP;
+	static const uint32_t BMAXY = MAXY >> BSHIFT;
 
 	// Flags
 	static const int WITH_DEPTH = (1 << 0);
@@ -63,12 +68,12 @@ private:
 	) const;
 
 	const Resource::BlockType * getBlockStateAt(block_idx_t x, y_idx_t y, block_idx_t z) const {
-		section_idx_t Y = y / SIZE;
+		section_idx_t Y = y >> HSHIFT;
 
 		if (Y >= sections.size() || !sections[Y])
 			return nullptr;
 
-		return sections[Y]->getBlockStateAt(x, y % SIZE, z);
+		return sections[Y]->getBlockStateAt(x, y & HMASK, z);
 	}