Move coord offset to new util module, make more efficient

div_floor_mod() generates inefficient code. For power-of-2 divisors,
shift and mask can be used instead.

src/bin/minedmap/tile_renderer.rs

@@ -8,39 +8,16 @@ use std::{
 use anyhow::{Context, Result};
 use glam::Vec3;
 use lru::LruCache;
-use num_integer::div_mod_floor;
 
 use minedmap::{
 	io::{fs, storage},
 	resource::{block_color, needs_biome},
 	types::*,
+	util::coord_offset,
 };
 
 use super::{common::*, region_group::RegionGroup};
 
-/// Offsets a chunk and block coordinate pair by a number of blocks
-///
-/// As the new coordinate may end up in a different region, a region offset
-/// is returned together with the new chunk and block coordinates.
-fn coord_offset<const AXIS: u8>(
-	chunk: ChunkCoord<AXIS>,
-	block: BlockCoord<AXIS>,
-	offset: i32,
-) -> (i8, ChunkCoord<AXIS>, BlockCoord<AXIS>) {
-	const CHUNKS: i32 = CHUNKS_PER_REGION as i32;
-	const BLOCKS: i32 = BLOCKS_PER_CHUNK as i32;
-	let coord = chunk.0 as i32 * BLOCKS + block.0 as i32 + offset;
-	let (region_chunk, block) = div_mod_floor(coord, BLOCKS);
-	let (region, chunk) = div_mod_floor(region_chunk, CHUNKS);
-	(
-		region
-			.try_into()
-			.expect("the region coordinate should be in the valid range"),
-		ChunkCoord::new(chunk),
-		BlockCoord::new(block),
-	)
-}
-
 fn biome_at(
 	region_group: &RegionGroup<Rc<ProcessedRegion>>,
 	chunk: ChunkCoords,
@@ -283,37 +260,3 @@ impl<'a> TileRenderer<'a> {
 		Ok(())
 	}
 }
-
-#[cfg(test)]
-mod test {
-	use super::*;
-
-	#[test]
-	fn test_coord_offset() {
-		const CHUNKS: i32 = CHUNKS_PER_REGION as i32;
-		const BLOCKS: i32 = BLOCKS_PER_CHUNK as i32;
-
-		for chunk in ChunkX::iter() {
-			for block in BlockX::iter() {
-				assert_eq!(coord_offset(chunk, block, 0), (0, chunk, block));
-				assert_eq!(
-					coord_offset(chunk, block, -(CHUNKS * BLOCKS)),
-					(-1, chunk, block)
-				);
-				assert_eq!(
-					coord_offset(chunk, block, CHUNKS * BLOCKS),
-					(1, chunk, block)
-				);
-
-				for offset in -(CHUNKS * BLOCKS)..(CHUNKS * BLOCKS) {
-					let (region2, chunk2, block2) = coord_offset(chunk, block, offset);
-					assert!((-1..=1).contains(&region2));
-					let coord = chunk.0 as i32 * BLOCKS + block.0 as i32 + offset;
-					let coord2 =
-						((region2 as i32 * CHUNKS) + chunk2.0 as i32) * BLOCKS + block2.0 as i32;
-					assert_eq!(coord2, coord);
-				}
-			}
-		}
-	}
-}

src/lib.rs

@@ -1,4 +1,5 @@
 pub mod io;
 pub mod resource;
 pub mod types;
+pub mod util;
 pub mod world;

src/types.rs

@@ -47,7 +47,8 @@ macro_rules! coord_type {
 	};
 }
 
-pub const BLOCKS_PER_CHUNK: usize = 16;
+pub const BLOCK_BITS: u8 = 4;
+pub const BLOCKS_PER_CHUNK: usize = 1 << BLOCK_BITS;
 coord_type!(BlockCoord, BLOCKS_PER_CHUNK);
 
 /// A block X coordinate relative to a chunk
@@ -136,7 +137,8 @@ impl Debug for SectionBlockCoords {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct SectionY(pub i32);
 
-pub const CHUNKS_PER_REGION: usize = 32;
+pub const CHUNK_BITS: u8 = 5;
+pub const CHUNKS_PER_REGION: usize = 1 << CHUNK_BITS;
 coord_type!(ChunkCoord, CHUNKS_PER_REGION);
 
 /// A chunk X coordinate relative to a region

src/util.rs

@@ -0,0 +1,84 @@
+use crate::types::*;
+
+pub trait ShiftMask: Sized {
+	type MaskedOutput;
+
+	/// Apply a right shift to a value, and return both the result and the
+	/// bytes that were shifted out
+	fn shift_mask(self, shift: u8) -> (Self, Self::MaskedOutput);
+}
+
+impl ShiftMask for u32 {
+	type MaskedOutput = u32;
+
+	fn shift_mask(self, shift: u8) -> (u32, u32) {
+		let mask = (1 << shift) - 1;
+		(self >> shift, self & mask)
+	}
+}
+
+impl ShiftMask for i32 {
+	type MaskedOutput = u32;
+
+	#[inline]
+	fn shift_mask(self, shift: u8) -> (i32, u32) {
+		let mask = (1 << shift) - 1;
+		(self >> shift, (self as u32) & mask)
+	}
+}
+
+/// Offsets a chunk and block coordinate pair by a number of blocks
+///
+/// As the new coordinate may end up in a different region, a region offset
+/// is returned together with the new chunk and block coordinates.
+#[inline]
+pub fn coord_offset<const AXIS: u8>(
+	chunk: ChunkCoord<AXIS>,
+	block: BlockCoord<AXIS>,
+	offset: i32,
+) -> (i8, ChunkCoord<AXIS>, BlockCoord<AXIS>) {
+	let coord = ((chunk.0 as i32) << BLOCK_BITS | block.0 as i32) + offset;
+	let (region_chunk, block) = coord.shift_mask(BLOCK_BITS);
+	let (region, chunk) = region_chunk.shift_mask(CHUNK_BITS);
+	(
+		region
+			.try_into()
+			.expect("the region coordinate should be in the valid range"),
+		ChunkCoord::new(chunk),
+		BlockCoord::new(block),
+	)
+}
+
+#[cfg(test)]
+mod test {
+	use super::*;
+
+	#[test]
+	fn test_coord_offset() {
+		const CHUNKS: i32 = CHUNKS_PER_REGION as i32;
+		const BLOCKS: i32 = BLOCKS_PER_CHUNK as i32;
+
+		for chunk in ChunkX::iter() {
+			for block in BlockX::iter() {
+				assert_eq!(coord_offset(chunk, block, 0), (0, chunk, block));
+				assert_eq!(
+					coord_offset(chunk, block, -(CHUNKS * BLOCKS)),
+					(-1, chunk, block)
+				);
+				assert_eq!(
+					coord_offset(chunk, block, CHUNKS * BLOCKS),
+					(1, chunk, block)
+				);
+
+				for offset in -(CHUNKS * BLOCKS)..(CHUNKS * BLOCKS) {
+					let (region2, chunk2, block2) = coord_offset(chunk, block, offset);
+					assert!((-1..=1).contains(&region2));
+					let coord = chunk.0 as i32 * BLOCKS + block.0 as i32 + offset;
+					let coord2 =
+						((region2 as i32 * CHUNKS) + chunk2.0 as i32) * BLOCKS + block2.0 as i32;
+					assert_eq!(coord2, coord);
+				}
+			}
+		}
+	}
+}