io/region: use ChunkArray to parse region header

A layout of a ChunkArray::<u32> matches the binary format of the region
header.

Cargo.lock

@@ -20,6 +20,12 @@ version = "1.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
 
+[[package]]
+name = "bytemuck"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c041d3eab048880cb0b86b256447da3f18859a163c3b8d8893f4e6368abe6393"
+
 [[package]]
 name = "byteorder"
 version = "1.4.3"
@@ -202,6 +208,7 @@ name = "minedmap"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "bytemuck",
  "clap",
  "fastnbt",
  "flate2",

Cargo.toml

@@ -9,6 +9,7 @@ default-run = "minedmap"
 
 [dependencies]
 anyhow = "1.0.68"
+bytemuck = "1.13.0"
 clap = { version = "4.1.4", features = ["derive"] }
 fastnbt = "2.3.2"
 flate2 = "1.0.25"

src/io/region.rs

@@ -19,20 +19,18 @@ struct ChunkDesc {
 	len: u8,
 }
 
-fn parse_header(header: &[u8; BLOCKSIZE]) -> HashMap<u32, ChunkDesc> {
+fn parse_header(header: &ChunkArray<u32>) -> HashMap<u32, ChunkDesc> {
 	let mut map = HashMap::new();
 
-	for coords in ChunkArray::<()>::keys() {
-		let chunk = &header[(4
-			* (usize::from(CHUNKS_PER_REGION) * usize::from(coords.z.0)
-				+ usize::from(coords.x.0)))..];
+	for (coords, &chunk) in header.iter() {
+		let offset_len = u32::from_be(chunk);
 
-		let offset = u32::from(chunk[0]) << 16 | u32::from(chunk[1]) << 8 | u32::from(chunk[2]);
+		let offset = offset_len >> 8;
 		if offset == 0 {
 			continue;
 		}
 
-		let len = chunk[3];
+		let len = offset_len as u8;
 
 		map.insert(offset, ChunkDesc { coords, len });
 	}
@@ -81,9 +79,9 @@ impl<R: Read + Seek> Region<R> {
 		let Region { mut reader } = self;
 
 		let mut chunk_map = {
-			let mut header = [0u8; BLOCKSIZE];
+			let mut header = ChunkArray::<u32>::default();
 			reader
-				.read_exact(&mut header)
+				.read_exact(bytemuck::cast_mut::<_, [u8; BLOCKSIZE]>(&mut header.0))
 				.context("Failed to read region header")?;
 
 			parse_header(&header)