world: add types to wrap common parts of section data

src/world/section.rs

@@ -0,0 +1,95 @@
+use anyhow::{bail, Context, Result};
+
+use super::de;
+
+fn palette_bits(len: usize, min: u8, max: u8) -> Option<u8> {
+	let mut bits = min;
+	while (1 << bits) < len {
+		bits += 1;
+
+		if bits > max {
+			return None;
+		}
+	}
+
+	Some(bits)
+}
+
+#[derive(Debug)]
+pub struct PaletteSectionBiomes<'a> {
+	biomes: Option<&'a fastnbt::LongArray>,
+	palette: &'a Vec<String>,
+	bits: u8,
+}
+
+impl<'a> PaletteSectionBiomes<'a> {
+	pub fn new(biomes: Option<&'a fastnbt::LongArray>, palette: &'a Vec<String>) -> Result<Self> {
+		let bits = palette_bits(palette.len(), 1, 6).context("Unsupported block palette size")?;
+
+		if let Some(biomes) = biomes {
+			let biomes_per_word = 64 / bits as usize;
+			let expected_length = (64 + biomes_per_word - 1) / biomes_per_word;
+			if biomes.len() != expected_length {
+				bail!("Invalid section biome data");
+			}
+		}
+
+		Ok(PaletteSectionBiomes {
+			biomes,
+			palette,
+			bits,
+		})
+	}
+}
+
+#[derive(Debug)]
+pub struct PaletteSection<'a> {
+	block_states: Option<&'a fastnbt::LongArray>,
+	palette: &'a Vec<de::BlockStatePaletteEntry>,
+	bits: u8,
+	aligned_blocks: bool,
+}
+
+impl<'a> PaletteSection<'a> {
+	pub fn new(
+		data_version: u32,
+		block_states: Option<&'a fastnbt::LongArray>,
+		palette: &'a Vec<de::BlockStatePaletteEntry>,
+	) -> Result<Self> {
+		let aligned_blocks = data_version >= 2529;
+
+		let bits = palette_bits(palette.len(), 4, 12).context("Unsupported block palette size")?;
+
+		if let Some(block_states) = block_states {
+			let expected_length = if aligned_blocks {
+				let blocks_per_word = 64 / bits as usize;
+				(4096 + blocks_per_word - 1) / blocks_per_word
+			} else {
+				64 * bits as usize
+			};
+			if block_states.len() != expected_length {
+				bail!("Invalid section block data");
+			}
+		}
+
+		Ok(Self {
+			block_states,
+			palette,
+			bits,
+			aligned_blocks,
+		})
+	}
+}
+
+#[derive(Debug)]
+pub struct OldSection<'a> {
+	blocks: &'a fastnbt::ByteArray,
+	data: &'a fastnbt::ByteArray,
+}
+
+impl<'a> OldSection<'a> {
+	pub fn new(blocks: &'a fastnbt::ByteArray, data: &'a fastnbt::ByteArray) -> Result<Self> {
+		// TODO: Check lengths
+		Ok(Self { blocks, data })
+	}
+}