world/section: add method to get block ID at coordinate

src/world/section.rs

@@ -1,6 +1,7 @@
 use anyhow::{bail, Context, Result};
 
 use super::de;
+use crate::{resource, types::*};
 
 fn palette_bits(len: usize, min: u8, max: u8) -> Option<u8> {
 	let mut bits = min;
@@ -15,7 +16,9 @@ fn palette_bits(len: usize, min: u8, max: u8) -> Option<u8> {
 	Some(bits)
 }
 
-pub trait Section {}
+pub trait Section {
+	fn get_block_id(&self, coords: BlockCoords) -> Result<&str>;
+}
 
 #[derive(Debug)]
 pub struct PaletteSectionBiomes<'a> {
@@ -81,9 +84,48 @@ impl<'a> PaletteSection<'a> {
 			aligned_blocks,
 		})
 	}
+
+	fn get_palette_index(&self, coords: BlockCoords) -> usize {
+		let Some(block_states) = self.block_states else {
+			return 0;
+		};
+
+		let bits = self.bits as usize;
+		let mask = (1 << bits) - 1;
+
+		let offset = coords.offset();
+
+		let shifted = if self.aligned_blocks {
+			let blocks_per_word = 64 / bits;
+			let (word, shift) = offset.div_rem(blocks_per_word);
+			block_states[word] as u64 >> (shift * bits)
+		} else {
+			let bit_offset = offset * bits;
+			let (word, bit_shift) = bit_offset.div_rem(64);
+
+			if bit_shift + bits <= 64 {
+				block_states[word] as u64 >> bit_shift
+			} else {
+				let tmp = (block_states[word + 1] as u64 as u128) << 64
+					| block_states[word] as u64 as u128;
+				(tmp >> bit_shift) as u64
+			}
+		};
+
+		(shifted & mask) as usize
+	}
 }
 
-impl<'a> Section for PaletteSection<'a> {}
+impl<'a> Section for PaletteSection<'a> {
+	fn get_block_id(&self, coords: BlockCoords) -> Result<&str> {
+		let index = self.get_palette_index(coords);
+		let entry = self
+			.palette
+			.get(index)
+			.context("Palette index out of bounds")?;
+		Ok(&entry.name)
+	}
+}
 
 #[derive(Debug)]
 pub struct OldSection<'a> {
@@ -93,9 +135,31 @@ pub struct OldSection<'a> {
 
 impl<'a> OldSection<'a> {
 	pub fn new(blocks: &'a fastnbt::ByteArray, data: &'a fastnbt::ByteArray) -> Result<Self> {
-		// TODO: Check lengths
-		Ok(Self { blocks, data })
+		const N: usize = BLOCKS_PER_CHUNK as usize;
+		if blocks.len() != N * N * N {
+			bail!("Invalid section block data");
+		}
+		if data.len() != N * N * N / 2 {
+			bail!("Invalid section extra data");
+		}
+
+		Ok(OldSection { blocks, data })
 	}
 }
 
-impl<'a> Section for OldSection<'a> {}
+impl<'a> Section for OldSection<'a> {
+	fn get_block_id(&self, coords: BlockCoords) -> Result<&str> {
+		let offset = coords.offset();
+		let block = self.blocks[offset] as u8;
+
+		let (data_offset, data_nibble) = offset.div_rem(2);
+		let data_byte = self.data[data_offset] as u8;
+		let data = if data_nibble == 1 {
+			data_byte >> 4
+		} else {
+			data_byte & 0xf
+		};
+
+		Ok(resource::LEGACY_BLOCK_TYPES[block as usize][data as usize])
+	}
+}