world: implement top_layer function

Implement one of the core functions of MinedMap: finding the topmost
visible block at each coordinate.

src/world/layer.rs

@@ -0,0 +1,130 @@
+use anyhow::{Context, Result};
+use itertools::iproduct;
+
+use super::chunk::Chunk;
+use crate::{
+	resource::{BlockFlag, BlockType, BlockTypeMap},
+	types::*,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct BlockHeight(i32);
+
+impl BlockHeight {
+	/// Constructs a new [BlockHeight] from section and block Y indices
+	///
+	/// Returns an error if the resulting coordindate does not fit into
+	/// an [i32].
+	pub fn new(section: SectionY, block: BlockY) -> Result<Self> {
+		let height = section
+			.0
+			.checked_mul(BLOCKS_PER_CHUNK as i32)
+			.and_then(|y| y.checked_add_unsigned(block.0.into()))
+			.context("Block height out of bounds")?;
+		Ok(BlockHeight(height))
+	}
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct BlockInfo {
+	block_type: BlockType,
+	y: BlockHeight,
+	depth: Option<BlockHeight>,
+}
+
+/// Helper methods for [BlockInfo]
+trait OptionBlockInfoExt {
+	/// Checks if a [BlockInfo] has been filled in completely
+	///
+	/// Helper used by [top_layer]
+	fn done(&self) -> bool;
+
+	/// Fills in a [BlockInfo] based on a [BlockType]
+	///
+	/// Only fills in data if the block is part of the visible top layer
+	/// of the rendered map.
+	///
+	/// Must be called on an incomplete [BlockInfo] entry. Returns `true`
+	/// if the entry has been filled in completely.
+	fn fill(&mut self, y: BlockHeight, block_type: BlockType) -> bool;
+}
+
+impl OptionBlockInfoExt for Option<BlockInfo> {
+	fn done(&self) -> bool {
+		let Some(info) = self else {
+			return false;
+		};
+
+		info.depth.is_some()
+	}
+
+	fn fill(&mut self, y: BlockHeight, block_type: BlockType) -> bool {
+		if !block_type.is(BlockFlag::Opaque) {
+			return false;
+		}
+
+		if self.is_none() {
+			*self = Some(BlockInfo {
+				block_type,
+				y,
+				depth: None,
+			});
+		}
+
+		if block_type.is(BlockFlag::Water) {
+			return false;
+		}
+
+		let info = self.as_mut().unwrap();
+		info.depth = Some(y);
+
+		true
+	}
+}
+
+pub type BlockInfoArray = LayerBlockArray<Option<BlockInfo>>;
+
+/// Fills in a [BlockInfoArray] with the information of the chunk's top
+/// block layer
+///
+/// For each (X, Z) coordinate pair, the topmost opaque block is
+/// determined as the block that should be visible on the rendered
+/// map. For water blocks, the height of the first non-water block
+/// is additionally filled in as the water depth.
+pub fn top_layer(chunk: &Chunk, block_types: &BlockTypeMap) -> Result<BlockInfoArray> {
+	use BLOCKS_PER_CHUNK as N;
+
+	let mut done = 0;
+	let mut ret = BlockInfoArray::default();
+
+	for ((section_y, section), y, xz) in iproduct!(
+		chunk.sections().rev(),
+		BlockY::iter().rev(),
+		BlockInfoArray::keys()
+	) {
+		let entry = &mut ret[xz];
+		if entry.done() {
+			continue;
+		}
+
+		let coords = SectionBlockCoords { xz, y };
+		let block_id = section.block_id_at(coords)?;
+		let Some(&block_type) = block_types.get(block_id) else {
+			eprintln!("Unknown block type: {}", block_id);
+			continue;
+		};
+		let height = BlockHeight::new(section_y, y)?;
+		if !entry.fill(height, block_type) {
+			continue;
+		}
+
+		assert!(entry.done());
+
+		done += 1;
+		if done == N * N {
+			break;
+		}
+	}
+
+	Ok(ret)
+}

src/world/mod.rs

@@ -1,3 +1,4 @@
 pub mod chunk;
 pub mod de;
+pub mod layer;
 pub mod section;