lazily-evaluated extractors

crates/pile-dataset/src/extract/mod.rs

@@ -0,0 +1,68 @@
+mod flac;
+use std::{collections::HashMap, rc::Rc};
+
+pub use flac::*;
+
+mod fs;
+pub use fs::*;
+
+mod map;
+pub use map::*;
+use pile_config::Label;
+
+/// An attachment that extracts metadata from an [Item].
+///
+/// Metadata is exposed as an immutable map of {label: value},
+/// much like a json object.
+pub trait Extractor<I: crate::Item> {
+	/// Get the field at `name` from `item`.
+	/// - returns `None` if `name` is not a valid field
+	/// - returns `Some(Null)` if `name` is not available
+	fn field<'a>(
+		&'a self,
+		name: &pile_config::Label,
+	) -> Result<Option<&'a crate::PileValue<'a, I>>, std::io::Error>;
+
+	/// Return all fields in this extractor.
+	/// `Self::field` must return [Some] for all these keys
+	/// and [None] for all others.
+	fn fields(&self) -> Result<Vec<Label>, std::io::Error>;
+}
+
+pub struct MetaExtractor<'a, I: crate::Item> {
+	inner: MapExtractor<'a, I>,
+}
+
+impl<'a> MetaExtractor<'a, crate::FileItem> {
+	#[expect(clippy::unwrap_used)]
+	pub fn new(item: &'a crate::FileItem) -> Self {
+		let inner = MapExtractor {
+			inner: HashMap::from([
+				(
+					Label::new("flac").unwrap(),
+					crate::PileValue::Extractor(Rc::new(FlacExtractor::new(item))),
+				),
+				(
+					Label::new("fs").unwrap(),
+					crate::PileValue::Extractor(Rc::new(FsExtractor::new(item))),
+				),
+			]),
+		};
+
+		Self { inner }
+	}
+}
+
+impl Extractor<crate::FileItem> for MetaExtractor<'_, crate::FileItem> {
+	fn field<'a>(
+		&'a self,
+		name: &pile_config::Label,
+	) -> Result<Option<&'a crate::PileValue<'a, crate::FileItem>>, std::io::Error> {
+		self.inner.field(name)
+	}
+
+	#[expect(clippy::unwrap_used)]
+	fn fields(&self) -> Result<Vec<Label>, std::io::Error> {
+		return Ok(vec![Label::new("flac").unwrap(), Label::new("fs").unwrap()]);
+	}
+}