Add the folded subject to LayoutMeta and derive reducer depth from it

node-graph/libraries/core-types/src/record.rs

@@ -284,6 +284,10 @@ pub struct LayoutMeta {
 	/// The depth change the node applies: `0` for elementwise and flip nodes,
 	/// `+1` for a creator, `-1` for a reducer.
 	pub level_delta: i8,
+	/// The materialized subject a reducer folds, as `(input, levels)`: the
+	/// output keeps the subject's levels above the folded ones, so the depth
+	/// derives from its layout even though it contributes no fields.
+	pub folded: Option<(u8, u8)>,
 }
 
 /// The attributes a node reads from one input, recorded on [`LayoutMeta`] for
@@ -314,6 +318,7 @@ impl LayoutMeta {
 			writes: Vec::new(),
 			removes: Vec::new(),
 			level_delta: 0,
+			folded: None,
 		}
 	}
 
@@ -328,7 +333,13 @@ impl LayoutMeta {
 			sources => Layout::union(sources),
 		}
 		.without(&self.removes);
-		let depth = (base.depth as i8 + self.level_delta).max(0) as u8;
+		let depth = match self.folded {
+			Some((input, levels)) => {
+				let subject = inputs[input as usize].expect("layout fold folded input has no layout");
+				(subject.depth.saturating_sub(levels) as i8 + self.level_delta).max(0) as u8
+			}
+			None => (base.depth as i8 + self.level_delta).max(0) as u8,
+		};
 		let element = match &self.element {
 			ElementSpec::Concrete(element) => *element,
 			ElementSpec::Carried => base.element,

node-graph/node-macro/src/codegen/ir.rs

@@ -236,6 +236,10 @@ pub(crate) fn layout_meta_tokens(node: &Node, element_spec: TokenStream2, core_t
 		quote!((<#marker as #core_types::attribute::Attribute>::NAME, #level))
 	});
 	let level_delta = level_delta(node);
+	let folded = match folded_subject(node) {
+		Some((index, levels)) => quote!(::core::option::Option::Some((#index, #levels))),
+		None => quote!(::core::option::Option::None),
+	};
 	quote! {
 		#core_types::record::LayoutMeta {
 			sources: ::std::vec![#(#sources),*],
@@ -244,10 +248,20 @@ pub(crate) fn layout_meta_tokens(node: &Node, element_spec: TokenStream2, core_t
 			writes: ::std::vec![#(#writes),*],
 			removes: ::std::vec![#(#removes),*],
 			level_delta: #level_delta,
+			folded: #folded,
 		}
 	}
 }
 
+/// The materialized subject a node folds, as `(input, levels)`.
+pub(crate) fn folded_subject(node: &Node) -> Option<(u8, u8)> {
+	node.inputs
+		.iter()
+		.enumerate()
+		.find(|(index, input)| input.subject && materialized_levels(node, *index) > 0)
+		.map(|(index, _)| (index as u8, materialized_levels(node, index)))
+}
+
 fn field_writes(attrs: &[LevelAttr], core_types: &TokenStream2) -> Vec<TokenStream2> {
 	attrs
 		.iter()

node-graph/nodes/gcore/src/record.rs

@@ -636,6 +636,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 1,
+			folded: None,
 		};
 		let node = install(
 			RepeatNode::new(RecordSource::new(bare_source(&base, 7.), &base, &base), count_edge, reverse_edge, &base, &count_layout, &reverse_layout),
@@ -671,6 +672,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 1,
+			folded: None,
 		};
 		let repeat = install(
 			RepeatNode::new(RecordSource::new(IndexSourceNode { layout: base.clone() }, &base, &base), count_edge, reverse_edge, &base, &count_layout, &reverse_layout),
@@ -712,6 +714,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 1,
+			folded: None,
 		};
 		let repeat = install(
 			RepeatNode::new(RecordSource::new(IndexSourceNode { layout: base.clone() }, &base, &base), count_edge, reverse_edge, &base, &count_layout, &reverse_layout),
@@ -755,6 +758,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 1,
+			folded: None,
 		};
 		let repeat = install(
 			RepeatNode::new(RecordSource::new(content, &leveled_content, &leveled_content), count_edge, reverse_edge, &leveled_content, &count_layout, &reverse_layout),
@@ -854,6 +858,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 0,
+			folded: None,
 		};
 		let node = install(
 			ExtendNode::new(RecordSource::new(base, &base_layout, &union), RecordSource::new(new, &new_layout, &union), &union),
@@ -903,6 +908,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 0,
+			folded: None,
 		};
 		let build = |index: f64| {
 			let content = LeveledSourceNode {
@@ -958,6 +964,7 @@ mod tests {
 				writes: vec![],
 				removes: vec![],
 				level_delta: 0,
+				folded: None,
 			};
 			install(
 				IndexElementsNode::new(RecordSource::new(content, &layout, &layout), index_edge, &layout, &index_layout),
@@ -1174,6 +1181,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 1,
+			folded: None,
 		};
 		let nested = install(
 			RepeatNode::new(RecordSource::new(content, &leveled_content, &leveled_content), count_edge, reverse_edge, &leveled_content, &count_layout, &reverse_layout),
@@ -1234,6 +1242,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta: 1,
+			folded: None,
 		};
 		let repeat = install(
 			RepeatNode::new(RecordSource::new(IndexSourceNode { layout: base.clone() }, &base, &base), count_edge, reverse_edge, &base, &count_layout, &reverse_layout),

node-graph/nodes/graphic/src/record.rs

@@ -409,6 +409,7 @@ mod tests {
 			writes: vec![],
 			removes: vec![],
 			level_delta,
+			folded: None,
 		}
 	}