ReverseTopological -> AdjacencyMap (vercel/turborepo#5430)

### Description

This PR renames the `ReverseTopological` graph store implementation to
`AdjacencyMap`. The reasoning here is that we can implement any graph
traversal algorithm on top of `AdjacencyMap`, not just reverse
topological.

This PR also adds a `reverse_topological_from_node` iterator method to
only traverse the subgraph from a given node.

Next.js PR:
https://github.com/vercel/next.js/compare/alexkirsz/web-866-adjacency-map?expand=1

### Testing Instructions

N/A

crates/turbo-tasks/src/graph/adjacency_map.rs

@@ -0,0 +1,203 @@
+use std::collections::{HashMap, HashSet};
+
+use super::graph_store::{GraphNode, GraphStore};
+
+/// A graph traversal that builds an adjacency map
+pub struct AdjacencyMap<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    adjacency_map: HashMap<T, Vec<T>>,
+    roots: Vec<T>,
+}
+
+impl<T> Default for AdjacencyMap<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T> AdjacencyMap<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    /// Creates a new adjacency map
+    pub fn new() -> Self {
+        Self {
+            adjacency_map: HashMap::new(),
+            roots: Vec::new(),
+        }
+    }
+
+    /// Returns an iterator over the root nodes of the graph
+    pub fn roots(&self) -> impl Iterator<Item = &T> {
+        self.roots.iter()
+    }
+
+    /// Returns an iterator over the children of the given node
+    pub fn get(&self, node: &T) -> Option<impl Iterator<Item = &T>> {
+        self.adjacency_map.get(node).map(|vec| vec.iter())
+    }
+}
+
+impl<T> GraphStore for AdjacencyMap<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    type Node = T;
+    type Handle = T;
+
+    fn insert(&mut self, from_handle: Option<T>, node: GraphNode<T>) -> Option<(Self::Handle, &T)> {
+        let vec = if let Some(from_handle) = from_handle {
+            self.adjacency_map
+                .entry(from_handle)
+                .or_insert_with(|| Vec::with_capacity(1))
+        } else {
+            &mut self.roots
+        };
+
+        vec.push(node.node().clone());
+        Some((node.into_node(), vec.last().unwrap()))
+    }
+}
+
+impl<T> AdjacencyMap<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    /// Returns an iterator over the nodes in reverse topological order,
+    /// starting from the roots.
+    pub fn into_reverse_topological(self) -> ReverseTopologicalIter<T> {
+        ReverseTopologicalIter {
+            adjacency_map: self.adjacency_map,
+            stack: self
+                .roots
+                .into_iter()
+                .map(|root| (ReverseTopologicalPass::Pre, root))
+                .collect(),
+            visited: HashSet::new(),
+        }
+    }
+
+    /// Returns an iterator over the nodes in reverse topological order,
+    /// starting from the given node.
+    pub fn reverse_topological_from_node<'graph>(
+        &'graph self,
+        node: &'graph T,
+    ) -> ReverseTopologicalFromNodeIter<'graph, T> {
+        ReverseTopologicalFromNodeIter {
+            adjacency_map: &self.adjacency_map,
+            stack: vec![(ReverseTopologicalPass::Pre, node)],
+            visited: HashSet::new(),
+        }
+    }
+}
+
+#[derive(Debug)]
+enum ReverseTopologicalPass {
+    Pre,
+    Post,
+}
+
+/// An iterator over the nodes of a graph in reverse topological order, starting
+/// from the roots.
+pub struct ReverseTopologicalIter<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    adjacency_map: HashMap<T, Vec<T>>,
+    stack: Vec<(ReverseTopologicalPass, T)>,
+    visited: HashSet<T>,
+}
+
+impl<T> Iterator for ReverseTopologicalIter<T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = loop {
+            let (pass, current) = self.stack.pop()?;
+
+            match pass {
+                ReverseTopologicalPass::Post => {
+                    break current;
+                }
+                ReverseTopologicalPass::Pre => {
+                    if self.visited.contains(&current) {
+                        continue;
+                    }
+
+                    self.visited.insert(current.clone());
+
+                    let Some(neighbors) = self.adjacency_map.get(&current) else {
+                        break current;
+                    };
+
+                    self.stack.push((ReverseTopologicalPass::Post, current));
+                    self.stack.extend(
+                        neighbors
+                            .iter()
+                            .map(|neighbor| (ReverseTopologicalPass::Pre, neighbor.clone())),
+                    );
+                }
+            }
+        };
+
+        Some(current)
+    }
+}
+
+/// An iterator over the nodes of a graph in reverse topological order, starting
+/// from a given node.
+pub struct ReverseTopologicalFromNodeIter<'graph, T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    adjacency_map: &'graph HashMap<T, Vec<T>>,
+    stack: Vec<(ReverseTopologicalPass, &'graph T)>,
+    visited: HashSet<&'graph T>,
+}
+
+impl<'graph, T> Iterator for ReverseTopologicalFromNodeIter<'graph, T>
+where
+    T: Eq + std::hash::Hash + Clone,
+{
+    type Item = &'graph T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = loop {
+            let (pass, current) = self.stack.pop()?;
+
+            match pass {
+                ReverseTopologicalPass::Post => {
+                    break current;
+                }
+                ReverseTopologicalPass::Pre => {
+                    if self.visited.contains(&current) {
+                        continue;
+                    }
+
+                    self.visited.insert(current);
+
+                    let Some(neighbors) = self.adjacency_map.get(current) else {
+                        break current;
+                    };
+
+                    self.stack.push((ReverseTopologicalPass::Post, current));
+                    self.stack.extend(
+                        neighbors
+                            .iter()
+                            .map(|neighbor| (ReverseTopologicalPass::Pre, neighbor)),
+                    );
+                }
+            }
+        };
+
+        Some(current)
+    }
+}

crates/turbo-tasks/src/graph/mod.rs

@@ -1,14 +1,14 @@
+mod adjacency_map;
 mod control_flow;
 mod graph_store;
 mod graph_traversal;
 mod non_deterministic;
-mod reverse_topological;
 mod visit;
 mod with_future;
 
+pub use adjacency_map::AdjacencyMap;
 pub use control_flow::VisitControlFlow;
 pub use graph_store::{GraphStore, SkipDuplicates};
 pub use graph_traversal::{GraphTraversal, GraphTraversalResult};
 pub use non_deterministic::NonDeterministic;
-pub use reverse_topological::ReverseTopological;
 pub use visit::Visit;

crates/turbopack-build/src/chunking_context.rs

@@ -1,7 +1,7 @@
 use anyhow::Result;
 use indexmap::IndexSet;
 use turbo_tasks::{
-    graph::{GraphTraversal, ReverseTopological},
+    graph::{AdjacencyMap, GraphTraversal},
     primitives::{BoolVc, StringVc},
     TryJoinIterExt, Value,
 };
@@ -322,7 +322,7 @@ async fn get_parallel_chunks<I>(entries: I) -> Result<impl Iterator<Item = Chunk
 where
     I: IntoIterator<Item = ChunkVc>,
 {
-    Ok(ReverseTopological::new()
+    Ok(AdjacencyMap::new()
         .skip_duplicates()
         .visit(entries, |chunk: ChunkVc| async move {
             Ok(chunk
@@ -336,7 +336,7 @@ where
         .await
         .completed()?
         .into_inner()
-        .into_iter())
+        .into_reverse_topological())
 }
 
 async fn get_optimized_chunks<I>(chunks: I) -> Result<ChunksVc>