Make sure metadata gets updated with congruence.

CHANGELOG.md

@@ -4,6 +4,11 @@
 
 ## [Unreleased] - ReleaseDate
 
+### Changed
+- **Fix:** An eclass's metadata will now get updated by
+  congruence. This unfortunately results in a little bit slower code,
+  but it's the right thing.
+
 ## [0.3.0] - 2020-02-27
 
 ### Added

src/eclass.rs

@@ -84,7 +84,7 @@ assert_eq!(runner.egraph.find(runner.roots[0]), runner.egraph.find(just_foo));
 [`math.rs`]: https://github.com/mwillsey/egg/blob/master/tests/math.rs
 [`prop.rs`]: https://github.com/mwillsey/egg/blob/master/tests/prop.rs
 */
-pub trait Metadata<L>: Sized + Debug {
+pub trait Metadata<L>: Sized + Debug + PartialEq + Eq {
     /// Unused for now, probably just make this `()`.
     type Error: Debug;
 

src/egraph.rs

@@ -372,6 +372,7 @@ impl<L: Language, M: Metadata<L>> EGraph<L, M> {
     fn rebuild_once(&mut self) -> usize {
         let mut new_memo = IndexMap::new();
         let mut to_union = Vec::new();
+        let mut new_metas = Vec::new();
 
         for (leader, class) in self.classes.iter() {
             for node in &class.nodes {
@@ -382,14 +383,26 @@ impl<L: Language, M: Metadata<L>> EGraph<L, M> {
                     }
                 }
             }
+
+            let mut metas = class.nodes.iter().map(|n| M::make(self, n));
+            let first_meta = metas.next().expect("eclass shouldn't be empty");
+            let meta = metas.fold(first_meta, |m1, m2| m1.merge(&m2));
+            if meta != class.metadata {
+                new_metas.push((class.id, meta))
+            }
         }
 
         let n_unions = to_union.len();
         for (id1, id2) in to_union {
             self.union(id1, id2);
         }
 
-        n_unions
+        let n_metas = new_metas.len();
+        for (id, meta) in new_metas {
+            self.classes.get_mut(id).metadata = meta;
+        }
+
+        n_unions + n_metas
     }
 
     fn rebuild_classes(&mut self) -> usize {
@@ -572,6 +585,11 @@ impl<L: Language, M: Metadata<L>> EGraph<L, M> {
                 warn!("Long time: {}, to do: {}", t, ids.len());
             }
 
+            let mut metas = self[id].nodes.iter().map(|n| M::make(self, n));
+            let first_meta = metas.next().expect("eclass shouldn't be empty");
+            let meta = metas.fold(first_meta, |m1, m2| m1.merge(&m2));
+            self.classes.get_mut(id).metadata = meta;
+
             let map = self[id]
                 .parents
                 .iter()

src/rewrite.rs

@@ -159,7 +159,7 @@ where
 ///
 /// // Our metadata in this case will be size of the smallest
 /// // represented expression in the eclass.
-/// #[derive(Debug)]
+/// #[derive(Debug, Clone, PartialEq, Eq)]
 /// struct Meta {
 ///     size: usize,
 /// }
@@ -194,11 +194,13 @@ where
 ///     }}),
 /// ];
 ///
+/// #[derive(Debug, Clone, PartialEq, Eq)]
 /// struct Funky {
 ///     a: Var,
 ///     b: Var,
 ///     c: Var,
 /// }
+///
 /// impl Applier<Math, Meta> for Funky {
 ///     fn apply_one(&self, egraph: &mut EGraph, matched_id: Id, subst: &Subst) -> Vec<Id> {
 ///         let a: Id = subst[&self.a];

src/run.rs

@@ -304,6 +304,7 @@ where
         // TODO check that we haven't
         loop {
             if let Err(stop_reason) = self.run_one(rules) {
+                info!("Stopping: {:?}", stop_reason);
                 self.stop_reason = Some(stop_reason);
                 break;
             }

tests/math.rs

@@ -46,7 +46,7 @@ impl egg::CostFunction<Math> for MathCostFn {
     }
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Meta {
     pub cost: usize,
     pub best: RecExpr<Math>,