Add ability to split e-classes for easier visualization (#14)

* Add ability to split e-classes for easier visualization

* Move comment

* Replace loop with partition

* Update docstring and name

* Only trigger as changed if we make a new node for the parents

src/algorithms.rs

@@ -1,4 +1,6 @@
-use crate::EGraph;
+use std::collections::HashMap;
+
+use crate::{Class, ClassId, EGraph, Node, NodeId};
 
 pub const MISSING_ARG_VALUE: &str = "·";
 
@@ -94,12 +96,12 @@ impl EGraph {
         // 5. Remove leaf nodes from egraph, class data, and root eclasses
         for (eclass, node_id) in &leaves {
             // If this node has no parents, don't remove it, since it wasn't inlined
-            if node_to_parents.get(node_id).is_none() {
+            if !node_to_parents.contains_key(node_id) {
                 continue;
             }
             n_inlined += 1;
-            self.nodes.remove(node_id);
-            self.class_data.remove(eclass);
+            self.nodes.swap_remove(node_id);
+            self.class_data.swap_remove(eclass);
             self.root_eclasses.retain(|root| root != eclass);
         }
         n_inlined
@@ -109,4 +111,111 @@ impl EGraph {
     pub fn saturate_inline_leaves(&mut self) {
         while self.inline_leaves() > 0 {}
     }
+
+    /// Given some function `should_split`, after calling this method, all nodes where it is true will have at most
+    /// one other node in their e-class and if they have parents, will no other nodes in their e-class.
+    ///
+    /// All nodes in an e-class shared with a split node will still be in a e-class with that node, but no longer
+    /// in an e-class with each other.
+    ///
+    /// This is used to help make the visualization easier to parse, by breaking cycles and allowing these split nodes
+    /// to be later inlined into their parents with `inline_leaves`, which only applies when there is a single node in an
+    /// e-class.
+    ///
+    /// For example, if we split on all "primitive" nodes or their wrappers, like Int(1), then those nodes
+    /// can then be inlined into their parents and are all nodes equal to them are no longer in single e-class,
+    /// making the graph layout easier to understand.
+    ///
+    /// Note that `should_split` should only ever be true for either a single node, or no nodes, in an e-class.
+    /// If it is true for multiple nodes in an e-class, then this method will panic.
+    ///
+    /// Another way to think about it is that any isomporphic function can be split, since if f(a) = f(b) then a = b,
+    /// in that case.
+    pub fn split_classes(&mut self, should_split: impl Fn(&NodeId, &Node) -> bool) {
+        // run till fixpoint since splitting a node might add more parents and require splitting the child down the line
+        let mut changed = true;
+        while changed {
+            changed = false;
+            // Mapping from class ID to all nodes that point to any node in that e-class
+            let parents: HashMap<ClassId, Vec<(NodeId, usize)>> =
+                self.nodes
+                    .iter()
+                    .fold(HashMap::new(), |mut parents, (node_id, node)| {
+                        for (position, child) in node.children.iter().enumerate() {
+                            let child_class = self.nodes[child].eclass.clone();
+                            parents
+                                .entry(child_class)
+                                .or_default()
+                                .push((node_id.clone(), position));
+                        }
+                        parents
+                    });
+
+            for Class { id, nodes } in self.classes().clone().into_values() {
+                let (unique_nodes, other_nodes): (Vec<_>, Vec<_>) = nodes
+                    .into_iter()
+                    .partition(|node_id| should_split(node_id, &self.nodes[node_id]));
+                if unique_nodes.len() > 1 {
+                    panic!(
+                        "Multiple nodes in one e-class should be split. E-class: {:?} Nodes: {:?}",
+                        id, unique_nodes
+                    );
+                }
+                let unique_node = unique_nodes.into_iter().next();
+                let class_data = self.class_data.get(&id).cloned();
+                if let Some(unique_node_id) = unique_node {
+                    let unique_node = self.nodes[&unique_node_id].clone();
+                    let n_other_nodes = other_nodes.len();
+                    let mut offset = 0;
+                    if n_other_nodes == 0 {
+                        continue;
+                    }
+                    // split out other nodes if there are multiple of them.
+                    // Leave one node in this e-class and make new e-classes for remaining nodes
+                    for other_node_id in other_nodes.into_iter().skip(1) {
+                        changed = true;
+                        // use same ID for new class and new node added to that class
+                        let new_id = format!("split-{}-{}", offset, unique_node_id);
+                        offset += 1;
+                        let new_class_id: ClassId = new_id.clone().into();
+                        // Copy the class data if it exists
+                        if let Some(class_data) = &class_data {
+                            self.class_data
+                                .insert(new_class_id.clone(), class_data.clone());
+                        }
+                        // Change the e-class of the other node
+                        self.nodes[&other_node_id].eclass = new_class_id.clone();
+                        // Create a new unique node with the same data
+                        let mut new_unique_node = unique_node.clone();
+                        new_unique_node.eclass = new_class_id;
+                        self.nodes.insert(new_id.into(), new_unique_node);
+                    }
+                    // If there are other nodes, then make one more copy and point all the parents at that
+                    let parents = parents.get(&id).cloned().unwrap_or_default();
+                    if parents.is_empty() {
+                        continue;
+                    }
+                    changed = true;
+                    let new_id = format!("split-{}-{}", offset, unique_node_id);
+                    let new_class_id: ClassId = new_id.clone().into();
+                    // Copy the class data if it exists
+                    if let Some(class_data) = &class_data {
+                        self.class_data
+                            .insert(new_class_id.clone(), class_data.clone());
+                    }
+                    // Create a new unique node with the same data
+                    let mut new_unique_node = unique_node.clone();
+                    new_unique_node.eclass = new_class_id;
+                    self.nodes.insert(new_id.clone().into(), new_unique_node);
+                    for (parent_id, position) in parents {
+                        // Change the child of the parent to the new node
+                        self.nodes.get_mut(&parent_id).unwrap().children[position] =
+                            new_id.clone().into();
+                    }
+                }
+            }
+            // reset the classes computation
+            self.once_cell_classes.take();
+        }
+    }
 }