Make graphviz colors consistent and fix color bug (#9)

* Make colors consistent regardless of expr ordering

Currently, if you have two graphs which use the same types, the colors
per type might be different, depending on which type comes first.

This makes comparing similar graphs harder visually.

This resolves that by using a sorted map for mapping types to values,
so that if the same set of types are used by two graphs, they will
have the same colors.

* Fix which collection needs to be sorted

* Fix color wraparound when number of types exceed the max colors

src/graphviz.rs

@@ -34,8 +34,8 @@ impl EGraph {
     }
 
     fn to_graphviz(&self) -> Graph {
-        // 1. Group nodes by typ and class
-        let mut class_nodes = std::collections::HashMap::new();
+        // 1. Group nodes by type and class (use BTreeMap to keep sorted so colors are consistent)
+        let mut class_nodes = std::collections::BTreeMap::new();
         //  and create mapping from each node ID to its class
         let mut node_to_class = std::collections::HashMap::new();
         for (node_id, node) in &self.nodes {
@@ -81,7 +81,7 @@ impl EGraph {
         let mut typ_colors = std::collections::HashMap::new();
 
         for (typ, class_to_node) in class_nodes {
-            let next_color = typ_colors.len() % N_COLORS + INITIAL_COLOR;
+            let next_color = (typ_colors.len() + INITIAL_COLOR) % N_COLORS;
             let color = typ_colors.entry(typ).or_insert(next_color);
             stmts.push(stmt!(attr!("fillcolor", color)));
             for (class_id, nodes) in class_to_node {