morphism base: find shortest path instead of just some path

src/morphism.rs

@@ -17,6 +17,10 @@ pub struct MorphismType {
 pub trait Morphism : Sized {
     fn get_type(&self) -> MorphismType;
     fn list_map_morphism(&self, list_typeid: TypeID) -> Option< Self >;
+
+    fn weight(&self) -> u64 {
+        1
+    }
 }
 
 #[derive(Clone)]
@@ -134,41 +138,47 @@ impl<M: Morphism + Clone> MorphismBase<M> {
         dst_types
     }
 
-    /* performs DFS to find a morphism-path for a given type
-     * will return the first matching path, not the shortest
+    /* try to find shortest morphism-path for a given type
      */
     pub fn find_morphism_path(&self, ty: MorphismType)
     -> Option< Vec<TypeTerm> >
     {
         let ty = ty.normalize();
-        let mut visited = Vec::new();
+
         let mut queue = vec![
-            vec![ ty.src_type.clone().normalize() ]
+            (0, vec![ ty.src_type.clone().normalize() ])
         ];
 
-        while let Some(current_path) = queue.pop() {       
-            let current_type = current_path.last().unwrap();
+        while ! queue.is_empty() {
+            queue.sort_by( |&(w1,_),&(w2,_)| w2.cmp(&w1));
 
-            if ! visited.contains( current_type ) {
-                visited.push( current_type.clone() );
+            if let Some((current_weight, current_path)) = queue.pop() {
+                let current_type = current_path.last().unwrap();
 
                 for (h, t) in self.enum_morphisms_with_subtyping(&current_type) {
                     let tt = TypeTerm::Ladder( vec![ h, t ] ).normalize();
 
-                    if ! visited.contains( &tt ) {
+                    if ! current_path.contains( &tt ) {
                         let unification_result = crate::unification::unify(&tt, &ty.dst_type);
-                        let mut new_path = current_path.clone();
 
+                        let morphism_weight = 1;
+                            /*
+                            {
+                                self.find_morphism( &tt ).unwrap().0.get_weight()
+                            };
+                            */
+
+                        let new_weight = current_weight + morphism_weight;
+                        let mut new_path = current_path.clone();
                         new_path.push( tt );
 
                         if let Ok(σ) = unification_result {
                             new_path = new_path.into_iter().map(
                                 |mut t: TypeTerm| t.apply_substitution(&|x| σ.get(x).cloned()).clone()
                             ).collect::<Vec<TypeTerm>>();
-
                             return Some(new_path);
                         } else {
-                            queue.push( new_path );
+                            queue.push( (new_weight, new_path) );
                         }
                     }
                 }