speed up unification

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "scryer-prolog"
-version = "0.8.69"
+version = "0.8.70"
 authors = ["Mark Thom <markjordanthom@gmail.com>"]
 repository = "https://github.com/mthom/scryer-prolog"
 description = "A modern Prolog implementation written mostly in Rust."

src/prolog/heap_print.rs

@@ -487,7 +487,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter>
         for _ in 0 .. arity {
             self.state_stack.push(TokenOrRedirect::FunctorRedirect);
             self.state_stack.push(TokenOrRedirect::Comma);
-        }        
+        }
 
         self.state_stack.pop();
         self.state_stack.push(TokenOrRedirect::Open);
@@ -778,7 +778,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter>
 
     fn print_string(&mut self, s: StringList) {
         match self.machine_st.machine_flags().double_quotes {
-            DoubleQuotes::Chars | DoubleQuotes::Codes => {
+            DoubleQuotes::Chars | DoubleQuotes::Codes =>
                 if !s.is_empty() {
                     if self.ignore_ops {
                         self.format_struct(2, clause_name!("."));
@@ -791,8 +791,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter>
                     }
                 } else if !self.at_cdr("") {
                     self.append_str("[]");
-                }
-            },
+                },            
             DoubleQuotes::Atom => {
                 let borrowed_str = s.borrow();
 
@@ -802,7 +801,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter>
             }
         }
     }
-
+    
     fn push_list(&mut self) {
         let cell = Rc::new(Cell::new(true));
 

src/prolog/machine/machine_state_impl.rs

@@ -342,7 +342,8 @@ impl MachineState {
 
     pub(super) fn unify(&mut self, a1: Addr, a2: Addr) {
         let mut pdl = vec![a1, a2];
-
+        let mut tabu_list: HashSet<(Addr, Addr)> = HashSet::new();
+        
         self.fail = false;
 
         while !(pdl.is_empty() || self.fail) {
@@ -350,7 +351,16 @@ impl MachineState {
             let d2 = self.deref(pdl.pop().unwrap());
 
             if d1 != d2 {
-                match (self.store(d1.clone()), self.store(d2.clone())) {
+                let d1 = self.store(d1);
+                let d2 = self.store(d2);
+
+                if tabu_list.contains(&(d1.clone(), d2.clone())) {
+                    continue;
+                } else {
+                    tabu_list.insert((d1.clone(), d2.clone()));
+                }
+                
+                match (d1.clone(), d2.clone()) {
                     (Addr::AttrVar(h), addr) | (addr, Addr::AttrVar(h)) =>
                         self.bind(Ref::AttrVar(h), addr),
                     (Addr::HeapCell(h), _) =>