add acyclic term

src/prolog/machine/machine_state.rs

@@ -1,6 +1,7 @@
 use prolog::and_stack::*;
 use prolog::ast::*;
 use prolog::copier::*;
+use prolog::heap_iter::*;
 use prolog::num::{BigInt, BigUint, Zero, One};
 use prolog::or_stack::*;
 use prolog::heap_print::*;
@@ -9,7 +10,7 @@ use prolog::tabled_rc::*;
 use downcast::Any;
 
 use std::cmp::Ordering;
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::mem::swap;
 use std::ops::{Index, IndexMut};
 use std::rc::Rc;
@@ -395,6 +396,33 @@ pub(crate) trait CallPolicy: Any {
                            -> CallResult
     {
         match ct {
+            &ClauseType::AcyclicTerm => {
+                let addr = machine_st[temp_v!(1)].clone();
+                let mut seen = HashSet::new();
+                let mut fail = false;
+                
+                {
+                    let mut iter = machine_st.pre_order_iter(addr);
+
+                    loop {
+                        if let Some(addr) = iter.stack().last() {
+                            if !seen.contains(addr) {                            
+                                seen.insert(addr.clone());
+                            } else {
+                                fail = true;
+                                break;
+                            }                            
+                        }
+
+                        if iter.next().is_none() {
+                            break;
+                        }
+                    }
+                }
+
+                machine_st.fail = fail;
+                return_from_clause!(lco, machine_st)
+            },
             &ClauseType::Arg => {
                 if !lco {
                     machine_st.cp = machine_st.p.clone() + 1;