Don’t add built-in signatures

org.aminb.alloy.catalyst/src/main/java/org/aminb/alloy/catalyst/SSRVisitor.java

@@ -2,6 +2,10 @@ package org.aminb.alloy.catalyst;
 
 import java.util.Map;
 import java.util.HashMap;
+import java.util.Set;
+import java.util.HashSet;
+import java.util.Arrays;
+import java.util.Optional;
 
 import edu.mit.csail.sdg.alloy4.Pos;
 import edu.mit.csail.sdg.ast.Decl;
@@ -29,22 +33,27 @@ import edu.mit.csail.sdg.ast.VisitReturn;
  */
 
 public class SSRVisitor extends VisitReturn {
-    // if come to var that's not used in a join field access, toss out its sig?
-
-    /**
-     *  new plan:
-     *
-     *  - gather list of candidate sigs in visit(Sig)
-     *    - all but those who a field of their own or another sig refers to them
-     *  - if in visit(ExprBinary) we detect a "field access" dot join involving
-     *    a sig, let it remain on the list if it is
-     *  - how about ExprQt? when to disallow those that are quantified over?
-     *  - any other cases where it should be allowed?
-     *  - otherwise, remove sig from candidates
-     */
+
+    Set<PrimSig> builtins =
+        new HashSet<>(Arrays.asList(Sig.UNIV, Sig.SIGINT, Sig.SEQIDX,
+                                    Sig.STRING, Sig.NONE, Sig.GHOST));
 
     Map<PrimSig, Boolean> candidates = new HashMap<>();
 
+    Optional<Boolean> putCandidate(PrimSig sig, Boolean viable) {
+        if (!builtins.contains(sig))
+            return Optional.of(candidates.put(sig, viable));
+        else
+            return Optional.empty();
+    }
+
+    Optional<Boolean> putCandidateIfAbsent(PrimSig sig, Boolean viable) {
+        if (!builtins.contains(sig))
+            return Optional.of(candidates.putIfAbsent(sig, viable));
+        else
+            return Optional.empty();
+    }
+
     PrimSig getFirstPrimSig(Expr e) {
         return
             e.type()            // :: Type
@@ -73,7 +82,7 @@ public class SSRVisitor extends VisitReturn {
         if (x.op == ExprBinary.Op.JOIN)
             if (x.left instanceof ExprUnary && x.right instanceof ExprUnary)
                 if (!isFieldAccess(x))
-                    candidates.put(getFirstPrimSig(x.left), false);
+                    putCandidate(getFirstPrimSig(x.left), false);
         visitThis(x.left);
         visitThis(x.right);
         return null;
@@ -133,7 +142,7 @@ public class SSRVisitor extends VisitReturn {
                     Expr subsub = getUnarySub(getUnarySub(s));
                     if (subsub instanceof PrimSig)
                         if (((PrimSig)subsub).getFieldDecls().size() > 1)
-                            candidates.put((PrimSig)subsub, false);
+                            putCandidate((PrimSig)subsub, false);
                 }
             }
             else if (s instanceof PrimSig)
@@ -190,7 +199,7 @@ public class SSRVisitor extends VisitReturn {
     public Object visit(Sig x) {
         // add the sig as a candidate if we haven't seen it before
         if (x instanceof PrimSig)
-            candidates.putIfAbsent((PrimSig)x, true);
+            putCandidateIfAbsent((PrimSig)x, true);
         return null;
     }
 
@@ -204,7 +213,7 @@ public class SSRVisitor extends VisitReturn {
         // after first column
         for (ProductType t : x.type())
             for (int i = 1; i < t.arity(); i++)
-                candidates.put(t.get(i), false);
+                putCandidate(t.get(i), false);
 
         // TODO: if type of field refers to another field, in this sig
         // or elsewhere, if there are no multiplicity constraints then