Bugfix: flattening for non-binary bitnor, bitnand, bitxnor

This fixes the flattening for bitnor, bitnand, bitxnor for the case that the
expression has one operand or more than two operands.

For one operand, the result is now the bit-wise negation.

For three or more operands, the result is now the bit-wise negation of
the bitor, bitand, bitxor with the same operands.

src/solvers/flattening/boolbv_bitwise.cpp

@@ -25,6 +25,17 @@ bvt boolbvt::convert_bitwise(const exprt &expr)
           expr.id()==ID_bitnand || expr.id()==ID_bitnor ||
           expr.id()==ID_bitxnor)
   {
+    std::function<literalt(literalt, literalt)> f;
+
+    if(expr.id() == ID_bitand || expr.id() == ID_bitnand)
+      f = [this](literalt a, literalt b) { return prop.land(a, b); };
+    else if(expr.id() == ID_bitor || expr.id() == ID_bitnor)
+      f = [this](literalt a, literalt b) { return prop.lor(a, b); };
+    else if(expr.id() == ID_bitxor || expr.id() == ID_bitxnor)
+      f = [this](literalt a, literalt b) { return prop.lxor(a, b); };
+    else
+      UNIMPLEMENTED;
+
     bvt bv;
     bv.resize(width);
 
@@ -38,25 +49,19 @@ bvt boolbvt::convert_bitwise(const exprt &expr)
       {
         for(std::size_t i=0; i<width; i++)
         {
-          if(expr.id()==ID_bitand)
-            bv[i]=prop.land(bv[i], op[i]);
-          else if(expr.id()==ID_bitor)
-            bv[i]=prop.lor(bv[i], op[i]);
-          else if(expr.id()==ID_bitxor)
-            bv[i]=prop.lxor(bv[i], op[i]);
-          else if(expr.id()==ID_bitnand)
-            bv[i]=prop.lnand(bv[i], op[i]);
-          else if(expr.id()==ID_bitnor)
-            bv[i]=prop.lnor(bv[i], op[i]);
-          else if(expr.id()==ID_bitxnor)
-            bv[i]=prop.lequal(bv[i], op[i]);
-          else
-            UNIMPLEMENTED;
+          bv[i] = f(bv[i], op[i]);
         }
       }
     }
 
-    return bv;
+    if(
+      expr.id() == ID_bitnand || expr.id() == ID_bitnor ||
+      expr.id() == ID_bitxnor)
+    {
+      return bv_utils.inverted(bv);
+    }
+    else
+      return bv;
   }
 
   UNIMPLEMENTED;