add the pdd evaluator and a unit test for it

Signed-off-by: Lev Nachmanson <levnach@hotmail.com>

src/math/dd/pdd_eval.h

@@ -0,0 +1,42 @@
+/*++
+Copyright (c) 2019 Microsoft Corporation
+
+Module Name:
+
+    dd_pdd.cpp
+
+Abstract:
+
+    Poly DD package 
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2019-12-23
+    Lev Nachmanson (levnach) 2019-12-23
+
+Revision History:
+
+--*/
+#include "math/dd/dd_pdd.h"
+
+namespace dd {
+// calculates the value of a pdd expression based on the given values of the variables
+class pdd_eval {
+    pdd_manager& m;
+    
+    std::function<rational (unsigned)> m_var2val;
+    
+public:
+    
+    pdd_eval(pdd_manager& m): m(m) {}
+    
+    void operator=(std::function<rational (unsigned)>& i2v) { m_var2val = i2v; }
+
+    rational operator()(pdd const& p) {
+        if (p.is_val()) {
+            return p.val();
+        }
+        return (*this)(p.hi()) * m_var2val(p.var()) + (*this)(p.lo());
+    }
+};
+}