CFL: A simple big speed optimization for random generation

cfl.py

@@ -826,9 +826,13 @@ class CFGRandom:
             weights: List[int] = []
 
             for prod in self.cfg.rules[sym]:
-                cand = sentence[:i] + prod + sentence[i + 1:]
-                w = self._materialize_prod(cand, length)
+                suff = prod + sentence[i + 1:]
+                # note: no nonterminals before i ~> the weight of the whole
+                # candidate is the same as the weight of its suffix starting at
+                # position i
+                w = self._materialize_prod(suff, length - i)
                 if w > 0:
+                    cand = sentence[:i] + suff
                     candidates.append(cand)
                     weights.append(w)