Fix corner case in evaluation at geometric progression

doc/source/nmod_poly.rst

@@ -742,7 +742,7 @@ Multiplication
     product of ``poly1`` and ``poly2``.
 
 .. function:: void _nmod_poly_mulmid(nn_ptr res, nn_srcptr poly1, slong len1, nn_srcptr poly2, slong len2, slong nlo, slong nhi, nmod_t mod)
-              void nmod_poly_mulmid(nn_ptr res, nn_srcptr poly1, slong len1, nn_srcptr poly2, slong len2, slong nlo, slong nhi, nmod_t mod)
+              void nmod_poly_mulmid(nmod_poly_t res, const nmod_poly_t poly1, const nmod_poly_t poly2, slong nlo, slong nhi)
               void _nmod_poly_mulmid_classical(nn_ptr res, nn_srcptr poly1, slong len1, nn_srcptr poly2, slong len2, slong nlo, slong nhi, nmod_t mod)
               void nmod_poly_mulmid_classical(nn_ptr res, nn_srcptr poly1, slong len1, nn_srcptr poly2, slong len2, slong nlo, slong nhi, nmod_t mod)
               void _nmod_poly_mulmid_KS(nn_ptr res, nn_srcptr poly1, slong len1, nn_srcptr poly2, slong len2, slong nlo, slong nhi, nmod_t mod)

src/nmod_poly/evaluate_geometric_nmod_vec.c

@@ -24,22 +24,22 @@ void _nmod_poly_evaluate_geometric_nmod_vec_fast_precomp(nn_ptr vs, nn_srcptr po
     FLINT_ASSERT(G->function & 1);
     FLINT_ASSERT(len <= G->len);
 
-    if (plen == 0)
-    {
-        _nmod_vec_zero(vs, len);
-        return;
-    }
-
     /* val = valuation of poly */
-    slong val;
-    for (val = 0; val < plen; val++)
+    slong val = 0;
+    for ( ; val < plen; val++)
     {
         if (poly[val] != 0)
         {
             break;
         }
     }
 
+    if (val == plen) /* covers the case plen == 0 */
+    {
+        _nmod_vec_zero(vs, len);
+        return;
+    }
+
     /** Formula, based on Bluestein's trick:
      * poly(q**j) = sum_{i=0}^{n-1} poly[i] q**{i*j}  is the coefficient x**{n+j-1} of the product
      *     (sum_{i=0}^{len-1} poly[i] * q**(-i*i / 2) x**{len-i-1}) * (sum_{i=0}^{2*len-2} q**{i*i/2} x**i)