add gr_poly_is_scalar

doc/source/gr_poly.rst

@@ -110,6 +110,7 @@ Basic manipulation
 .. function:: truth_t gr_poly_is_zero(const gr_poly_t poly, gr_ctx_t ctx)
               truth_t gr_poly_is_one(const gr_poly_t poly, gr_ctx_t ctx)
               truth_t gr_poly_is_gen(const gr_poly_t poly, gr_ctx_t ctx)
+              truth_t gr_poly_is_scalar(const gr_poly_t poly, gr_ctx_t ctx)
 
 .. function:: int gr_poly_set_scalar(gr_poly_t poly, gr_srcptr c, gr_ctx_t ctx)
               int gr_poly_set_si(gr_poly_t poly, slong c, gr_ctx_t ctx)

src/gr_poly.h

@@ -93,6 +93,7 @@ truth_t gr_poly_equal(const gr_poly_t poly1, const gr_poly_t poly2, gr_ctx_t ctx
 truth_t gr_poly_is_zero(const gr_poly_t poly, gr_ctx_t ctx);
 truth_t gr_poly_is_one(const gr_poly_t poly, gr_ctx_t ctx);
 truth_t gr_poly_is_gen(const gr_poly_t poly, gr_ctx_t ctx);
+truth_t gr_poly_is_scalar(const gr_poly_t poly, gr_ctx_t ctx);
 
 WARN_UNUSED_RESULT int gr_poly_set_scalar(gr_poly_t poly, gr_srcptr x, gr_ctx_t ctx);
 WARN_UNUSED_RESULT int gr_poly_set_si(gr_poly_t poly, slong x, gr_ctx_t ctx);

src/gr_poly/is_scalar.c

@@ -0,0 +1,23 @@
+/*
+    Copyright (C) 2023 Fredrik Johansson
+
+    This file is part of FLINT.
+
+    FLINT is free software: you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License (LGPL) as published
+    by the Free Software Foundation; either version 2.1 of the License, or
+    (at your option) any later version.  See <http://www.gnu.org/licenses/>.
+*/
+
+#include "gr_poly.h"
+
+truth_t
+gr_poly_is_scalar(const gr_poly_t poly, gr_ctx_t ctx)
+{
+    slong len = poly->length;
+
+    if (len <= 1)
+        return T_TRUE;
+
+    return _gr_vec_is_zero(GR_ENTRY(poly->coeffs, 1, ctx->sizeof_elem), len - 1, ctx);
+}