+/** @file remainder.h
+ *
+ * Functions calculating remainders. */
+
+/*
+ * GiNaC Copyright (C) 1999-2015 Johannes Gutenberg University Mainz, Germany
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "remainder.h"
+#include "ring_traits.h"
+#include "upoly_io.h"
+#include "debug.h"
+
+namespace GiNaC {
+
+/**
+ * @brief Polynomial remainder for univariate polynomials over fields
+ *
+ * Given two univariate polynomials \f$a, b \in F[x]\f$, where F is
+ * a finite field (presumably Z/p) computes the remainder @a r, which is
+ * defined as \f$a = b q + r\f$. Returns true if the remainder is zero
+ * and false otherwise.
+ */
+bool
+remainder_in_field(umodpoly& r, const umodpoly& a, const umodpoly& b)
+{
+ typedef cln::cl_MI field_t;
+
+ if (degree(a) < degree(b)) {
+ r = a;
+ return false;
+ }
+ // The coefficient ring is a field, so any 0 degree polynomial
+ // divides any other polynomial.
+ if (degree(b) == 0) {
+ r.clear();
+ return true;
+ }
+
+ r = a;
+ const field_t b_lcoeff = lcoeff(b);
+ for (std::size_t k = a.size(); k-- >= b.size(); ) {
+
+ // r -= r_k/b_n x^{k - n} b(x)
+ if (zerop(r[k]))
+ continue;
+
+ field_t qk = div(r[k], b_lcoeff);
+ bug_on(zerop(qk), "division in a field yield zero: "
+ << r[k] << '/' << b_lcoeff);
+
+ // Why C++ is so off-by-one prone?
+ for (std::size_t j = k, i = b.size(); i-- != 0; --j) {
+ if (zerop(b[i]))
+ continue;
+ r[j] = r[j] - qk*b[i];
+ }
+ bug_on(!zerop(r[k]), "polynomial division in field failed: " <<
+ "r[" << k << "] = " << r[k] << ", " <<
+ "r = " << r << ", b = " << b);
+
+ }
+
+ // Canonicalize the remainder: remove leading zeros. Give a hint
+ // to canonicalize(): we know degree(remainder) < degree(b)
+ // (because the coefficient ring is a field), so
+ // c_{degree(b)} \ldots c_{degree(a)} are definitely zero.
+ std::size_t from = degree(b) - 1;
+ canonicalize(r, from);
+ return r.empty();
+}
+
+} // namespace GiNaC