--- /dev/null
+/* gcd_subdiv_step.c.
+
+ THE FUNCTIONS IN THIS FILE ARE INTERNAL WITH MUTABLE INTERFACES. IT IS ONLY
+ SAFE TO REACH THEM THROUGH DOCUMENTED INTERFACES. IN FACT, IT IS ALMOST
+ GUARANTEED THAT THEY'LL CHANGE OR DISAPPEAR IN A FUTURE GNU MP RELEASE.
+
+Copyright 2003, 2004, 2005, 2008 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 3 of the License, or (at your
+option) any later version.
+
+The GNU MP Library 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 Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+#include "longlong.h"
+
+/* Used when mpn_hgcd or mpn_hgcd2 has failed. Then either one of a or
+ b is small, or the difference is small. Perform one subtraction
+ followed by one division. If the gcd is found, stores it in gp and
+ *gn, and returns zero. Otherwise, compute the reduced a and b, and
+ return the new size. */
+
+/* FIXME: Check when the smaller number is a single limb, and invoke
+ * mpn_gcd_1. */
+mp_size_t
+mpn_gcd_subdiv_step (mp_ptr gp, mp_size_t *gn,
+ mp_ptr ap, mp_ptr bp, mp_size_t n, mp_ptr tp)
+{
+ mp_size_t an, bn;
+
+ ASSERT (n > 0);
+ ASSERT (ap[n-1] > 0 || bp[n-1] > 0);
+
+ an = bn = n;
+ MPN_NORMALIZE (ap, an);
+ MPN_NORMALIZE (bp, bn);
+
+ if (UNLIKELY (an == 0))
+ {
+ return_b:
+ MPN_COPY (gp, bp, bn);
+ *gn = bn;
+ return 0;
+ }
+ else if (UNLIKELY (bn == 0))
+ {
+ return_a:
+ MPN_COPY (gp, ap, an);
+ *gn = an;
+ return 0;
+ }
+
+ /* Arrange so that a > b, subtract an -= bn, and maintain
+ normalization. */
+ if (an < bn)
+ MPN_PTR_SWAP (ap, an, bp, bn);
+ else if (an == bn)
+ {
+ int c;
+ MPN_CMP (c, ap, bp, an);
+ if (UNLIKELY (c == 0))
+ goto return_a;
+ else if (c < 0)
+ MP_PTR_SWAP (ap, bp);
+ }
+
+ ASSERT_NOCARRY (mpn_sub (ap, ap, an, bp, bn));
+ MPN_NORMALIZE (ap, an);
+ ASSERT (an > 0);
+
+ /* Arrange so that a > b, and divide a = q b + r */
+ /* FIXME: an < bn happens when we have cancellation. If that is the
+ common case, then we could reverse the roles of a and b to avoid
+ the swap. */
+ if (an < bn)
+ MPN_PTR_SWAP (ap, an, bp, bn);
+ else if (an == bn)
+ {
+ int c;
+ MPN_CMP (c, ap, bp, an);
+ if (UNLIKELY (c == 0))
+ goto return_a;
+ else if (c < 0)
+ MP_PTR_SWAP (ap, bp);
+ }
+
+ mpn_tdiv_qr (tp, ap, 0, ap, an, bp, bn);
+
+ if (mpn_zero_p (ap, bn))
+ goto return_b;
+
+ return bn;
+}