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+/* mpn_mul -- Multiply two natural numbers.
+
+Copyright (C) 1991, 1993, 1994, 1996 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 2.1 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; see the file COPYING.LIB. If not, write to
+the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+MA 02111-1307, USA. */
+
+#include <gmp.h>
+#include "gmp-impl.h"
+
+/* Multiply the natural numbers u (pointed to by UP, with USIZE limbs)
+ and v (pointed to by VP, with VSIZE limbs), and store the result at
+ PRODP. USIZE + VSIZE limbs are always stored, but if the input
+ operands are normalized. Return the most significant limb of the
+ result.
+
+ NOTE: The space pointed to by PRODP is overwritten before finished
+ with U and V, so overlap is an error.
+
+ Argument constraints:
+ 1. USIZE >= VSIZE.
+ 2. PRODP != UP and PRODP != VP, i.e. the destination
+ must be distinct from the multiplier and the multiplicand. */
+
+/* If KARATSUBA_THRESHOLD is not already defined, define it to a
+ value which is good on most machines. */
+#ifndef KARATSUBA_THRESHOLD
+#define KARATSUBA_THRESHOLD 32
+#endif
+
+mp_limb_t
+#if __STDC__
+mpn_mul (mp_ptr prodp,
+ mp_srcptr up, mp_size_t usize,
+ mp_srcptr vp, mp_size_t vsize)
+#else
+mpn_mul (prodp, up, usize, vp, vsize)
+ mp_ptr prodp;
+ mp_srcptr up;
+ mp_size_t usize;
+ mp_srcptr vp;
+ mp_size_t vsize;
+#endif
+{
+ mp_ptr prod_endp = prodp + usize + vsize - 1;
+ mp_limb_t cy;
+ mp_ptr tspace;
+ TMP_DECL (marker);
+
+ if (vsize < KARATSUBA_THRESHOLD)
+ {
+ /* Handle simple cases with traditional multiplication.
+
+ This is the most critical code of the entire function. All
+ multiplies rely on this, both small and huge. Small ones arrive
+ here immediately. Huge ones arrive here as this is the base case
+ for Karatsuba's recursive algorithm below. */
+ mp_size_t i;
+ mp_limb_t cy_limb;
+ mp_limb_t v_limb;
+
+ if (vsize == 0)
+ return 0;
+
+ /* Multiply by the first limb in V separately, as the result can be
+ stored (not added) to PROD. We also avoid a loop for zeroing. */
+ v_limb = vp[0];
+ if (v_limb <= 1)
+ {
+ if (v_limb == 1)
+ MPN_COPY (prodp, up, usize);
+ else
+ MPN_ZERO (prodp, usize);
+ cy_limb = 0;
+ }
+ else
+ cy_limb = mpn_mul_1 (prodp, up, usize, v_limb);
+
+ prodp[usize] = cy_limb;
+ prodp++;
+
+ /* For each iteration in the outer loop, multiply one limb from
+ U with one limb from V, and add it to PROD. */
+ for (i = 1; i < vsize; i++)
+ {
+ v_limb = vp[i];
+ if (v_limb <= 1)
+ {
+ cy_limb = 0;
+ if (v_limb == 1)
+ cy_limb = mpn_add_n (prodp, prodp, up, usize);
+ }
+ else
+ cy_limb = mpn_addmul_1 (prodp, up, usize, v_limb);
+
+ prodp[usize] = cy_limb;
+ prodp++;
+ }
+ return cy_limb;
+ }
+
+ TMP_MARK (marker);
+
+ tspace = (mp_ptr) TMP_ALLOC (2 * vsize * BYTES_PER_MP_LIMB);
+ MPN_MUL_N_RECURSE (prodp, up, vp, vsize, tspace);
+
+ prodp += vsize;
+ up += vsize;
+ usize -= vsize;
+ if (usize >= vsize)
+ {
+ mp_ptr tp = (mp_ptr) TMP_ALLOC (2 * vsize * BYTES_PER_MP_LIMB);
+ do
+ {
+ MPN_MUL_N_RECURSE (tp, up, vp, vsize, tspace);
+ cy = mpn_add_n (prodp, prodp, tp, vsize);
+ mpn_add_1 (prodp + vsize, tp + vsize, vsize, cy);
+ prodp += vsize;
+ up += vsize;
+ usize -= vsize;
+ }
+ while (usize >= vsize);
+ }
+
+ /* True: usize < vsize. */
+
+ /* Make life simple: Recurse. */
+
+ if (usize != 0)
+ {
+ mpn_mul (tspace, vp, vsize, up, usize);
+ cy = mpn_add_n (prodp, prodp, tspace, vsize);
+ mpn_add_1 (prodp + vsize, tspace + vsize, usize, cy);
+ }
+
+ TMP_FREE (marker);
+ return *prod_endp;
+}