--- /dev/null
+/* mpz_powm(res,base,exp,mod) -- Set RES to (base**exp) mod MOD.
+
+Copyright (C) 1991, 1992 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 General Public License as published by
+the Free Software Foundation; either version 2, 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with the GNU MP Library; see the file COPYING. If not, write to
+the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+#include "longlong.h"
+
+#ifndef BERKELEY_MP
+void
+#ifdef __STDC__
+mpz_powm (MP_INT *res, const MP_INT *base, const MP_INT *exp,
+ const MP_INT *mod)
+#else
+mpz_powm (res, base, exp, mod)
+ MP_INT *res;
+ const MP_INT *base;
+ const MP_INT *exp;
+ const MP_INT *mod;
+#endif
+#else /* BERKELEY_MP */
+void
+#ifdef __STDC__
+pow (const MP_INT *base, const MP_INT *exp, const MP_INT *mod, MP_INT *res)
+#else
+pow (base, exp, mod, res)
+ const MP_INT *base;
+ const MP_INT *exp;
+ const MP_INT *mod;
+ MP_INT *res;
+#endif
+#endif /* BERKELEY_MP */
+{
+ mp_ptr rp, ep, mp, bp;
+ mp_size esize, msize, bsize, rsize;
+ mp_size size;
+ int mod_shift_cnt;
+ int negative_result;
+ mp_limb *free_me = NULL;
+ size_t free_me_size;
+
+ esize = ABS (exp->size);
+ msize = ABS (mod->size);
+ size = 2 * msize;
+
+ rp = res->d;
+ ep = exp->d;
+
+ /* Normalize MOD (i.e. make its most significant bit set) as required by
+ mpn_div. This will make the intermediate values in the calculation
+ slightly larger, but the correct result is obtained after a final
+ reduction using the original MOD value. */
+
+ mp = (mp_ptr) alloca (msize * BYTES_PER_MP_LIMB);
+ count_leading_zeros (mod_shift_cnt, mod->d[msize - 1]);
+ if (mod_shift_cnt != 0)
+ (void) mpn_lshift (mp, mod->d, msize, mod_shift_cnt);
+ else
+ MPN_COPY (mp, mod->d, msize);
+
+ bsize = ABS (base->size);
+ if (bsize > msize)
+ {
+ /* The base is larger than the module. Reduce it. */
+
+ /* Allocate (BSIZE + 1) with space for remainder and quotient.
+ (The quotient is (bsize - msize + 1) limbs.) */
+ bp = (mp_ptr) alloca ((bsize + 1) * BYTES_PER_MP_LIMB);
+ MPN_COPY (bp, base->d, bsize);
+ /* We don't care about the quotient, store it above the remainder,
+ at BP + MSIZE. */
+ mpn_div (bp + msize, bp, bsize, mp, msize);
+ bsize = msize;
+ while (bsize > 0 && bp[bsize - 1] == 0)
+ bsize--;
+ }
+ else
+ {
+ bp = base->d;
+ bsize = ABS (base->size);
+ }
+
+ if (res->alloc < size)
+ {
+ /* We have to allocate more space for RES. If any of the input
+ parameters are identical to RES, defer deallocation of the old
+ space. */
+
+ if (rp == ep || rp == mp || rp == bp)
+ {
+ free_me = rp;
+ free_me_size = res->alloc;
+ }
+ else
+ (*_mp_free_func) (rp, res->alloc * BYTES_PER_MP_LIMB);
+
+ rp = (mp_ptr) (*_mp_allocate_func) (size * BYTES_PER_MP_LIMB);
+ res->alloc = size;
+ res->d = rp;
+ }
+ else
+ {
+ /* Make BASE, EXP and MOD not overlap with RES. */
+ if (rp == bp)
+ {
+ /* RES and BASE are identical. Allocate temp. space for BASE. */
+ bp = (mp_ptr) alloca (bsize * BYTES_PER_MP_LIMB);
+ MPN_COPY (bp, rp, bsize);
+ }
+ if (rp == ep)
+ {
+ /* RES and EXP are identical. Allocate temp. space for EXP. */
+ ep = (mp_ptr) alloca (esize * BYTES_PER_MP_LIMB);
+ MPN_COPY (ep, rp, esize);
+ }
+ if (rp == mp)
+ {
+ /* RES and MOD are identical. Allocate temporary space for MOD. */
+ mp = (mp_ptr) alloca (msize * BYTES_PER_MP_LIMB);
+ MPN_COPY (mp, rp, msize);
+ }
+ }
+
+ if (esize == 0)
+ {
+ rp[0] = 1;
+ res->size = 1;
+ return;
+ }
+
+ MPN_COPY (rp, bp, bsize);
+ rsize = bsize;
+
+ {
+ mp_size i;
+ mp_size xsize;
+ mp_ptr dummyp = (mp_ptr) alloca ((msize + 1) * BYTES_PER_MP_LIMB);
+ mp_ptr xp = (mp_ptr) alloca (2 * (msize + 1) * BYTES_PER_MP_LIMB);
+ int c;
+ mp_limb e;
+ mp_limb carry_limb;
+
+ negative_result = (ep[0] & 1) && base->size < 0;
+
+ i = esize - 1;
+ e = ep[i];
+ count_leading_zeros (c, e);
+ e <<= (c + 1); /* shift the exp bits to the left, loose msb */
+ c = BITS_PER_MP_LIMB - 1 - c;
+
+ /* Main loop.
+
+ Make the result be pointed to alternatingly by XP and RP. This
+ helps us avoid block copying, which would otherwise be necessary
+ with the overlap restrictions of mpn_div. With 50% probability
+ the result after this loop will be in the area originally pointed
+ by RP (==RES->D), and with 50% probability in the area originally
+ pointed to by XP. */
+
+ for (;;)
+ {
+ while (c != 0)
+ {
+ mp_ptr tp;
+ mp_size tsize;
+
+ xsize = mpn_mul (xp, rp, rsize, rp, rsize);
+ mpn_div (dummyp, xp, xsize, mp, msize);
+
+ /* Remove any leading zero words from the result. */
+ if (xsize > msize)
+ xsize = msize;
+ while (xsize > 0 && xp[xsize - 1] == 0)
+ xsize--;
+
+ tp = rp; rp = xp; xp = tp;
+ tsize = rsize; rsize = xsize; xsize = tsize;
+
+ if ((mp_limb_signed) e < 0)
+ {
+ if (rsize > bsize)
+ xsize = mpn_mul (xp, rp, rsize, bp, bsize);
+ else
+ xsize = mpn_mul (xp, bp, bsize, rp, rsize);
+ mpn_div (dummyp, xp, xsize, mp, msize);
+
+ /* Remove any leading zero words from the result. */
+ if (xsize > msize)
+ xsize = msize;
+ while (xsize > 0 && xp[xsize - 1] == 0)
+ xsize--;
+
+ tp = rp; rp = xp; xp = tp;
+ tsize = rsize; rsize = xsize; xsize = tsize;
+ }
+ e <<= 1;
+ c--;
+ }
+
+ i--;
+ if (i < 0)
+ break;
+ e = ep[i];
+ c = BITS_PER_MP_LIMB;
+ }
+
+ /* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT
+ steps. Adjust the result by reducing it with the original MOD.
+
+ Also make sure the result is put in RES->D (where it already
+ might be, see above). */
+
+ carry_limb = mpn_lshift (res->d, rp, rsize, mod_shift_cnt);
+ rp = res->d;
+ if (carry_limb != 0)
+ {
+ rp[rsize] = carry_limb;
+ rsize++;
+ }
+ mpn_div (dummyp, rp, rsize, mp, msize);
+ /* Remove any leading zero words from the result. */
+ if (rsize > msize)
+ rsize = msize;
+ while (rsize > 0 && rp[rsize - 1] == 0)
+ rsize--;
+ rsize = mpn_rshift (rp, rp, rsize, mod_shift_cnt);
+ }
+
+ res->size = negative_result >= 0 ? rsize : -rsize;
+
+ if (free_me != NULL)
+ (*_mp_free_func) (free_me, free_me_size * BYTES_PER_MP_LIMB);
+
+ alloca (0);
+}
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