--- /dev/null
+/* mpn_powlo -- Compute R = U^E mod R^n, where R is the limb base.
+
+Copyright 2007, 2008, 2009 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"
+
+
+#define getbit(p,bi) \
+ ((p[(bi - 1) / GMP_LIMB_BITS] >> (bi - 1) % GMP_LIMB_BITS) & 1)
+
+static inline mp_limb_t
+getbits (const mp_limb_t *p, unsigned long bi, int nbits)
+{
+ int nbits_in_r;
+ mp_limb_t r;
+ mp_size_t i;
+
+ if (bi < nbits)
+ {
+ return p[0] & (((mp_limb_t) 1 << bi) - 1);
+ }
+ else
+ {
+ bi -= nbits; /* bit index of low bit to extract */
+ i = bi / GMP_LIMB_BITS; /* word index of low bit to extract */
+ bi %= GMP_LIMB_BITS; /* bit index in low word */
+ r = p[i] >> bi; /* extract (low) bits */
+ nbits_in_r = GMP_LIMB_BITS - bi; /* number of bits now in r */
+ if (nbits_in_r < nbits) /* did we get enough bits? */
+ r += p[i + 1] << nbits_in_r; /* prepend bits from higher word */
+ return r & (((mp_limb_t ) 1 << nbits) - 1);
+ }
+}
+
+static inline int
+win_size (unsigned long eb)
+{
+ int k;
+ static unsigned long x[] = {1,7,25,81,241,673,1793,4609,11521,28161,~0ul};
+ for (k = 0; eb > x[k]; k++)
+ ;
+ return k;
+}
+
+/* rp[n-1..0] = bp[n-1..0] ^ ep[en-1..0] mod R^n, R is the limb base.
+ Requires that ep[en-1] is non-zero.
+ Uses scratch space tp[3n-1..0], i.e., 3n words. */
+void
+mpn_powlo (mp_ptr rp, mp_srcptr bp,
+ mp_srcptr ep, mp_size_t en,
+ mp_size_t n, mp_ptr tp)
+{
+ int cnt;
+ long ebi;
+ int windowsize, this_windowsize;
+ mp_limb_t expbits;
+ mp_limb_t *pp, *this_pp, *last_pp;
+ mp_limb_t *b2p;
+ long i;
+ TMP_DECL;
+
+ ASSERT (en > 1 || (en == 1 && ep[0] > 1));
+
+ TMP_MARK;
+
+ count_leading_zeros (cnt, ep[en - 1]);
+ ebi = en * GMP_LIMB_BITS - cnt;
+
+ windowsize = win_size (ebi);
+
+ pp = TMP_ALLOC_LIMBS ((n << (windowsize - 1)) + n); /* + n is for mullow ign part */
+
+ this_pp = pp;
+
+ MPN_COPY (this_pp, bp, n);
+
+ b2p = tp + 2*n;
+
+ /* Store b^2 in b2. */
+ mpn_sqr_n (tp, bp, n); /* FIXME: Use "mpn_sqrlo" */
+ MPN_COPY (b2p, tp, n);
+
+ /* Precompute odd powers of b and put them in the temporary area at pp. */
+ for (i = (1 << (windowsize - 1)) - 1; i > 0; i--)
+ {
+ last_pp = this_pp;
+ this_pp += n;
+ mpn_mullow_n (this_pp, last_pp, b2p, n);
+ }
+
+ expbits = getbits (ep, ebi, windowsize);
+ ebi -= windowsize;
+ if (ebi < 0)
+ ebi = 0;
+
+ count_trailing_zeros (cnt, expbits);
+ ebi += cnt;
+ expbits >>= cnt;
+
+ MPN_COPY (rp, pp + n * (expbits >> 1), n);
+
+ while (ebi != 0)
+ {
+ while (getbit (ep, ebi) == 0)
+ {
+ mpn_sqr_n (tp, rp, n); /* FIXME: Use "mpn_sqrlo" */
+ MPN_COPY (rp, tp, n);
+ ebi--;
+ if (ebi == 0)
+ goto done;
+ }
+
+ /* The next bit of the exponent is 1. Now extract the largest block of
+ bits <= windowsize, and such that the least significant bit is 1. */
+
+ expbits = getbits (ep, ebi, windowsize);
+ ebi -= windowsize;
+ this_windowsize = windowsize;
+ if (ebi < 0)
+ {
+ this_windowsize += ebi;
+ ebi = 0;
+ }
+
+ count_trailing_zeros (cnt, expbits);
+ this_windowsize -= cnt;
+ ebi += cnt;
+ expbits >>= cnt;
+
+ do
+ {
+ mpn_sqr_n (tp, rp, n);
+ MPN_COPY (rp, tp, n);
+ this_windowsize--;
+ }
+ while (this_windowsize != 0);
+
+ mpn_mullow_n (tp, rp, pp + n * (expbits >> 1), n);
+ MPN_COPY (rp, tp, n);
+ }
+
+ done:
+ TMP_FREE;
+}
projects / msp430-gcc.git / blobdiff