Coverage Report

Created: 2025-03-06 06:58

/src/gmp/mpn/powlo.c
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/* mpn_powlo -- Compute R = U^E mod B^n, where B is the limb base.
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Copyright 2007-2009, 2012, 2015, 2016, 2018, 2020 Free Software
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Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of either:
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  * the GNU Lesser General Public License as published by the Free
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    Software Foundation; either version 3 of the License, or (at your
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    option) any later version.
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or
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  * the GNU General Public License as published by the Free Software
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    Foundation; either version 2 of the License, or (at your option) any
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    later version.
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or both in parallel, as here.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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for more details.
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You should have received copies of the GNU General Public License and the
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GNU Lesser General Public License along with the GNU MP Library.  If not,
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see https://www.gnu.org/licenses/.  */
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#include "gmp-impl.h"
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#include "longlong.h"
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#define getbit(p,bi) \
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  ((p[(bi - 1) / GMP_LIMB_BITS] >> (bi - 1) % GMP_LIMB_BITS) & 1)
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static inline mp_limb_t
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getbits (const mp_limb_t *p, mp_bitcnt_t bi, unsigned nbits)
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{
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  unsigned nbits_in_r;
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  mp_limb_t r;
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  mp_size_t i;
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  if (bi <= nbits)
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    {
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      return p[0] & (((mp_limb_t) 1 << bi) - 1);
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    }
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  else
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    {
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      bi -= nbits;      /* bit index of low bit to extract */
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      i = bi / GMP_NUMB_BITS;   /* word index of low bit to extract */
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      bi %= GMP_NUMB_BITS;   /* bit index in low word */
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      r = p[i] >> bi;     /* extract (low) bits */
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      nbits_in_r = GMP_NUMB_BITS - bi;  /* number of bits now in r */
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      if (nbits_in_r < nbits)   /* did we get enough bits? */
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  r += p[i + 1] << nbits_in_r; /* prepend bits from higher word */
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      return r & (((mp_limb_t) 1 << nbits) - 1);
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    }
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}
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static inline unsigned
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win_size (mp_bitcnt_t eb)
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{
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  unsigned k;
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  static mp_bitcnt_t x[] = {7,25,81,241,673,1793,4609,11521,28161,~(mp_bitcnt_t)0};
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  ASSERT (eb > 1);
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  for (k = 0; eb > x[k++];)
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    ;
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  return k;
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}
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/* rp[n-1..0] = bp[n-1..0] ^ ep[en-1..0] mod B^n, B is the limb base.
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   Requires that ep[en-1] is non-zero.
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   Uses scratch space tp[3n-1..0], i.e., 3n words.  */
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/* We only use n words in the scratch space, we should pass tp + n to
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   mullo/sqrlo as a temporary area, it is needed. */
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void
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mpn_powlo (mp_ptr rp, mp_srcptr bp,
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     mp_srcptr ep, mp_size_t en,
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     mp_size_t n, mp_ptr tp)
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{
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  unsigned cnt;
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  mp_bitcnt_t ebi;
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  unsigned windowsize, this_windowsize;
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  mp_limb_t expbits;
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  mp_limb_t *pp;
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  long i;
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  int flipflop;
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  TMP_DECL;
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  ASSERT (en > 1 || (en == 1 && ep[0] > 1));
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  TMP_MARK;
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  MPN_SIZEINBASE_2EXP(ebi, ep, en, 1);
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  windowsize = win_size (ebi);
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  if (windowsize > 1)
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    {
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      mp_limb_t *this_pp, *last_pp;
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      ASSERT (windowsize < ebi);
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      pp = TMP_ALLOC_LIMBS ((n << (windowsize - 1)));
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      this_pp = pp;
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      MPN_COPY (this_pp, bp, n);
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      /* Store b^2 in tp.  */
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      mpn_sqrlo (tp, bp, n);
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      /* Precompute odd powers of b and put them in the temporary area at pp.  */
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      i = (1 << (windowsize - 1)) - 1;
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      do
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  {
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    last_pp = this_pp;
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    this_pp += n;
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    mpn_mullo_n (this_pp, last_pp, tp, n);
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  } while (--i != 0);
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      expbits = getbits (ep, ebi, windowsize);
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      ebi -= windowsize;
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      /* THINK: Should we initialise the case expbits % 4 == 0 with a mullo? */
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      count_trailing_zeros (cnt, expbits);
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      ebi += cnt;
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      expbits >>= cnt;
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      MPN_COPY (rp, pp + n * (expbits >> 1), n);
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    }
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  else
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    {
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      pp = tp + n;
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      MPN_COPY (pp, bp, n);
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      MPN_COPY (rp, bp, n);
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      --ebi;
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    }
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  flipflop = 0;
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  do
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    {
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      while (getbit (ep, ebi) == 0)
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  {
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    mpn_sqrlo (tp, rp, n);
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    MP_PTR_SWAP (rp, tp);
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    flipflop = ! flipflop;
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    if (--ebi == 0)
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      goto done;
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  }
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      /* The next bit of the exponent is 1.  Now extract the largest block of
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   bits <= windowsize, and such that the least significant bit is 1.  */
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      expbits = getbits (ep, ebi, windowsize);
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      this_windowsize = MIN (windowsize, ebi);
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      count_trailing_zeros (cnt, expbits);
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      this_windowsize -= cnt;
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      ebi -= this_windowsize;
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      expbits >>= cnt;
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      while (this_windowsize > 1)
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  {
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    mpn_sqrlo (tp, rp, n);
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    mpn_sqrlo (rp, tp, n);
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    this_windowsize -= 2;
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  }
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      if (this_windowsize != 0)
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  mpn_sqrlo (tp, rp, n);
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      else
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  {
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    MP_PTR_SWAP (rp, tp);
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    flipflop = ! flipflop;
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  }
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      mpn_mullo_n (rp, tp, pp + n * (expbits >> 1), n);
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    } while (ebi != 0);
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 done:
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  if (flipflop)
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    MPN_COPY (tp, rp, n);
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  TMP_FREE;
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}