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Annotation of OpenXM_contrib/gmp/mpz/cong_2exp.c, Revision 1.1.1.1

1.1       ohara       1: /* mpz_congruent_2exp_p -- test congruence of mpz mod 2^n.
                      2:
                      3: Copyright 2001, 2002 Free Software Foundation, Inc.
                      4:
                      5: This file is part of the GNU MP Library.
                      6:
                      7: The GNU MP Library is free software; you can redistribute it and/or modify
                      8: it under the terms of the GNU Lesser General Public License as published by
                      9: the Free Software Foundation; either version 2.1 of the License, or (at your
                     10: option) any later version.
                     11:
                     12: The GNU MP Library is distributed in the hope that it will be useful, but
                     13: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
                     14: or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
                     15: License for more details.
                     16:
                     17: You should have received a copy of the GNU Lesser General Public License
                     18: along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
                     19: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
                     20: MA 02111-1307, USA. */
                     21:
                     22: #include "gmp.h"
                     23: #include "gmp-impl.h"
                     24:
                     25:
                     26: int
                     27: mpz_congruent_2exp_p (mpz_srcptr a, mpz_srcptr c, unsigned long d)
                     28: {
                     29:   unsigned long  i, dlimbs, dbits;
                     30:   mp_ptr         ap, cp;
                     31:   mp_limb_t      dmask, alimb, climb, sum;
                     32:   mp_size_t      asize_signed, csize_signed, asize, csize;
                     33:
                     34:   if (ABSIZ(a) < ABSIZ(c))
                     35:     MPZ_SRCPTR_SWAP (a, c);
                     36:
                     37:   dlimbs = d / GMP_NUMB_BITS;
                     38:   dbits = d % GMP_NUMB_BITS;
                     39:   dmask = (CNST_LIMB(1) << dbits) - 1;
                     40:
                     41:   ap = PTR(a);
                     42:   cp = PTR(c);
                     43:
                     44:   asize_signed = SIZ(a);
                     45:   asize = ABS(asize_signed);
                     46:
                     47:   csize_signed = SIZ(c);
                     48:   csize = ABS(csize_signed);
                     49:
                     50:   if (csize_signed == 0)
                     51:     goto a_zeros;
                     52:
                     53:   if ((asize_signed ^ csize_signed) >= 0)
                     54:     {
                     55:       /* same signs, direct comparison */
                     56:
                     57:       /* a==c for limbs in common */
                     58:       if (mpn_cmp (ap, cp, MIN (csize, dlimbs)) != 0)
                     59:         return 0;
                     60:
                     61:       /* if that's all of dlimbs, then a==c for remaining bits */
                     62:       if (csize > dlimbs)
                     63:         return ((ap[dlimbs]-cp[dlimbs]) & dmask) == 0;
                     64:
                     65:     a_zeros:
                     66:       /* a remains, need all zero bits */
                     67:
                     68:       /* if d covers all of a and c, then must be exactly equal */
                     69:       if (asize <= dlimbs)
                     70:         return asize == csize;
                     71:
                     72:       /* whole limbs zero */
                     73:       for (i = csize; i < dlimbs; i++)
                     74:         if (ap[i] != 0)
                     75:           return 0;
                     76:
                     77:       /* partial limb zero */
                     78:       return (ap[dlimbs] & dmask) == 0;
                     79:     }
                     80:   else
                     81:     {
                     82:       /* different signs, negated comparison */
                     83:
                     84:       /* common low zero limbs, stopping at first non-zeros, which must
                     85:          match twos complement */
                     86:       i = 0;
                     87:       for (;;)
                     88:         {
                     89:           ASSERT (i < csize);  /* always have a non-zero limb on c */
                     90:           alimb = ap[i];
                     91:           climb = cp[i];
                     92:           sum = (alimb + climb) & GMP_NUMB_MASK;
                     93:
                     94:           if (i >= dlimbs)
                     95:             return (sum & dmask) == 0;
                     96:           i++;
                     97:
                     98:           /* require both zero, or first non-zeros as twos-complements */
                     99:           if (sum != 0)
                    100:             return 0;
                    101:
                    102:           if (alimb != 0)
                    103:             break;
                    104:         }
                    105:
                    106:       /* further limbs matching as ones-complement */
                    107:       for (;;)
                    108:         {
                    109:           if (i >= csize)
                    110:             break;
                    111:
                    112:           alimb = ap[i];
                    113:           climb = cp[i];
                    114:           sum = (alimb + climb + 1) & GMP_NUMB_MASK;
                    115:
                    116:           if (i >= dlimbs)
                    117:             return (sum & dmask) == 0;
                    118:
                    119:           if (sum != 0)
                    120:             return 0;
                    121:
                    122:           i++;
                    123:         }
                    124:
                    125:       /* no more c, so require all 1 bits in a */
                    126:
                    127:       if (asize < dlimbs)
                    128:         return 0;   /* not enough a */
                    129:
                    130:       /* whole limbs */
                    131:       for ( ; i < dlimbs; i++)
                    132:         if (ap[i] != GMP_NUMB_MAX)
                    133:           return 0;
                    134:
                    135:       /* if only whole limbs, no further fetches from a */
                    136:       if (dbits == 0)
                    137:         return 1;
                    138:
                    139:       /* need enough a */
                    140:       if (asize == dlimbs)
                    141:         return 0;
                    142:
                    143:       return ((ap[dlimbs]+1) & dmask) == 0;
                    144:     }
                    145: }