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Annotation of OpenXM_contrib/gmp/mpz/tstbit.c, Revision 1.1.1.2

1.1.1.2 ! ohara       1: /* mpz_tstbit -- test a specified bit.
1.1       maekawa     2:
1.1.1.2 ! ohara       3: Copyright 2000, 2002 Free Software Foundation, Inc.
1.1       maekawa     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:
1.1.1.2 ! ohara      25:
        !            26: /* For negatives the effective twos complement is achieved by negating the
        !            27:    limb tested, either with a ones or twos complement.  Twos complement
        !            28:    ("-") is used if there's only zero limbs below the one being tested.
        !            29:    Ones complement ("~") is used if there's a non-zero below.  Note that "-"
        !            30:    is correct even if the limb examined is 0 (and the true beginning of twos
        !            31:    complement is further up).
        !            32:
        !            33:    Testing the limbs below p is unavoidable on negatives, but will usually
        !            34:    need to examine only *(p-1).  The search is done from *(p-1) down to
        !            35:    *u_ptr, since that might give better cache locality, and because a
        !            36:    non-zero limb is perhaps a touch more likely in the middle of a number
        !            37:    than at the low end.
        !            38:
        !            39:    Bits past the end of available data simply follow sign of u.  Notice that
        !            40:    the limb_index >= abs_size test covers u=0 too.  */
        !            41:
1.1       maekawa    42: int
1.1.1.2 ! ohara      43: mpz_tstbit (mpz_srcptr u, unsigned long bit_index)
1.1       maekawa    44: {
1.1.1.2 ! ohara      45:   mp_srcptr      u_ptr      = PTR(u);
        !            46:   mp_size_t      size       = SIZ(u);
        !            47:   unsigned       abs_size   = ABS(size);
        !            48:   unsigned long  limb_index = bit_index / GMP_NUMB_BITS;
        !            49:   mp_srcptr      p          = u_ptr + limb_index;
        !            50:   mp_limb_t      limb;
1.1       maekawa    51:
1.1.1.2 ! ohara      52:   if (limb_index >= abs_size)
        !            53:     return (size < 0);
1.1       maekawa    54:
1.1.1.2 ! ohara      55:   limb = *p;
        !            56:   if (size < 0)
        !            57:     {
        !            58:       limb = -limb;     /* twos complement */
1.1       maekawa    59:
1.1.1.2 ! ohara      60:       while (p != u_ptr)
        !            61:         {
        !            62:           p--;
        !            63:           if (*p != 0)
        !            64:             {
        !            65:               limb--;   /* make it a ones complement instead */
        !            66:               break;
        !            67:             }
        !            68:         }
1.1       maekawa    69:     }
1.1.1.2 ! ohara      70:
        !            71:   return (limb >> (bit_index % GMP_NUMB_BITS)) & 1;
1.1       maekawa    72: }