source/libs/gmp/gmp-src/mpz/tstbit.c

   1 /* mpz_tstbit -- test a specified bit.
   2
   3 Copyright 2000, 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 either:
   9
  10   * the GNU Lesser General Public License as published by the Free
  11     Software Foundation; either version 3 of the License, or (at your
  12     option) any later version.
  13
  14 or
  15
  16   * the GNU General Public License as published by the Free Software
  17     Foundation; either version 2 of the License, or (at your option) any
  18     later version.
  19
  20 or both in parallel, as here.
  21
  22 The GNU MP Library is distributed in the hope that it will be useful, but
  23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  24 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  25 for more details.
  26
  27 You should have received copies of the GNU General Public License and the
  28 GNU Lesser General Public License along with the GNU MP Library.  If not,
  29 see https://www.gnu.org/licenses/.  */
  30
  31 #include "gmp.h"
  32 #include "gmp-impl.h"
  33
  34
  35 /* For negatives the effective twos complement is achieved by negating the
  36    limb tested, either with a ones or twos complement.  Twos complement
  37    ("-") is used if there's only zero limbs below the one being tested.
  38    Ones complement ("~") is used if there's a non-zero below.  Note that "-"
  39    is correct even if the limb examined is 0 (and the true beginning of twos
  40    complement is further up).
  41
  42    Testing the limbs below p is unavoidable on negatives, but will usually
  43    need to examine only *(p-1).  The search is done from *(p-1) down to
  44    *u_ptr, since that might give better cache locality, and because a
  45    non-zero limb is perhaps a touch more likely in the middle of a number
  46    than at the low end.
  47
  48    Bits past the end of available data simply follow sign of u.  Notice that
  49    the limb_index >= abs_size test covers u=0 too.  */
  50
  51 int
  52 mpz_tstbit (mpz_srcptr u, mp_bitcnt_t bit_index) __GMP_NOTHROW
  53 {
  54   mp_srcptr      u_ptr      = PTR(u);
  55   mp_size_t      size       = SIZ(u);
  56   unsigned       abs_size   = ABS(size);
  57   mp_size_t      limb_index = bit_index / GMP_NUMB_BITS;
  58   mp_srcptr      p          = u_ptr + limb_index;
  59   mp_limb_t      limb;
  60
  61   if (limb_index >= abs_size)
  62     return (size < 0);
  63
  64   limb = *p;
  65   if (size < 0)
  66     {
  67       limb = -limb;     /* twos complement */
  68
  69       while (p != u_ptr)
  70         {
  71           p--;
  72           if (*p != 0)
  73             {
  74               limb--;   /* make it a ones complement instead */
  75               break;
  76             }
  77         }
  78     }
  79
  80   return (limb >> (bit_index % GMP_NUMB_BITS)) & 1;
  81 }