source/libs/gmp/gmp-src/mpz/scan1.c

   1 /* mpz_scan1 -- search for a 1 bit.
   2
   3 Copyright 2000-2002, 2004, 2012, 2015 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 #include "longlong.h"
  34
  35
  36 /* mpn_scan0 can't be used for the inverted u<0 search since there might not
  37    be a 0 bit before the end of the data.  mpn_scan1 could be used under u>0
  38    (except when in the high limb), but usually the search won't go very far
  39    so it seems reasonable to inline that code.  */
  40
  41 mp_bitcnt_t
  42 mpz_scan1 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW
  43 {
  44   mp_srcptr      u_ptr = PTR(u);
  45   mp_size_t      size = SIZ(u);
  46   mp_size_t      abs_size = ABS(size);
  47   mp_srcptr      u_end = u_ptr + abs_size - 1;
  48   mp_size_t      starting_limb = starting_bit / GMP_NUMB_BITS;
  49   mp_srcptr      p = u_ptr + starting_limb;
  50   mp_limb_t      limb;
  51   int            cnt;
  52
  53   /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
  54      Notice this test picks up any u==0 too. */
  55   if (starting_limb >= abs_size)
  56     return (size >= 0 ? ~(mp_bitcnt_t) 0 : starting_bit);
  57
  58   /* This is an important case, where sign is not relevant! */
  59   if (starting_bit == 0)
  60     goto short_cut;
  61
  62   limb = *p;
  63
  64   if (size >= 0)
  65     {
  66       /* Mask to 0 all bits before starting_bit, thus ignoring them. */
  67       limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
  68
  69       if (limb == 0)
  70         {
  71           /* If it's the high limb which is zero after masking, then there's
  72              no 1 bits after starting_bit.  */
  73           if (p == u_end)
  74             return ~(mp_bitcnt_t) 0;
  75
  76           /* Otherwise search further for a non-zero limb.  The high limb is
  77              non-zero, if nothing else.  */
  78         search_nonzero:
  79           do
  80             {
  81               ASSERT (p != u_end);
  82               p++;
  83             short_cut:
  84               limb = *p;
  85             }
  86           while (limb == 0);
  87         }
  88     }
  89   else
  90     {
  91       /* If there's a non-zero limb before ours then we're in the ones
  92          complement region.  */
  93       if (starting_limb == 0 || mpn_zero_p (u_ptr, starting_limb)) {
  94         if (limb == 0)
  95           /* Seeking for the first non-zero bit, it is the same for u and -u. */
  96           goto search_nonzero;
  97
  98         /* Adjust so ~limb implied by searching for 0 bit becomes -limb.  */
  99         limb--;
 100       }
 101
 102       /* Now seeking a 0 bit. */
 103
 104       /* Mask to 1 all bits before starting_bit, thus ignoring them. */
 105       limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
 106
 107       /* Search for a limb which is not all ones.  If the end is reached
 108          then the zero immediately past the end is the result.  */
 109       while (limb == GMP_NUMB_MAX)
 110         {
 111           if (p == u_end)
 112             return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
 113           p++;
 114           limb = *p;
 115         }
 116
 117       /* Now seeking low 1 bit. */
 118       limb = ~limb;
 119     }
 120
 121   ASSERT (limb != 0);
 122   count_trailing_zeros (cnt, limb);
 123   return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
 124 }