github.com/aergoio/aergo@v1.3.1/libtool/src/gmp-6.1.2/mpz/scan0.c (about)

     1  /* mpz_scan0 -- search for a 0 bit.
     2  
     3  Copyright 2000-2002, 2004, 2012 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 u>0 search since there might not be a 0
    37     bit before the end of the data.  mpn_scan1 could be used for the inverted
    38     search under u<0, but usually the search won't go very far so it seems
    39     reasonable to inline that code.  */
    40  
    41  mp_bitcnt_t
    42  mpz_scan0 (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;
    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    /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
    54       u<0.  Notice this test picks up all cases of u==0 too. */
    55    if (starting_limb >= abs_size)
    56      return (size >= 0 ? starting_bit : ~(mp_bitcnt_t) 0);
    57  
    58    limb = *p;
    59  
    60    if (size >= 0)
    61      {
    62        /* Mask to 1 all bits before starting_bit, thus ignoring them. */
    63        limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
    64  
    65        /* Search for a limb which isn't all ones.  If the end is reached then
    66  	 the zero bit immediately past the end is returned.  */
    67        while (limb == GMP_NUMB_MAX)
    68  	{
    69  	  p++;
    70  	  if (p == u_end)
    71  	    return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
    72  	  limb = *p;
    73  	}
    74  
    75        /* Now seek low 1 bit. */
    76        limb = ~limb;
    77      }
    78    else
    79      {
    80        mp_srcptr  q;
    81  
    82        /* If there's a non-zero limb before ours then we're in the ones
    83  	 complement region.  Search from *(p-1) downwards since that might
    84  	 give better cache locality, and since a non-zero in the middle of a
    85  	 number is perhaps a touch more likely than at the end.  */
    86        q = p;
    87        while (q != u_ptr)
    88  	{
    89  	  q--;
    90  	  if (*q != 0)
    91  	    goto inverted;
    92  	}
    93  
    94        /* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
    95  	 If limb==0 here then this isn't the beginning of twos complement
    96  	 inversion, but that doesn't matter because limb==0 is a zero bit
    97  	 immediately (-1 is all ones for below).  */
    98        limb--;
    99  
   100      inverted:
   101        /* Now seeking a 1 bit. */
   102  
   103        /* Mask to 0 all bits before starting_bit, thus ignoring them. */
   104        limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
   105  
   106        if (limb == 0)
   107  	{
   108  	  /* If the high limb is zero after masking, then no 1 bits past
   109  	     starting_bit.  */
   110  	  p++;
   111  	  if (p == u_end)
   112  	    return ~(mp_bitcnt_t) 0;
   113  
   114  	  /* Search further for a non-zero limb.  The high limb is non-zero,
   115  	     if nothing else.  */
   116  	  for (;;)
   117  	    {
   118  	      limb = *p;
   119  	      if (limb != 0)
   120  		break;
   121  	      p++;
   122  	      ASSERT (p < u_end);
   123  	    }
   124  	}
   125      }
   126  
   127    ASSERT (limb != 0);
   128    count_trailing_zeros (cnt, limb);
   129    return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
   130  }