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      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-impl.h"
     32 #include "longlong.h"
     33 
     34 
     35 /* mpn_scan0 can't be used for the u>0 search since there might not be a 0
     36    bit before the end of the data.  mpn_scan1 could be used for the inverted
     37    search under u<0, but usually the search won't go very far so it seems
     38    reasonable to inline that code.  */
     39 
     40 mp_bitcnt_t
     41 mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW
     42 {
     43   mp_srcptr      u_ptr = PTR(u);
     44   mp_size_t      size = SIZ(u);
     45   mp_size_t      abs_size = ABS(size);
     46   mp_srcptr      u_end = u_ptr + abs_size;
     47   mp_size_t      starting_limb = starting_bit / GMP_NUMB_BITS;
     48   mp_srcptr      p = u_ptr + starting_limb;
     49   mp_limb_t      limb;
     50   int            cnt;
     51 
     52   /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
     53      u<0.  Notice this test picks up all cases of u==0 too. */
     54   if (starting_limb >= abs_size)
     55     return (size >= 0 ? starting_bit : ~(mp_bitcnt_t) 0);
     56 
     57   limb = *p;
     58 
     59   if (size >= 0)
     60     {
     61       /* Mask to 1 all bits before starting_bit, thus ignoring them. */
     62       limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
     63 
     64       /* Search for a limb which isn't all ones.  If the end is reached then
     65 	 the zero bit immediately past the end is returned.  */
     66       while (limb == GMP_NUMB_MAX)
     67 	{
     68 	  p++;
     69 	  if (p == u_end)
     70 	    return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
     71 	  limb = *p;
     72 	}
     73 
     74       /* Now seek low 1 bit. */
     75       limb = ~limb;
     76     }
     77   else
     78     {
     79       mp_srcptr  q;
     80 
     81       /* If there's a non-zero limb before ours then we're in the ones
     82 	 complement region.  Search from *(p-1) downwards since that might
     83 	 give better cache locality, and since a non-zero in the middle of a
     84 	 number is perhaps a touch more likely than at the end.  */
     85       q = p;
     86       while (q != u_ptr)
     87 	{
     88 	  q--;
     89 	  if (*q != 0)
     90 	    goto inverted;
     91 	}
     92 
     93       /* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
     94 	 If limb==0 here then this isn't the beginning of twos complement
     95 	 inversion, but that doesn't matter because limb==0 is a zero bit
     96 	 immediately (-1 is all ones for below).  */
     97       limb--;
     98 
     99     inverted:
    100       /* Now seeking a 1 bit. */
    101 
    102       /* Mask to 0 all bits before starting_bit, thus ignoring them. */
    103       limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
    104 
    105       if (limb == 0)
    106 	{
    107 	  /* If the high limb is zero after masking, then no 1 bits past
    108 	     starting_bit.  */
    109 	  p++;
    110 	  if (p == u_end)
    111 	    return ~(mp_bitcnt_t) 0;
    112 
    113 	  /* Search further for a non-zero limb.  The high limb is non-zero,
    114 	     if nothing else.  */
    115 	  for (;;)
    116 	    {
    117 	      limb = *p;
    118 	      if (limb != 0)
    119 		break;
    120 	      p++;
    121 	      ASSERT (p < u_end);
    122 	    }
    123 	}
    124     }
    125 
    126   ASSERT (limb != 0);
    127   count_trailing_zeros (cnt, limb);
    128   return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
    129 }
    130