source/libs/gmp/gmp-src/mpz/scan0.c

   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 }