contrib/gmp/mpz/scan0.c

   1 /* mpz_scan0 -- search for a 0 bit.
   2
   3 Copyright 2000, 2001, 2002, 2004 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 the GNU Lesser General Public License as published by
   9 the Free Software Foundation; either version 3 of the License, or (at your
  10 option) any later version.
  11
  12 The GNU MP Library is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  14 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  15 License for more details.
  16
  17 You should have received a copy of the GNU Lesser General Public License
  18 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  19
  20 #include "gmp.h"
  21 #include "gmp-impl.h"
  22 #include "longlong.h"
  23
  24
  25 /* mpn_scan0 can't be used for the u>0 search since there might not be a 0
  26    bit before the end of the data.  mpn_scan1 could be used for the inverted
  27    search under u<0, but usually the search won't go very far so it seems
  28    reasonable to inline that code.  */
  29
  30 unsigned long
  31 mpz_scan0 (mpz_srcptr u, unsigned long starting_bit)
  32 {
  33   mp_srcptr      u_ptr = PTR(u);
  34   mp_size_t      size = SIZ(u);
  35   mp_size_t      abs_size = ABS(size);
  36   mp_srcptr      u_end = u_ptr + abs_size;
  37   unsigned long  starting_limb = starting_bit / GMP_NUMB_BITS;
  38   mp_srcptr      p = u_ptr + starting_limb;
  39   mp_limb_t      limb;
  40   int            cnt;
  41
  42   /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
  43      u<0.  Notice this test picks up all cases of u==0 too. */
  44   if (starting_limb >= abs_size)
  45     return (size >= 0 ? starting_bit : ULONG_MAX);
  46
  47   limb = *p;
  48
  49   if (size >= 0)
  50     {
  51       /* Mask to 1 all bits before starting_bit, thus ignoring them. */
  52       limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
  53
  54       /* Search for a limb which isn't all ones.  If the end is reached then
  55          the zero bit immediately past the end is returned.  */
  56       while (limb == GMP_NUMB_MAX)
  57         {
  58           p++;
  59           if (p == u_end)
  60             return (unsigned long) abs_size * GMP_NUMB_BITS;
  61           limb = *p;
  62         }
  63
  64       /* Now seek low 1 bit. */
  65       limb = ~limb;
  66     }
  67   else
  68     {
  69       mp_srcptr  q;
  70
  71       /* If there's a non-zero limb before ours then we're in the ones
  72          complement region.  Search from *(p-1) downwards since that might
  73          give better cache locality, and since a non-zero in the middle of a
  74          number is perhaps a touch more likely than at the end.  */
  75       q = p;
  76       while (q != u_ptr)
  77         {
  78           q--;
  79           if (*q != 0)
  80             goto inverted;
  81         }
  82
  83       /* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
  84          If limb==0 here then this isn't the beginning of twos complement
  85          inversion, but that doesn't matter because limb==0 is a zero bit
  86          immediately (-1 is all ones for below).  */
  87       limb--;
  88
  89     inverted:
  90       /* Now seeking a 1 bit. */
  91
  92       /* Mask to 0 all bits before starting_bit, thus ignoring them. */
  93       limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
  94
  95       if (limb == 0)
  96         {
  97           /* If the high limb is zero after masking, then no 1 bits past
  98              starting_bit.  */
  99           p++;
 100           if (p == u_end)
 101             return ULONG_MAX;
 102
 103           /* Search further for a non-zero limb.  The high limb is non-zero,
 104              if nothing else.  */
 105           for (;;)
 106             {
 107               limb = *p;
 108               if (limb != 0)
 109                 break;
 110               p++;
 111               ASSERT (p < u_end);
 112             }
 113         }
 114     }
 115
 116   ASSERT (limb != 0);
 117   count_trailing_zeros (cnt, limb);
 118   return (p - u_ptr) * GMP_NUMB_BITS + cnt;
 119 }