contrib/gmp/mpz/scan1.c

   1 /* mpz_scan1 -- search for a 1 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 inverted u<0 search since there might not
  26    be a 0 bit before the end of the data.  mpn_scan1 could be used under u>0
  27    (except when in the high limb), but usually the search won't go very far
  28    so it seems reasonable to inline that code.  */
  29
  30 unsigned long
  31 mpz_scan1 (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   /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
  43      Notice this test picks up any u==0 too. */
  44   if (starting_limb >= abs_size)
  45     return (size >= 0 ? ULONG_MAX : starting_bit);
  46
  47   limb = *p;
  48
  49   if (size >= 0)
  50     {
  51       /* Mask to 0 all bits before starting_bit, thus ignoring them. */
  52       limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
  53
  54       if (limb == 0)
  55         {
  56           /* If it's the high limb which is zero after masking, then there's
  57              no 1 bits after starting_bit.  */
  58           p++;
  59           if (p == u_end)
  60             return ULONG_MAX;
  61
  62           /* Otherwise search further for a non-zero limb.  The high limb is
  63              non-zero, if nothing else.  */
  64           for (;;)
  65             {
  66               limb = *p;
  67               if (limb != 0)
  68                 break;
  69               p++;
  70               ASSERT (p < u_end);
  71             }
  72         }
  73     }
  74   else
  75     {
  76       mp_srcptr  q;
  77
  78       /* If there's a non-zero limb before ours then we're in the ones
  79          complement region.  Search from *(p-1) downwards since that might
  80          give better cache locality, and since a non-zero in the middle of a
  81          number is perhaps a touch more likely than at the end.  */
  82       q = p;
  83       while (q != u_ptr)
  84         {
  85           q--;
  86           if (*q != 0)
  87             goto inverted;
  88         }
  89
  90       if (limb == 0)
  91         {
  92           /* Skip zero limbs, to find the start of twos complement.  The
  93              high limb is non-zero, if nothing else.  This search is
  94              necessary so the -limb is applied at the right spot. */
  95           do
  96             {
  97               p++;
  98               ASSERT (p < u_end);
  99               limb = *p;
 100             }
 101           while (limb == 0);
 102
 103           /* Apply twos complement, and look for a 1 bit in that.  Since
 104              limb!=0 here, also have (-limb)!=0 so there's certainly a 1
 105              bit.  */
 106           limb = -limb;
 107           goto got_limb;
 108         }
 109
 110       /* Adjust so ~limb implied by searching for 0 bit becomes -limb.  */
 111       limb--;
 112
 113     inverted:
 114       /* Now seeking a 0 bit. */
 115
 116       /* Mask to 1 all bits before starting_bit, thus ignoring them. */
 117       limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
 118
 119       /* Search for a limb which is not all ones.  If the end is reached
 120          then the zero immediately past the end is the result.  */
 121       while (limb == GMP_NUMB_MAX)
 122         {
 123           p++;
 124           if (p == u_end)
 125             return abs_size * GMP_NUMB_BITS;
 126           limb = *p;
 127         }
 128
 129       /* Now seeking low 1 bit. */
 130       limb = ~limb;
 131     }
 132
 133  got_limb:
 134   ASSERT (limb != 0);
 135   count_trailing_zeros (cnt, limb);
 136   return (p - u_ptr) * GMP_NUMB_BITS + cnt;
 137 }