1  /* mpn_scan1 -- Scan from a given bit position for the next set bit.
       2  
       3  Copyright 1994, 1996, 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 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  /* Argument constraints:
      35     1. U must sooner or later have a limb != 0.
      36   */
      37  
      38  mp_bitcnt_t
      39  mpn_scan1 (mp_srcptr up, mp_bitcnt_t starting_bit)
      40  {
      41    mp_size_t starting_word;
      42    mp_limb_t alimb;
      43    int cnt;
      44    mp_srcptr p;
      45  
      46    /* Start at the word implied by STARTING_BIT.  */
      47    starting_word = starting_bit / GMP_NUMB_BITS;
      48    p = up + starting_word;
      49    alimb = *p++;
      50  
      51    /* Mask off any bits before STARTING_BIT in the first limb.  */
      52    alimb &= - (mp_limb_t) 1 << (starting_bit % GMP_NUMB_BITS);
      53  
      54    while (alimb == 0)
      55      alimb = *p++;
      56  
      57    count_trailing_zeros (cnt, alimb);
      58    return (p - up - 1) * GMP_NUMB_BITS + cnt;
      59  }