| /* mpz_scan1 -- search for a 1 bit. |
| |
| Copyright 2000, 2001, 2002, 2004 Free Software Foundation, Inc. |
| |
| This file is part of the GNU MP Library. |
| |
| The GNU MP Library is free software; you can redistribute it and/or modify |
| it under the terms of the GNU Lesser General Public License as published by |
| the Free Software Foundation; either version 3 of the License, or (at your |
| option) any later version. |
| |
| The GNU MP Library is distributed in the hope that it will be useful, but |
| WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY |
| or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public |
| License for more details. |
| |
| You should have received a copy of the GNU Lesser General Public License |
| along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */ |
| |
| #include "gmp.h" |
| #include "gmp-impl.h" |
| #include "longlong.h" |
| |
| |
| /* mpn_scan0 can't be used for the inverted u<0 search since there might not |
| be a 0 bit before the end of the data. mpn_scan1 could be used under u>0 |
| (except when in the high limb), but usually the search won't go very far |
| so it seems reasonable to inline that code. */ |
| |
| unsigned long |
| mpz_scan1 (mpz_srcptr u, unsigned long starting_bit) |
| { |
| mp_srcptr u_ptr = PTR(u); |
| mp_size_t size = SIZ(u); |
| mp_size_t abs_size = ABS(size); |
| mp_srcptr u_end = u_ptr + abs_size; |
| unsigned long starting_limb = starting_bit / GMP_NUMB_BITS; |
| mp_srcptr p = u_ptr + starting_limb; |
| mp_limb_t limb; |
| int cnt; |
| |
| /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0. |
| Notice this test picks up any u==0 too. */ |
| if (starting_limb >= abs_size) |
| return (size >= 0 ? ULONG_MAX : starting_bit); |
| |
| limb = *p; |
| |
| if (size >= 0) |
| { |
| /* Mask to 0 all bits before starting_bit, thus ignoring them. */ |
| limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS)); |
| |
| if (limb == 0) |
| { |
| /* If it's the high limb which is zero after masking, then there's |
| no 1 bits after starting_bit. */ |
| p++; |
| if (p == u_end) |
| return ULONG_MAX; |
| |
| /* Otherwise search further for a non-zero limb. The high limb is |
| non-zero, if nothing else. */ |
| for (;;) |
| { |
| limb = *p; |
| if (limb != 0) |
| break; |
| p++; |
| ASSERT (p < u_end); |
| } |
| } |
| } |
| else |
| { |
| mp_srcptr q; |
| |
| /* If there's a non-zero limb before ours then we're in the ones |
| complement region. Search from *(p-1) downwards since that might |
| give better cache locality, and since a non-zero in the middle of a |
| number is perhaps a touch more likely than at the end. */ |
| q = p; |
| while (q != u_ptr) |
| { |
| q--; |
| if (*q != 0) |
| goto inverted; |
| } |
| |
| if (limb == 0) |
| { |
| /* Skip zero limbs, to find the start of twos complement. The |
| high limb is non-zero, if nothing else. This search is |
| necessary so the -limb is applied at the right spot. */ |
| do |
| { |
| p++; |
| ASSERT (p < u_end); |
| limb = *p; |
| } |
| while (limb == 0); |
| |
| /* Apply twos complement, and look for a 1 bit in that. Since |
| limb!=0 here, also have (-limb)!=0 so there's certainly a 1 |
| bit. */ |
| limb = -limb; |
| goto got_limb; |
| } |
| |
| /* Adjust so ~limb implied by searching for 0 bit becomes -limb. */ |
| limb--; |
| |
| inverted: |
| /* Now seeking a 0 bit. */ |
| |
| /* Mask to 1 all bits before starting_bit, thus ignoring them. */ |
| limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1; |
| |
| /* Search for a limb which is not all ones. If the end is reached |
| then the zero immediately past the end is the result. */ |
| while (limb == GMP_NUMB_MAX) |
| { |
| p++; |
| if (p == u_end) |
| return abs_size * GMP_NUMB_BITS; |
| limb = *p; |
| } |
| |
| /* Now seeking low 1 bit. */ |
| limb = ~limb; |
| } |
| |
| got_limb: |
| ASSERT (limb != 0); |
| count_trailing_zeros (cnt, limb); |
| return (p - u_ptr) * GMP_NUMB_BITS + cnt; |
| } |