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Diffstat (limited to 'gmp/mpz/scan0.c')
| -rw-r--r-- | gmp/mpz/scan0.c | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/gmp/mpz/scan0.c b/gmp/mpz/scan0.c new file mode 100644 index 0000000000..de24595cfe --- /dev/null +++ b/gmp/mpz/scan0.c @@ -0,0 +1,130 @@ +/* mpz_scan0 -- search for a 0 bit. + +Copyright 2000-2002, 2004, 2012 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 either: + + * 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. + +or + + * the GNU General Public License as published by the Free Software + Foundation; either version 2 of the License, or (at your option) any + later version. + +or both in parallel, as here. + +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 General Public License +for more details. + +You should have received copies of the GNU General Public License and the +GNU Lesser General Public License along with the GNU MP Library. If not, +see https://www.gnu.org/licenses/. */ + +#include "gmp.h" +#include "gmp-impl.h" +#include "longlong.h" + + +/* mpn_scan0 can't be used for the u>0 search since there might not be a 0 + bit before the end of the data. mpn_scan1 could be used for the inverted + search under u<0, but usually the search won't go very far so it seems + reasonable to inline that code. */ + +mp_bitcnt_t +mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW +{ + 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; + mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS; + mp_srcptr p = u_ptr + starting_limb; + mp_limb_t limb; + int cnt; + + /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for + u<0. Notice this test picks up all cases of u==0 too. */ + if (starting_limb >= abs_size) + return (size >= 0 ? starting_bit : ~(mp_bitcnt_t) 0); + + limb = *p; + + if (size >= 0) + { + /* 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 isn't all ones. If the end is reached then + the zero bit immediately past the end is returned. */ + while (limb == GMP_NUMB_MAX) + { + p++; + if (p == u_end) + return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS; + limb = *p; + } + + /* Now seek low 1 bit. */ + limb = ~limb; + } + 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; + } + + /* Adjust so ~limb implied by searching for 1 bit below becomes -limb. + If limb==0 here then this isn't the beginning of twos complement + inversion, but that doesn't matter because limb==0 is a zero bit + immediately (-1 is all ones for below). */ + limb--; + + inverted: + /* Now seeking a 1 bit. */ + + /* 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 the high limb is zero after masking, then no 1 bits past + starting_bit. */ + p++; + if (p == u_end) + return ~(mp_bitcnt_t) 0; + + /* 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); + } + } + } + + ASSERT (limb != 0); + count_trailing_zeros (cnt, limb); + return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt; +} |