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Annotation of OpenXM_contrib/gmp/mpz/scan1.c, Revision 1.1.1.3

1.1.1.3 ! ohara       1: /* mpz_scan1 -- search for a 1 bit.
1.1       maekawa     2:
1.1.1.3 ! ohara       3: Copyright 2000, 2001, 2002 Free Software Foundation, Inc.
1.1       maekawa     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
1.1.1.2   maekawa     8: it under the terms of the GNU Lesser General Public License as published by
                      9: the Free Software Foundation; either version 2.1 of the License, or (at your
1.1       maekawa    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
1.1.1.2   maekawa    14: or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
1.1       maekawa    15: License for more details.
                     16:
1.1.1.2   maekawa    17: You should have received a copy of the GNU Lesser General Public License
1.1       maekawa    18: along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
                     19: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
                     20: MA 02111-1307, USA. */
                     21:
                     22: #include "gmp.h"
                     23: #include "gmp-impl.h"
1.1.1.3 ! ohara      24: #include "longlong.h"
1.1       maekawa    25:
1.1.1.3 ! ohara      26:
        !            27: /* mpn_scan0 can't be used for the inverted u<0 search since there might not
        !            28:    be a 0 bit before the end of the data.  mpn_scan1 could be used under u>0
        !            29:    (except when in the high limb), but usually the search won't go very far
        !            30:    so it seems reasonable to inline that code.  */
        !            31:
        !            32: unsigned long
        !            33: mpz_scan1 (mpz_srcptr u, unsigned long starting_bit)
1.1       maekawa    34: {
1.1.1.3 ! ohara      35:   mp_srcptr      u_ptr = PTR(u);
        !            36:   mp_size_t      size = SIZ(u);
        !            37:   mp_size_t      abs_size = ABS(size);
        !            38:   mp_srcptr      u_end = u_ptr + abs_size;
        !            39:   unsigned long  starting_limb = starting_bit / GMP_NUMB_BITS;
        !            40:   mp_srcptr      p = u_ptr + starting_limb;
        !            41:   mp_limb_t      limb;
        !            42:   int            cnt;
        !            43:
        !            44:   /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
        !            45:      Notice this test picks up any u==0 too. */
        !            46:   if (starting_limb >= abs_size)
        !            47:     return (size >= 0 ? ULONG_MAX : starting_bit);
        !            48:
        !            49:   limb = *p;
        !            50:
        !            51:   if (size >= 0)
        !            52:     {
        !            53:       /* Mask to 0 all bits before starting_bit, thus ignoring them. */
        !            54:       limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
        !            55:
        !            56:       if (limb == 0)
        !            57:         {
        !            58:           /* If it's the high limb which is zero after masking, then there's
        !            59:              no 1 bits after starting_bit.  */
        !            60:           p++;
        !            61:           if (p == u_end)
        !            62:             return ULONG_MAX;
        !            63:
        !            64:           /* Otherwise search further for a non-zero limb.  The high limb is
        !            65:              non-zero, if nothing else.  */
        !            66:           for (;;)
        !            67:             {
        !            68:               limb = *p;
        !            69:               if (limb != 0)
        !            70:                 break;
        !            71:               p++;
        !            72:               ASSERT (p < u_end);
        !            73:             }
        !            74:         }
        !            75:     }
        !            76:   else
        !            77:     {
        !            78:       mp_srcptr  q;
        !            79:
        !            80:       /* If there's a non-zero limb before ours then we're in the ones
        !            81:          complement region.  Search from *(p-1) downwards since that might
        !            82:          give better cache locality, and since a non-zero in the middle of a
        !            83:          number is perhaps a touch more likely than at the end.  */
        !            84:       q = p;
        !            85:       while (q != u_ptr)
        !            86:         {
        !            87:           q--;
        !            88:           if (*q != 0)
        !            89:             goto inverted;
        !            90:         }
        !            91:
        !            92:       if (limb == 0)
        !            93:         {
        !            94:           /* Skip zero limbs, to find the start of twos complement.  The
        !            95:              high limb is non-zero, if nothing else.  This search is
        !            96:              necessary so the -limb is applied at the right spot. */
        !            97:           do
        !            98:             {
        !            99:               p++;
        !           100:               ASSERT (p < u_end);
        !           101:               limb = *p;
        !           102:             }
        !           103:           while (limb == 0);
        !           104:
        !           105:           /* Apply twos complement, and look for a 1 bit in that.  Since
        !           106:              limb!=0 here, also have (-limb)!=0 so there's certainly a 1
        !           107:              bit.  */
        !           108:           limb = -limb;
        !           109:           goto got_limb;
        !           110:         }
        !           111:
        !           112:       /* Adjust so ~limb implied by searching for 0 bit becomes -limb.  */
        !           113:       limb--;
        !           114:
        !           115:     inverted:
        !           116:       /* Now seeking a 0 bit. */
        !           117:
        !           118:       /* Mask to 1 all bits before starting_bit, thus ignoring them. */
        !           119:       limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
        !           120:
        !           121:       /* Search for a limb which is not all ones.  If the end is reached
        !           122:          then the zero immediately past the end is the result.  */
        !           123:       while (limb == GMP_NUMB_MAX)
        !           124:         {
        !           125:           p++;
        !           126:           if (p == u_end)
        !           127:             return abs_size * GMP_NUMB_BITS;
        !           128:           limb = *p;
        !           129:         }
        !           130:
        !           131:       /* Now seeking low 1 bit. */
        !           132:       limb = ~limb;
        !           133:     }
        !           134:
        !           135:  got_limb:
        !           136:   /* Mask to 0 all bits above the lowest 1 bit. */
        !           137:   ASSERT (limb != 0);
        !           138:   limb &= -limb;
        !           139:
        !           140:   count_leading_zeros (cnt, limb);
        !           141:   return (p - u_ptr) * GMP_NUMB_BITS + GMP_LIMB_BITS - 1 - cnt;
1.1       maekawa   142: }