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Annotation of OpenXM_contrib/gmp/mpn/generic/diveby3.c, Revision 1.1

1.1     ! maekawa     1: /* mpn_divexact_by3 -- mpn division by 3, expecting no remainder. */
        !             2:
        !             3: /*
        !             4: Copyright (C) 2000 Free Software Foundation, Inc.
        !             5:
        !             6: This file is part of the GNU MP Library.
        !             7:
        !             8: The GNU MP Library is free software; you can redistribute it and/or modify
        !             9: it under the terms of the GNU Lesser General Public License as published by
        !            10: the Free Software Foundation; either version 2.1 of the License, or (at your
        !            11: option) any later version.
        !            12:
        !            13: The GNU MP Library is distributed in the hope that it will be useful, but
        !            14: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
        !            15: or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
        !            16: License for more details.
        !            17:
        !            18: You should have received a copy of the GNU Lesser General Public License
        !            19: along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
        !            20: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
        !            21: MA 02111-1307, USA.
        !            22: */
        !            23:
        !            24:
        !            25: #include "gmp.h"
        !            26: #include "gmp-impl.h"
        !            27:
        !            28:
        !            29: /* Multiplicative inverse of 3, modulo 2^BITS_PER_MP_LIMB.
        !            30:    0xAAAAAAAB for 32 bits, 0xAAAAAAAAAAAAAAAB for 64 bits. */
        !            31: #define INVERSE_3      ((MP_LIMB_T_MAX / 3) * 2 + 1)
        !            32:
        !            33:
        !            34: /* The "c += ..."s are adding the high limb of 3*l to c.  That high limb
        !            35:    will be 0, 1 or 2.  Doing two separate "+="s seems to turn out better
        !            36:    code on gcc (as of 2.95.2 at least).
        !            37:
        !            38:    When a subtraction of a 0,1,2 carry value causes a borrow, that leaves a
        !            39:    limb value of either 0xFF...FF or 0xFF...FE and the multiply by INVERSE_3
        !            40:    gives 0x55...55 or 0xAA...AA respectively, producing a further borrow of
        !            41:    only 0 or 1 respectively.  Hence the carry out of each stage and for the
        !            42:    return value is always only 0, 1 or 2.  */
        !            43:
        !            44: mp_limb_t
        !            45: #if __STDC__
        !            46: mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t c)
        !            47: #else
        !            48: mpn_divexact_by3c (dst, src, size, c)
        !            49:      mp_ptr    dst;
        !            50:      mp_srcptr src;
        !            51:      mp_size_t size;
        !            52:      mp_limb_t c;
        !            53: #endif
        !            54: {
        !            55:   mp_size_t  i;
        !            56:
        !            57:   ASSERT (size >= 1);
        !            58:
        !            59:   i = 0;
        !            60:   do
        !            61:     {
        !            62:       mp_limb_t  l, s;
        !            63:
        !            64:       s = src[i];
        !            65:       l = s - c;
        !            66:       c = (l > s);
        !            67:
        !            68:       l *= INVERSE_3;
        !            69:       dst[i] = l;
        !            70:
        !            71:       c += (l > MP_LIMB_T_MAX/3);
        !            72:       c += (l > (MP_LIMB_T_MAX/3)*2);
        !            73:     }
        !            74:   while (++i < size);
        !            75:
        !            76:   return c;
        !            77: }