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/* mpn_divexact_by3 -- mpn division by 3, expecting no remainder. */
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Copyright (C) 2000 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
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/* Multiplicative inverse of 3, modulo 2^BITS_PER_MP_LIMB.
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0xAAAAAAAB for 32 bits, 0xAAAAAAAAAAAAAAAB for 64 bits. */
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#define INVERSE_3 ((MP_LIMB_T_MAX / 3) * 2 + 1)
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/* The "c += ..."s are adding the high limb of 3*l to c. That high limb
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will be 0, 1 or 2. Doing two separate "+="s seems to turn out better
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code on gcc (as of 2.95.2 at least).
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When a subtraction of a 0,1,2 carry value causes a borrow, that leaves a
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limb value of either 0xFF...FF or 0xFF...FE and the multiply by INVERSE_3
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gives 0x55...55 or 0xAA...AA respectively, producing a further borrow of
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only 0 or 1 respectively. Hence the carry out of each stage and for the
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return value is always only 0, 1 or 2. */
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mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t c)
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mpn_divexact_by3c (dst, src, size, c)
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c += (l > MP_LIMB_T_MAX/3);
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c += (l > (MP_LIMB_T_MAX/3)*2);