mirror of
https://github.com/vale981/arb
synced 2025-03-06 09:51:39 -05:00
100 lines
2.9 KiB
C
100 lines
2.9 KiB
C
/*=============================================================================
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This file is part of ARB.
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ARB is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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ARB is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with ARB; if not, write to the Free Software
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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=============================================================================*/
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/******************************************************************************
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Copyright (C) 2014 Fredrik Johansson
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******************************************************************************/
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#include "mag.h"
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#include "arf.h"
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void
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mag_sub_lower(mag_t z, const mag_t x, const mag_t y)
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{
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if (mag_is_special(x) || mag_is_special(y))
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{
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if (mag_is_zero(y))
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mag_set(z, x);
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else if (mag_is_zero(x) || mag_is_inf(y))
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mag_zero(z);
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else
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mag_inf(z);
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}
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else
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{
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slong shift, fix;
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shift = _fmpz_sub_small(MAG_EXPREF(x), MAG_EXPREF(y));
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/* y > x */
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if (shift < 0)
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{
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mag_zero(z);
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}
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else if (shift == 0)
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{
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if (MAG_MAN(y) >= MAG_MAN(x))
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{
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mag_zero(z);
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}
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else
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{
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MAG_MAN(z) = MAG_MAN(x) - MAG_MAN(y);
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fix = MAG_BITS - FLINT_BIT_COUNT(MAG_MAN(z));
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MAG_MAN(z) <<= fix;
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_fmpz_add_fast(MAG_EXPREF(z), MAG_EXPREF(x), -fix);
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}
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}
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else
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{
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if (shift <= MAG_BITS)
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{
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mp_limb_t c = MAG_MAN(x) - (MAG_MAN(y) >> shift) - 1;
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/* too much cancellation -- compute precisely */
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if (c < (UWORD(1) << (MAG_BITS - 4)))
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{
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arf_t t, u;
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arf_init(t);
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arf_init(u);
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arf_set_mag(t, x);
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arf_set_mag(u, y);
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arf_sub(t, t, u, MAG_BITS, ARF_RND_DOWN);
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arf_get_mag_lower(z, t);
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arf_clear(t);
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arf_clear(u);
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return;
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}
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MAG_MAN(z) = c;
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}
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else
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{
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MAG_MAN(z) = MAG_MAN(x) - 1;
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}
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fix = MAG_BITS - FLINT_BIT_COUNT(MAG_MAN(z));
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MAG_MAN(z) <<= fix;
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_fmpz_add_fast(MAG_EXPREF(z), MAG_EXPREF(x), -fix);
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}
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}
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}
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