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/*
Copyright (C) 2014 Fredrik Johansson
This file is part of Arb.
Arb is free software: you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License (LGPL) as published
by the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version. See <http://www.gnu.org/licenses/>.
*/
#include "mag.h"
#include "arf.h"
void
mag_sub_lower(mag_t z, const mag_t x, const mag_t y)
{
if (mag_is_special(x) || mag_is_special(y))
{
if (mag_is_zero(y))
mag_set(z, x);
else if (mag_is_zero(x) || mag_is_inf(y))
mag_zero(z);
else
mag_inf(z);
}
else
{
slong shift, fix;
shift = _fmpz_sub_small(MAG_EXPREF(x), MAG_EXPREF(y));
/* y > x */
if (shift < 0)
{
mag_zero(z);
}
else if (shift == 0)
{
if (MAG_MAN(y) >= MAG_MAN(x))
{
mag_zero(z);
}
else
{
MAG_MAN(z) = MAG_MAN(x) - MAG_MAN(y);
fix = MAG_BITS - FLINT_BIT_COUNT(MAG_MAN(z));
MAG_MAN(z) <<= fix;
_fmpz_add_fast(MAG_EXPREF(z), MAG_EXPREF(x), -fix);
}
}
else
{
if (shift <= MAG_BITS)
{
mp_limb_t c = MAG_MAN(x) - (MAG_MAN(y) >> shift) - 1;
/* too much cancellation -- compute precisely */
if (c < (UWORD(1) << (MAG_BITS - 4)))
{
arf_t t, u;
arf_init(t);
arf_init(u);
arf_set_mag(t, x);
arf_set_mag(u, y);
arf_sub(t, t, u, MAG_BITS, ARF_RND_DOWN);
arf_get_mag_lower(z, t);
arf_clear(t);
arf_clear(u);
return;
}
MAG_MAN(z) = c;
}
else
{
MAG_MAN(z) = MAG_MAN(x) - 1;
}
fix = MAG_BITS - FLINT_BIT_COUNT(MAG_MAN(z));
MAG_MAN(z) <<= fix;
_fmpz_add_fast(MAG_EXPREF(z), MAG_EXPREF(x), -fix);
}
}
}
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