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/* mpfr_cbrt -- cube root function.
Copyright 2002, 2003, 2004 Free Software Foundation.
Contributed by the Spaces project, INRIA Lorraine.
This file is part of the MPFR Library.
The MPFR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.
The MPFR Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
License for more details.
You should have received a copy of the GNU Lesser General Public License
along with the MPFR Library; see the file COPYING.LIB. If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */
#include <stdio.h>
#include <stdlib.h>
#include "mpfr-impl.h"
/* The computation of y = x^(1/3) is done as follows:
Let x = sign * m * 2^(3*e) where m is an integer
with 2^(3n-3) <= m < 2^(3n) where n = PREC(y)
and m = s^3 + r where 0 <= r and m < (s+1)^3
we want that s has n bits i.e. s >= 2^(n-1), or m >= 2^(3n-3)
i.e. m must have at least 3n-2 bits
then x^(1/3) = s * 2^e if r=0
x^(1/3) = (s+1) * 2^e if round up
x^(1/3) = (s-1) * 2^e if round down
x^(1/3) = s * 2^e if nearest and r < 3/2*s^2+3/4*s+1/8
(s+1) * 2^e otherwise
*/
int
mpfr_cbrt (mpfr_ptr y, mpfr_srcptr x, mp_rnd_t rnd_mode)
{
mpz_t m;
mp_exp_t e, r, sh;
mp_prec_t n, size_m;
int inexact, negative;
/* special values */
if (MPFR_UNLIKELY( MPFR_IS_SINGULAR(x) ))
{
if (MPFR_IS_NAN(x))
{
MPFR_SET_NAN(y);
MPFR_RET_NAN;
}
else if (MPFR_IS_INF(x))
{
MPFR_SET_INF(y);
MPFR_SET_SAME_SIGN (y, x);
return 0;
}
/* case 0: cbrt(+/- 0) = +/- 0 */
else /* x is necessaryly 0 */
{
MPFR_ASSERTD(MPFR_IS_ZERO(x));
MPFR_SET_ZERO(y);
MPFR_SET_SAME_SIGN (y, x);
return 0;
}
}
/* Useless due to mpz_init
MPFR_CLEAR_FLAGS(y);*/
mpz_init (m);
e = mpfr_get_z_exp (m, x); /* x = m * 2^e */
if ((negative = MPFR_IS_NEG(x)))
mpz_neg (m, m);
r = e % 3;
if (r < 0)
r += 3;
/* x = (m*2^r) * 2^(e-r) = (m*2^r) * 2^(3*q) */
size_m = mpz_sizeinbase (m, 2);
n = MPFR_PREC(y);
if (rnd_mode == GMP_RNDN)
n ++;
/* we want 3*n-2 <= size_m + 3*sh + r <= 3*n
i.e. 3*sh + size_m + r <= 3*n */
sh = (3 * (mp_exp_t) n - (mp_exp_t) size_m - r) / 3;
sh = 3 * sh + r;
if (sh >= 0)
{
mpz_mul_2exp (m, m, sh);
e = e - sh;
}
else if (r > 0)
{
mpz_mul_2exp (m, m, r);
e = e - r;
}
/* invariant: x = m*2^e, with e divisible by 3 */
/* we reuse the variable m to store the cube root, since it is not needed
any more: we just need to know if the root is exact */
inexact = mpz_root (m, m, 3) == 0;
sh = mpz_sizeinbase (m, 2) - n;
if (sh > 0) /* we have to flush to 0 the last sh bits from m */
{
inexact = inexact || ((mp_exp_t) mpz_scan1 (m, 0) < sh);
mpz_div_2exp (m, m, sh);
e += 3 * sh;
}
if (inexact)
{
if (negative)
rnd_mode = MPFR_INVERT_RND(rnd_mode);
if ((rnd_mode == GMP_RNDU) ||
((rnd_mode == GMP_RNDN) && mpz_tstbit (m, 0)))
mpz_add_ui (m, m, inexact = 1);
else
inexact = -1;
}
/* either inexact is not zero, and the conversion is exact, i.e. inexact
is not changed; or inexact=0, and inexact is set only when
rnd_mode=GMP_RNDN and bit (n+1) from m is 1 */
inexact += mpfr_set_z (y, m, GMP_RNDN);
MPFR_SET_EXP (y, MPFR_GET_EXP (y) + e / 3);
if (negative)
{
MPFR_CHANGE_SIGN(y);
inexact = -inexact;
}
mpz_clear (m);
return inexact;
}
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