--- /dev/null
+/* mpfr_cbrt -- cube root function.
+
+Copyright 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
+Contributed by the Arenaire and Cacao projects, INRIA.
+
+This file is part of the GNU MPFR Library.
+
+The GNU 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 GNU 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 GNU MPFR Library; see the file COPYING.LIB. If not, write to
+the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
+MA 02110-1301, USA. */
+
+#define MPFR_NEED_LONGLONG_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, tmp;
+ int inexact, negative;
+ MPFR_SAVE_EXPO_DECL (expo);
+
+ /* 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);
+ MPFR_RET (0);
+ }
+ /* case 0: cbrt(+/- 0) = +/- 0 */
+ else /* x is necessarily 0 */
+ {
+ MPFR_ASSERTD (MPFR_IS_ZERO (x));
+ MPFR_SET_ZERO (y);
+ MPFR_SET_SAME_SIGN (y, x);
+ MPFR_RET (0);
+ }
+ }
+
+ /* General case */
+ MPFR_SAVE_EXPO_MARK (expo);
+ 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) */
+
+ MPFR_MPZ_SIZEINBASE2 (size_m, m);
+ n = MPFR_PREC (y) + (rnd_mode == GMP_RNDN);
+
+ /* 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;
+
+ MPFR_MPZ_SIZEINBASE2 (tmp, m);
+ sh = tmp - 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)))
+ inexact = 1, mpz_add_ui (m, m, 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);
+ MPFR_SAVE_EXPO_FREE (expo);
+ return mpfr_check_range (y, inexact, rnd_mode);
+}
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