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/* mpc_norm -- Square of the norm of a complex number.
Copyright (C) 2002, 2005, 2008, 2009, 2010, 2011, 2012 INRIA
This file is part of GNU MPC.
GNU MPC 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 3 of the License, or (at your
option) any later version.
GNU MPC 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 this program. If not, see http://www.gnu.org/licenses/ .
*/
#include <stdio.h> /* for MPC_ASSERT */
#include "mpc-impl.h"
/* a <- norm(b) = b * conj(b)
(the rounding mode is mpfr_rnd_t here since we return an mpfr number) */
int
mpc_norm (mpfr_ptr a, mpc_srcptr b, mpfr_rnd_t rnd)
{
int inexact;
int saved_underflow, saved_overflow;
/* handling of special values; consistent with abs in that
norm = abs^2; so norm (+-inf, xxx) = norm (xxx, +-inf) = +inf */
if (!mpc_fin_p (b))
return mpc_abs (a, b, rnd);
else if (mpfr_zero_p (mpc_realref (b))) {
if (mpfr_zero_p (mpc_imagref (b)))
return mpfr_set_ui (a, 0, rnd); /* +0 */
else
return mpfr_sqr (a, mpc_imagref (b), rnd);
}
else if (mpfr_zero_p (mpc_imagref (b)))
return mpfr_sqr (a, mpc_realref (b), rnd); /* Re(b) <> 0 */
else /* everything finite and non-zero */ {
mpfr_t u, v, res;
mpfr_prec_t prec, prec_u, prec_v;
int loops;
const int max_loops = 2;
/* switch to exact squarings when loops==max_loops */
prec = mpfr_get_prec (a);
mpfr_init (u);
mpfr_init (v);
mpfr_init (res);
/* save the underflow or overflow flags from MPFR */
saved_underflow = mpfr_underflow_p ();
saved_overflow = mpfr_overflow_p ();
loops = 0;
mpfr_clear_underflow ();
mpfr_clear_overflow ();
do {
loops++;
prec += mpc_ceil_log2 (prec) + 3;
if (loops >= max_loops) {
prec_u = 2 * MPC_PREC_RE (b);
prec_v = 2 * MPC_PREC_IM (b);
}
else {
prec_u = MPC_MIN (prec, 2 * MPC_PREC_RE (b));
prec_v = MPC_MIN (prec, 2 * MPC_PREC_IM (b));
}
mpfr_set_prec (u, prec_u);
mpfr_set_prec (v, prec_v);
inexact = mpfr_sqr (u, mpc_realref(b), MPFR_RNDD); /* err <= 1 ulp in prec */
inexact |= mpfr_sqr (v, mpc_imagref(b), MPFR_RNDD); /* err <= 1 ulp in prec */
/* If loops = max_loops, inexact should be 0 here, except in case
of underflow or overflow.
If loops < max_loops and inexact is zero, we can exit the
while-loop since it only remains to add u and v into a. */
if (inexact) {
mpfr_set_prec (res, prec);
mpfr_add (res, u, v, MPFR_RNDD); /* err <= 3 ulp in prec */
}
} while (loops < max_loops && inexact != 0
&& !mpfr_can_round (res, prec - 2, MPFR_RNDD, MPFR_RNDU,
mpfr_get_prec (a) + (rnd == MPFR_RNDN)));
if (!inexact)
/* squarings were exact, neither underflow nor overflow */
inexact = mpfr_add (a, u, v, rnd);
/* if there was an overflow in Re(b)^2 or Im(b)^2 or their sum,
since the norm is larger, there is an overflow for the norm */
else if (mpfr_overflow_p ()) {
/* replace by "correctly rounded overflow" */
mpfr_set_ui (a, 1ul, MPFR_RNDN);
inexact = mpfr_mul_2ui (a, a, mpfr_get_emax (), rnd);
}
else if (mpfr_underflow_p ()) {
/* necessarily one of the squarings did underflow (otherwise their
sum could not underflow), thus one of u, v is zero. */
mpfr_exp_t emin = mpfr_get_emin ();
/* Now either both u and v are zero, or u is zero and v exact,
or v is zero and u exact.
In the latter case, Im(b)^2 < 2^(emin-1).
If ulp(u) >= 2^(emin+1) and norm(b) is not exactly
representable at the target precision, then rounding u+Im(b)^2
is equivalent to rounding u+2^(emin-1).
For instance, if exp(u)>0 and the target precision is smaller
than about |emin|, the norm is not representable. To make the
scaling in the "else" case work without underflow, we test
whether exp(u) is larger than a small negative number instead.
The second case is handled analogously. */
if (!mpfr_zero_p (u)
&& mpfr_get_exp (u) - 2 * (mpfr_exp_t) prec_u > emin
&& mpfr_get_exp (u) > -10) {
mpfr_set_prec (v, MPFR_PREC_MIN);
mpfr_set_ui_2exp (v, 1, emin - 1, MPFR_RNDZ);
inexact = mpfr_add (a, u, v, rnd);
}
else if (!mpfr_zero_p (v)
&& mpfr_get_exp (v) - 2 * (mpfr_exp_t) prec_v > emin
&& mpfr_get_exp (v) > -10) {
mpfr_set_prec (u, MPFR_PREC_MIN);
mpfr_set_ui_2exp (u, 1, emin - 1, MPFR_RNDZ);
inexact = mpfr_add (a, u, v, rnd);
}
else {
unsigned long int scale, exp_re, exp_im;
int inex_underflow;
/* scale the input to an average exponent close to 0 */
exp_re = (unsigned long int) (-mpfr_get_exp (mpc_realref (b)));
exp_im = (unsigned long int) (-mpfr_get_exp (mpc_imagref (b)));
scale = exp_re / 2 + exp_im / 2 + (exp_re % 2 + exp_im % 2) / 2;
/* (exp_re + exp_im) / 2, computed in a way avoiding
integer overflow */
if (mpfr_zero_p (u)) {
/* recompute the scaled value exactly */
mpfr_mul_2ui (u, mpc_realref (b), scale, MPFR_RNDN);
mpfr_sqr (u, u, MPFR_RNDN);
}
else /* just scale */
mpfr_mul_2ui (u, u, 2*scale, MPFR_RNDN);
if (mpfr_zero_p (v)) {
mpfr_mul_2ui (v, mpc_imagref (b), scale, MPFR_RNDN);
mpfr_sqr (v, v, MPFR_RNDN);
}
else
mpfr_mul_2ui (v, v, 2*scale, MPFR_RNDN);
inexact = mpfr_add (a, u, v, rnd);
mpfr_clear_underflow ();
inex_underflow = mpfr_div_2ui (a, a, 2*scale, rnd);
if (mpfr_underflow_p ())
inexact = inex_underflow;
}
}
else /* no problems, ternary value due to mpfr_can_round trick */
inexact = mpfr_set (a, res, rnd);
/* restore underflow and overflow flags from MPFR */
if (saved_underflow)
mpfr_set_underflow ();
if (saved_overflow)
mpfr_set_overflow ();
mpfr_clear (u);
mpfr_clear (v);
mpfr_clear (res);
}
return inexact;
}
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