/* mpc_atan -- arctangent of a complex number. Copyright (C) 2009, 2010, 2011, 2012, 2013, 2017 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 #include "mpc-impl.h" /* set rop to -pi/2 if s < 0 +pi/2 else rounded in the direction rnd */ int set_pi_over_2 (mpfr_ptr rop, int s, mpfr_rnd_t rnd) { int inex; inex = mpfr_const_pi (rop, s < 0 ? INV_RND (rnd) : rnd); mpfr_div_2ui (rop, rop, 1, MPFR_RNDN); if (s < 0) { inex = -inex; mpfr_neg (rop, rop, MPFR_RNDN); } return inex; } int mpc_atan (mpc_ptr rop, mpc_srcptr op, mpc_rnd_t rnd) { int s_re; int s_im; int inex_re; int inex_im; int inex; inex_re = 0; inex_im = 0; s_re = mpfr_signbit (mpc_realref (op)); s_im = mpfr_signbit (mpc_imagref (op)); /* special values */ if (mpfr_nan_p (mpc_realref (op)) || mpfr_nan_p (mpc_imagref (op))) { if (mpfr_nan_p (mpc_realref (op))) { mpfr_set_nan (mpc_realref (rop)); if (mpfr_zero_p (mpc_imagref (op)) || mpfr_inf_p (mpc_imagref (op))) { mpfr_set_ui (mpc_imagref (rop), 0, MPFR_RNDN); if (s_im) mpc_conj (rop, rop, MPC_RNDNN); } else mpfr_set_nan (mpc_imagref (rop)); } else { if (mpfr_inf_p (mpc_realref (op))) { inex_re = set_pi_over_2 (mpc_realref (rop), -s_re, MPC_RND_RE (rnd)); mpfr_set_ui (mpc_imagref (rop), 0, MPFR_RNDN); } else { mpfr_set_nan (mpc_realref (rop)); mpfr_set_nan (mpc_imagref (rop)); } } return MPC_INEX (inex_re, 0); } if (mpfr_inf_p (mpc_realref (op)) || mpfr_inf_p (mpc_imagref (op))) { inex_re = set_pi_over_2 (mpc_realref (rop), -s_re, MPC_RND_RE (rnd)); mpfr_set_ui (mpc_imagref (rop), 0, MPFR_RNDN); if (s_im) mpc_conj (rop, rop, MPFR_RNDN); return MPC_INEX (inex_re, 0); } /* pure real argument */ if (mpfr_zero_p (mpc_imagref (op))) { inex_re = mpfr_atan (mpc_realref (rop), mpc_realref (op), MPC_RND_RE (rnd)); mpfr_set_ui (mpc_imagref (rop), 0, MPFR_RNDN); if (s_im) mpc_conj (rop, rop, MPFR_RNDN); return MPC_INEX (inex_re, 0); } /* pure imaginary argument */ if (mpfr_zero_p (mpc_realref (op))) { int cmp_1; if (s_im) cmp_1 = -mpfr_cmp_si (mpc_imagref (op), -1); else cmp_1 = mpfr_cmp_ui (mpc_imagref (op), +1); if (cmp_1 < 0) { /* atan(+0+iy) = +0 +i*atanh(y), if |y| < 1 atan(-0+iy) = -0 +i*atanh(y), if |y| < 1 */ mpfr_set_ui (mpc_realref (rop), 0, MPFR_RNDN); if (s_re) mpfr_neg (mpc_realref (rop), mpc_realref (rop), MPFR_RNDN); inex_im = mpfr_atanh (mpc_imagref (rop), mpc_imagref (op), MPC_RND_IM (rnd)); } else if (cmp_1 == 0) { /* atan(+/-0 +i) = +/-0 +i*inf atan(+/-0 -i) = +/-0 -i*inf */ mpfr_set_zero (mpc_realref (rop), s_re ? -1 : +1); mpfr_set_inf (mpc_imagref (rop), s_im ? -1 : +1); } else { /* atan(+0+iy) = +pi/2 +i*atanh(1/y), if |y| > 1 atan(-0+iy) = -pi/2 +i*atanh(1/y), if |y| > 1 */ mpfr_rnd_t rnd_im, rnd_away; mpfr_t y, z; mpfr_prec_t p, p_im; int ok; rnd_im = MPC_RND_IM (rnd); mpfr_init (y); mpfr_init (z); p_im = mpfr_get_prec (mpc_imagref (rop)); p = p_im; /* a = o(1/y) with error(a) < ulp(a), rounded away b = o(atanh(a)) with error(b) < ulp(b) + 1/|a^2-1|*ulp(a), since if a = 1/y + eps, then atanh(a) = atanh(1/y) + eps * atanh'(t) with t in (1/y, a). Since a is rounded away, we have 1/y <= a <= 1 if y > 1, and -1 <= a <= 1/y if y < -1, thus |atanh'(t)| = 1/|t^2-1| <= 1/|a^2-1|. We round atanh(1/y) away from 0. */ do { mpfr_exp_t err, exp_a; p += mpc_ceil_log2 (p) + 2; mpfr_set_prec (y, p); mpfr_set_prec (z, p); rnd_away = s_im == 0 ? MPFR_RNDU : MPFR_RNDD; inex_im = mpfr_ui_div (y, 1, mpc_imagref (op), rnd_away); exp_a = mpfr_get_exp (y); /* FIXME: should we consider the case with unreasonably huge precision prec(y)>3*exp_min, where atanh(1/Im(op)) could be representable while 1/Im(op) underflows ? This corresponds to |y| = 0.5*2^emin, in which case the result may be wrong. */ /* We would like to compute a rounded-up error bound 1/|a^2-1|, so we need to round down |a^2-1|, which means rounding up a^2 since |a|<1. */ mpfr_sqr (z, y, MPFR_RNDU); /* since |y| > 1, we should have |a| <= 1, thus a^2 <= 1 */ MPC_ASSERT(mpfr_cmp_ui (z, 1) <= 0); /* in case z=1, we should try again with more precision */ if (mpfr_cmp_ui (z, 1) == 0) continue; /* now z < 1 */ mpfr_ui_sub (z, 1, z, MPFR_RNDZ); /* atanh cannot underflow: |atanh(x)| > |x| for |x| < 1 */ inex_im |= mpfr_atanh (y, y, rnd_away); /* the error is now bounded by ulp(b) + 1/z*ulp(a), thus ulp(b) + 2^(exp(a) - exp(b) + 1 - exp(z)) * ulp(b) */ err = exp_a - mpfr_get_exp (y) + 1 - mpfr_get_exp (z); if (err >= 0) /* 1 + 2^err <= 2^(err+1) */ err = err + 1; else err = 1; /* 1 + 2^err <= 2^1 */ /* the error is bounded by 2^err ulps */ ok = inex_im == 0 || mpfr_can_round (y, p - err, rnd_away, MPFR_RNDZ, p_im + (rnd_im == MPFR_RNDN)); } while (ok == 0); inex_re = set_pi_over_2 (mpc_realref (rop), -s_re, MPC_RND_RE (rnd)); inex_im = mpfr_set (mpc_imagref (rop), y, rnd_im); mpfr_clear (y); mpfr_clear (z); } return MPC_INEX (inex_re, inex_im); } /* regular number argument */ { mpfr_t a, b, x, y; mpfr_prec_t prec, p; mpfr_exp_t err, expo; int ok = 0; mpfr_t minus_op_re; mpfr_exp_t op_re_exp, op_im_exp; mpfr_rnd_t rnd1, rnd2; mpfr_inits2 (MPFR_PREC_MIN, a, b, x, y, (mpfr_ptr) 0); /* real part: Re(arctan(x+i*y)) = [arctan2(x,1-y) - arctan2(-x,1+y)]/2 */ minus_op_re[0] = mpc_realref (op)[0]; MPFR_CHANGE_SIGN (minus_op_re); op_re_exp = mpfr_get_exp (mpc_realref (op)); op_im_exp = mpfr_get_exp (mpc_imagref (op)); prec = mpfr_get_prec (mpc_realref (rop)); /* result precision */ /* a = o(1-y) error(a) < 1 ulp(a) b = o(atan2(x,a)) error(b) < [1+2^{3+Exp(x)-Exp(a)-Exp(b)}] ulp(b) = kb ulp(b) c = o(1+y) error(c) < 1 ulp(c) d = o(atan2(-x,c)) error(d) < [1+2^{3+Exp(x)-Exp(c)-Exp(d)}] ulp(d) = kd ulp(d) e = o(b - d) error(e) < [1 + kb*2^{Exp(b}-Exp(e)} + kd*2^{Exp(d)-Exp(e)}] ulp(e) error(e) < [1 + 2^{4+Exp(x)-Exp(a)-Exp(e)} + 2^{4+Exp(x)-Exp(c)-Exp(e)}] ulp(e) because |atan(u)| < |u| < [1 + 2^{5+Exp(x)-min(Exp(a),Exp(c)) -Exp(e)}] ulp(e) f = e/2 exact */ /* p: working precision */ p = (op_im_exp > 0 || prec > SAFE_ABS (mpfr_prec_t, op_im_exp)) ? prec : (prec - op_im_exp); rnd1 = mpfr_sgn (mpc_realref (op)) > 0 ? MPFR_RNDD : MPFR_RNDU; rnd2 = mpfr_sgn (mpc_realref (op)) < 0 ? MPFR_RNDU : MPFR_RNDD; do { p += mpc_ceil_log2 (p) + 2; mpfr_set_prec (a, p); mpfr_set_prec (b, p); mpfr_set_prec (x, p); /* x = upper bound for atan (x/(1-y)). Since atan is increasing, we need an upper bound on x/(1-y), i.e., a lower bound on 1-y for x positive, and an upper bound on 1-y for x negative */ mpfr_ui_sub (a, 1, mpc_imagref (op), rnd1); if (mpfr_sgn (a) == 0) /* y is near 1, thus 1+y is near 2, and expo will be 1 or 2 below */ { MPC_ASSERT (mpfr_cmp_ui (mpc_imagref(op), 1) == 0); /* check for intermediate underflow */ err = 2; /* ensures err will be expo below */ } else err = mpfr_get_exp (a); /* err = Exp(a) with the notations above */ mpfr_atan2 (x, mpc_realref (op), a, MPFR_RNDU); /* b = lower bound for atan (-x/(1+y)): for x negative, we need a lower bound on -x/(1+y), i.e., an upper bound on 1+y */ mpfr_add_ui (a, mpc_imagref(op), 1, rnd2); /* if a is exactly zero, i.e., Im(op) = -1, then the error on a is 0, and we can simply ignore the terms involving Exp(a) in the error */ if (mpfr_sgn (a) == 0) { MPC_ASSERT (mpfr_cmp_si (mpc_imagref(op), -1) == 0); /* check for intermediate underflow */ expo = err; /* will leave err unchanged below */ } else expo = mpfr_get_exp (a); /* expo = Exp(c) with the notations above */ mpfr_atan2 (b, minus_op_re, a, MPFR_RNDD); err = err < expo ? err : expo; /* err = min(Exp(a),Exp(c)) */ mpfr_sub (x, x, b, MPFR_RNDU); err = 5 + op_re_exp - err - mpfr_get_exp (x); /* error is bounded by [1 + 2^err] ulp(e) */ err = err < 0 ? 1 : err + 1; mpfr_div_2ui (x, x, 1, MPFR_RNDU); /* Note: using RND2=RNDD guarantees that if x is exactly representable on prec + ... bits, mpfr_can_round will return 0 */ ok = mpfr_can_round (x, p - err, MPFR_RNDU, MPFR_RNDD, prec + (MPC_RND_RE (rnd) == MPFR_RNDN)); } while (ok == 0); /* Imaginary part Im(atan(x+I*y)) = 1/4 * [log(x^2+(1+y)^2) - log (x^2 +(1-y)^2)] */ prec = mpfr_get_prec (mpc_imagref (rop)); /* result precision */ /* a = o(1+y) error(a) < 1 ulp(a) b = o(a^2) error(b) < 5 ulp(b) c = o(x^2) error(c) < 1 ulp(c) d = o(b+c) error(d) < 7 ulp(d) e = o(log(d)) error(e) < [1 + 7*2^{2-Exp(e)}] ulp(e) = ke ulp(e) f = o(1-y) error(f) < 1 ulp(f) g = o(f^2) error(g) < 5 ulp(g) h = o(c+f) error(h) < 7 ulp(h) i = o(log(h)) error(i) < [1 + 7*2^{2-Exp(i)}] ulp(i) = ki ulp(i) j = o(e-i) error(j) < [1 + ke*2^{Exp(e)-Exp(j)} + ki*2^{Exp(i)-Exp(j)}] ulp(j) error(j) < [1 + 2^{Exp(e)-Exp(j)} + 2^{Exp(i)-Exp(j)} + 7*2^{3-Exp(j)}] ulp(j) < [1 + 2^{max(Exp(e),Exp(i))-Exp(j)+1} + 7*2^{3-Exp(j)}] ulp(j) k = j/4 exact */ err = 2; p = prec; /* working precision */ do { p += mpc_ceil_log2 (p) + err; mpfr_set_prec (a, p); mpfr_set_prec (b, p); mpfr_set_prec (y, p); /* a = upper bound for log(x^2 + (1+y)^2) */ mpfr_add_ui (a, mpc_imagref (op), 1, MPFR_RNDA); mpfr_sqr (a, a, MPFR_RNDU); mpfr_sqr (y, mpc_realref (op), MPFR_RNDU); mpfr_add (a, a, y, MPFR_RNDU); mpfr_log (a, a, MPFR_RNDU); /* b = lower bound for log(x^2 + (1-y)^2) */ mpfr_ui_sub (b, 1, mpc_imagref (op), MPFR_RNDZ); /* round to zero */ mpfr_sqr (b, b, MPFR_RNDZ); /* we could write mpfr_sqr (y, mpc_realref (op), MPFR_RNDZ) but it is more efficient to reuse the value of y (x^2) above and subtract one ulp */ mpfr_nextbelow (y); mpfr_add (b, b, y, MPFR_RNDZ); mpfr_log (b, b, MPFR_RNDZ); mpfr_sub (y, a, b, MPFR_RNDU); if (mpfr_zero_p (y)) /* FIXME: happens when x and y have very different magnitudes; could be handled more efficiently */ ok = 0; else { expo = MPC_MAX (mpfr_get_exp (a), mpfr_get_exp (b)); expo = expo - mpfr_get_exp (y) + 1; err = 3 - mpfr_get_exp (y); /* error(j) <= [1 + 2^expo + 7*2^err] ulp(j) */ if (expo <= err) /* error(j) <= [1 + 2^{err+1}] ulp(j) */ err = (err < 0) ? 1 : err + 2; else err = (expo < 0) ? 1 : expo + 2; mpfr_div_2ui (y, y, 2, MPFR_RNDN); MPC_ASSERT (!mpfr_zero_p (y)); /* FIXME: underflow. Since the main term of the Taylor series in y=0 is 1/(x^2+1) * y, this means that y is very small and/or x very large; but then the mpfr_zero_p (y) above should be true. This needs a proof, or better yet, special code. */ ok = mpfr_can_round (y, p - err, MPFR_RNDU, MPFR_RNDD, prec + (MPC_RND_IM (rnd) == MPFR_RNDN)); } } while (ok == 0); inex = mpc_set_fr_fr (rop, x, y, rnd); mpfr_clears (a, b, x, y, (mpfr_ptr) 0); return inex; } }