/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */

/*
 * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
 */
/*
 * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma weak __acos = acos

/* INDENT OFF */
/*
 * acos(x)
 * Method :
 *      acos(x)  = pi/2 - asin(x)
 *      acos(-x) = pi/2 + asin(x)
 * For |x|<=0.5
 *      acos(x) = pi/2 - (x + x*x^2*R(x^2))     (see asin.c)
 * For x>0.5
 *      acos(x) = pi/2 - (pi/2 - 2asin(sqrt((1-x)/2)))
 *              = 2asin(sqrt((1-x)/2))
 *              = 2s + 2s*z*R(z)        ...z=(1-x)/2, s=sqrt(z)
 *              = 2f + (2c + 2s*z*R(z))
 *     where f=hi part of s, and c = (z-f*f)/(s+f) is the correction term
 *     for f so that f+c ~ sqrt(z).
 * For x<-0.5
 *      acos(x) = pi - 2asin(sqrt((1-|x|)/2))
 *              = pi - 0.5*(s+s*z*R(z)), where z=(1-|x|)/2,s=sqrt(z)
 *
 * Special cases:
 *      if x is NaN, return x itself;
 *      if |x|>1, return NaN with invalid signal.
 *
 * Function needed: sqrt
 */
/* INDENT ON */

#include "libm_protos.h"        /* _SVID_libm_error */
#include "libm_macros.h"
#include <math.h>

/* INDENT OFF */
static const double xxx[] = {
/* one */        1.00000000000000000000e+00,    /* 3FF00000, 00000000 */
/* pi */         3.14159265358979311600e+00,    /* 400921FB, 54442D18 */
/* pio2_hi */    1.57079632679489655800e+00,    /* 3FF921FB, 54442D18 */
/* pio2_lo */    6.12323399573676603587e-17,    /* 3C91A626, 33145C07 */
/* pS0 */        1.66666666666666657415e-01,    /* 3FC55555, 55555555 */
/* pS1 */       -3.25565818622400915405e-01,    /* BFD4D612, 03EB6F7D */
/* pS2 */        2.01212532134862925881e-01,    /* 3FC9C155, 0E884455 */
/* pS3 */       -4.00555345006794114027e-02,    /* BFA48228, B5688F3B */
/* pS4 */        7.91534994289814532176e-04,    /* 3F49EFE0, 7501B288 */
/* pS5 */        3.47933107596021167570e-05,    /* 3F023DE1, 0DFDF709 */
/* qS1 */       -2.40339491173441421878e+00,    /* C0033A27, 1C8A2D4B */
/* qS2 */        2.02094576023350569471e+00,    /* 40002AE5, 9C598AC8 */
/* qS3 */       -6.88283971605453293030e-01,    /* BFE6066C, 1B8D0159 */
/* qS4 */        7.70381505559019352791e-02     /* 3FB3B8C5, B12E9282 */
};
#define one     xxx[0]
#define pi      xxx[1]
#define pio2_hi xxx[2]
#define pio2_lo xxx[3]
#define pS0     xxx[4]
#define pS1     xxx[5]
#define pS2     xxx[6]
#define pS3     xxx[7]
#define pS4     xxx[8]
#define pS5     xxx[9]
#define qS1     xxx[10]
#define qS2     xxx[11]
#define qS3     xxx[12]
#define qS4     xxx[13]
/* INDENT ON */

double
acos(double x) {
        double z, p, q, r, w, s, c, df;
        int hx, ix;

        hx = ((int *) &x)[HIWORD];
        ix = hx & 0x7fffffff;
        if (ix >= 0x3ff00000) { /* |x| >= 1 */
                if (((ix - 0x3ff00000) | ((int *) &x)[LOWORD]) == 0) {
                        /* |x| == 1 */
                        if (hx > 0)     /* acos(1) = 0 */
                                return (0.0);
                        else            /* acos(-1) = pi */
                                return (pi + 2.0 * pio2_lo);
                } else if (isnan(x))
#if defined(FPADD_TRAPS_INCOMPLETE_ON_NAN)
                        return (ix >= 0x7ff80000 ? x : (x - x) / (x - x));
                        /* assumes sparc-like QNaN */
#else
                        return (x - x) / (x - x);       /* acos(|x|>1) is NaN */
#endif
                else
                        return (_SVID_libm_err(x, x, 1));
        }
        if (ix < 0x3fe00000) {  /* |x| < 0.5 */
                if (ix <= 0x3c600000)
                        return (pio2_hi + pio2_lo);     /* if |x| < 2**-57 */
                z = x * x;
                p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 +
                        z * (pS4 + z * pS5)))));
                q = one + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
                r = p / q;
                return (pio2_hi - (x - (pio2_lo - x * r)));
        } else if (hx < 0) {
                /* x < -0.5 */
                z = (one + x) * 0.5;
                p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 +
                        z * (pS4 + z * pS5)))));
                q = one + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
                s = sqrt(z);
                r = p / q;
                w = r * s - pio2_lo;
                return (pi - 2.0 * (s + w));
        } else {
                /* x > 0.5 */
                z = (one - x) * 0.5;
                s = sqrt(z);
                df = s;
                ((int *) &df)[LOWORD] = 0;
                c = (z - df * df) / (s + df);
                p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 +
                        z * (pS4 + z * pS5)))));
                q = one + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
                r = p / q;
                w = r * s + c;
                return (2.0 * (df + w));
        }
}