# code: 9ferno

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```/* derived from /netlib/fdlibm */

/* @(#)e_j0.c 1.3 95/01/18 */
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunSoft, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/

/* __ieee754_j0(x), __ieee754_y0(x)
* Bessel function of the first and second kinds of order zero.
* Method -- j0(x):
*	1. For tiny x, we use j0(x) = 1 - x^2/4 + x^4/64 - ...
*	2. Reduce x to |x| since j0(x)=j0(-x),  and
*	   for x in (0,2)
*		j0(x) = 1-z/4+ z^2*R0/S0,  where z = x*x;
*	   (precision:  |j0-1+z/4-z^2R0/S0 |<2**-63.67 )
*	   for x in (2,inf)
* 		j0(x) = sqrt(2/(pi*x))*(p0(x)*cos(x0)-q0(x)*sin(x0))
* 	   where x0 = x-pi/4. It is better to compute sin(x0),cos(x0)
*	   as follow:
*		cos(x0) = cos(x)cos(pi/4)+sin(x)sin(pi/4)
*			= 1/sqrt(2) * (cos(x) + sin(x))
*		sin(x0) = sin(x)cos(pi/4)-cos(x)sin(pi/4)
*			= 1/sqrt(2) * (sin(x) - cos(x))
* 	   (To avoid cancellation, use
*		sin(x) +- cos(x) = -cos(2x)/(sin(x) -+ cos(x))
* 	    to compute the worse one.)
*
*	3 Special cases
*		j0(nan)= nan
*		j0(0) = 1
*		j0(inf) = 0
*
* Method -- y0(x):
*	1. For x<2.
*	   Since
*		y0(x) = 2/pi*(j0(x)*(ln(x/2)+Euler) + x^2/4 - ...)
*	   therefore y0(x)-2/pi*j0(x)*ln(x) is an even function.
*	   We use the following function to approximate y0,
*		y0(x) = U(z)/V(z) + (2/pi)*(j0(x)*ln(x)), z= x^2
*	   where
*		U(z) = u00 + u01*z + ... + u06*z^6
*		V(z) = 1  + v01*z + ... + v04*z^4
*	   with absolute approximation error bounded by 2**-72.
*	   Note: For tiny x, U/V = u0 and j0(x)~1, hence
*		y0(tiny) = u0 + (2/pi)*ln(tiny), (choose tiny<2**-27)
*	2. For x>=2.
* 		y0(x) = sqrt(2/(pi*x))*(p0(x)*cos(x0)+q0(x)*sin(x0))
* 	   where x0 = x-pi/4. It is better to compute sin(x0),cos(x0)
*	   by the method mentioned above.
*	3. Special cases: y0(0)=-inf, y0(x<0)=NaN, y0(inf)=0.
*/

#include "fdlibm.h"

static double pzero(double), qzero(double);

static const double
Huge 	= 1e300,
one	= 1.0,
invsqrtpi=  5.64189583547756279280e-01, /* 0x3FE20DD7, 0x50429B6D */
tpi      =  6.36619772367581382433e-01, /* 0x3FE45F30, 0x6DC9C883 */
/* R0/S0 on [0, 2.00] */
R02  =  1.56249999999999947958e-02, /* 0x3F8FFFFF, 0xFFFFFFFD */
R03  = -1.89979294238854721751e-04, /* 0xBF28E6A5, 0xB61AC6E9 */
R04  =  1.82954049532700665670e-06, /* 0x3EBEB1D1, 0x0C503919 */
R05  = -4.61832688532103189199e-09, /* 0xBE33D5E7, 0x73D63FCE */
S01  =  1.56191029464890010492e-02, /* 0x3F8FFCE8, 0x82C8C2A4 */
S02  =  1.16926784663337450260e-04, /* 0x3F1EA6D2, 0xDD57DBF4 */
S03  =  5.13546550207318111446e-07, /* 0x3EA13B54, 0xCE84D5A9 */
S04  =  1.16614003333790000205e-09; /* 0x3E1408BC, 0xF4745D8F */

static double zero = 0.0;

double __ieee754_j0(double x)
{
double z, s,c,ss,cc,r,u,v;
int hx,ix;

hx = __HI(x);
ix = hx&0x7fffffff;
if(ix>=0x7ff00000) return one/(x*x);
x = fabs(x);
if(ix >= 0x40000000) {	/* |x| >= 2.0 */
s = sin(x);
c = cos(x);
ss = s-c;
cc = s+c;
if(ix<0x7fe00000) {  /* make sure x+x not overflow */
z = -cos(x+x);
if ((s*c)<zero) cc = z/ss;
else 	    ss = z/cc;
}
/*
* j0(x) = 1/sqrt(pi) * (P(0,x)*cc - Q(0,x)*ss) / sqrt(x)
* y0(x) = 1/sqrt(pi) * (P(0,x)*ss + Q(0,x)*cc) / sqrt(x)
*/
if(ix>0x48000000) z = (invsqrtpi*cc)/sqrt(x);
else {
u = pzero(x); v = qzero(x);
z = invsqrtpi*(u*cc-v*ss)/sqrt(x);
}
return z;
}
if(ix<0x3f200000) {	/* |x| < 2**-13 */
if(Huge+x>one) {	/* raise inexact if x != 0 */
if(ix<0x3e400000) return one;	/* |x|<2**-27 */
else 	      return one - 0.25*x*x;
}
}
z = x*x;
r =  z*(R02+z*(R03+z*(R04+z*R05)));
s =  one+z*(S01+z*(S02+z*(S03+z*S04)));
if(ix < 0x3FF00000) {	/* |x| < 1.00 */
return one + z*(-0.25+(r/s));
} else {
u = 0.5*x;
return((one+u)*(one-u)+z*(r/s));
}
}

static const double
u00  = -7.38042951086872317523e-02, /* 0xBFB2E4D6, 0x99CBD01F */
u01  =  1.76666452509181115538e-01, /* 0x3FC69D01, 0x9DE9E3FC */
u02  = -1.38185671945596898896e-02, /* 0xBF8C4CE8, 0xB16CFA97 */
u03  =  3.47453432093683650238e-04, /* 0x3F36C54D, 0x20B29B6B */
u04  = -3.81407053724364161125e-06, /* 0xBECFFEA7, 0x73D25CAD */
u05  =  1.95590137035022920206e-08, /* 0x3E550057, 0x3B4EABD4 */
u06  = -3.98205194132103398453e-11, /* 0xBDC5E43D, 0x693FB3C8 */
v01  =  1.27304834834123699328e-02, /* 0x3F8A1270, 0x91C9C71A */
v02  =  7.60068627350353253702e-05, /* 0x3F13ECBB, 0xF578C6C1 */
v03  =  2.59150851840457805467e-07, /* 0x3E91642D, 0x7FF202FD */
v04  =  4.41110311332675467403e-10; /* 0x3DFE5018, 0x3BD6D9EF */

double __ieee754_y0(double x)
{
double z, s,c,ss,cc,u,v;
int hx,ix,lx;

hx = __HI(x);
ix = 0x7fffffff&hx;
lx = __LO(x);
/* Y0(NaN) is NaN, y0(-inf) is Nan, y0(inf) is 0  */
if(ix>=0x7ff00000) return  one/(x+x*x);
if((ix|lx)==0) return -one/zero;
if(hx<0) return zero/zero;
if(ix >= 0x40000000) {  /* |x| >= 2.0 */
/* y0(x) = sqrt(2/(pi*x))*(p0(x)*sin(x0)+q0(x)*cos(x0))
* where x0 = x-pi/4
*      Better formula:
*              cos(x0) = cos(x)cos(pi/4)+sin(x)sin(pi/4)
*                      =  1/sqrt(2) * (sin(x) + cos(x))
*              sin(x0) = sin(x)cos(3pi/4)-cos(x)sin(3pi/4)
*                      =  1/sqrt(2) * (sin(x) - cos(x))
* To avoid cancellation, use
*              sin(x) +- cos(x) = -cos(2x)/(sin(x) -+ cos(x))
* to compute the worse one.
*/
s = sin(x);
c = cos(x);
ss = s-c;
cc = s+c;
/*
* j0(x) = 1/sqrt(pi) * (P(0,x)*cc - Q(0,x)*ss) / sqrt(x)
* y0(x) = 1/sqrt(pi) * (P(0,x)*ss + Q(0,x)*cc) / sqrt(x)
*/
if(ix<0x7fe00000) {  /* make sure x+x not overflow */
z = -cos(x+x);
if ((s*c)<zero) cc = z/ss;
else            ss = z/cc;
}
if(ix>0x48000000) z = (invsqrtpi*ss)/sqrt(x);
else {
u = pzero(x); v = qzero(x);
z = invsqrtpi*(u*ss+v*cc)/sqrt(x);
}
return z;
}
if(ix<=0x3e400000) {	/* x < 2**-27 */
return(u00 + tpi*__ieee754_log(x));
}
z = x*x;
u = u00+z*(u01+z*(u02+z*(u03+z*(u04+z*(u05+z*u06)))));
v = one+z*(v01+z*(v02+z*(v03+z*v04)));
return(u/v + tpi*(__ieee754_j0(x)*__ieee754_log(x)));
}

/* The asymptotic expansions of pzero is
*	1 - 9/128 s^2 + 11025/98304 s^4 - ...,	where s = 1/x.
* For x >= 2, We approximate pzero by
* 	pzero(x) = 1 + (R/S)
* where  R = pR0 + pR1*s^2 + pR2*s^4 + ... + pR5*s^10
* 	  S = 1 + pS0*s^2 + ... + pS4*s^10
* and
*	| pzero(x)-1-R/S | <= 2  ** ( -60.26)
*/
static const double pR8[6] = { /* for x in [inf, 8]=1/[0,0.125] */
0.00000000000000000000e+00, /* 0x00000000, 0x00000000 */
-7.03124999999900357484e-02, /* 0xBFB1FFFF, 0xFFFFFD32 */
-8.08167041275349795626e+00, /* 0xC02029D0, 0xB44FA779 */
-2.57063105679704847262e+02, /* 0xC0701102, 0x7B19E863 */
-2.48521641009428822144e+03, /* 0xC0A36A6E, 0xCD4DCAFC */
-5.25304380490729545272e+03, /* 0xC0B4850B, 0x36CC643D */
};
static const double pS8[5] = {
1.16534364619668181717e+02, /* 0x405D2233, 0x07A96751 */
3.83374475364121826715e+03, /* 0x40ADF37D, 0x50596938 */
4.05978572648472545552e+04, /* 0x40E3D2BB, 0x6EB6B05F */
1.16752972564375915681e+05, /* 0x40FC810F, 0x8F9FA9BD */
4.76277284146730962675e+04, /* 0x40E74177, 0x4F2C49DC */
};

static const double pR5[6] = { /* for x in [8,4.5454]=1/[0.125,0.22001] */
-1.14125464691894502584e-11, /* 0xBDA918B1, 0x47E495CC */
-7.03124940873599280078e-02, /* 0xBFB1FFFF, 0xE69AFBC6 */
-4.15961064470587782438e+00, /* 0xC010A370, 0xF90C6BBF */
-6.76747652265167261021e+01, /* 0xC050EB2F, 0x5A7D1783 */
-3.31231299649172967747e+02, /* 0xC074B3B3, 0x6742CC63 */
-3.46433388365604912451e+02, /* 0xC075A6EF, 0x28A38BD7 */
};
static const double pS5[5] = {
6.07539382692300335975e+01, /* 0x404E6081, 0x0C98C5DE */
1.05125230595704579173e+03, /* 0x40906D02, 0x5C7E2864 */
5.97897094333855784498e+03, /* 0x40B75AF8, 0x8FBE1D60 */
9.62544514357774460223e+03, /* 0x40C2CCB8, 0xFA76FA38 */
2.40605815922939109441e+03, /* 0x40A2CC1D, 0xC70BE864 */
};

static const double pR3[6] = {/* for x in [4.547,2.8571]=1/[0.2199,0.35001] */
-2.54704601771951915620e-09, /* 0xBE25E103, 0x6FE1AA86 */
-7.03119616381481654654e-02, /* 0xBFB1FFF6, 0xF7C0E24B */
-2.40903221549529611423e+00, /* 0xC00345B2, 0xAEA48074 */
-2.19659774734883086467e+01, /* 0xC035F74A, 0x4CB94E14 */
-5.80791704701737572236e+01, /* 0xC04D0A22, 0x420A1A45 */
-3.14479470594888503854e+01, /* 0xC03F72AC, 0xA892D80F */
};
static const double pS3[5] = {
3.58560338055209726349e+01, /* 0x4041ED92, 0x84077DD3 */
3.61513983050303863820e+02, /* 0x40769839, 0x464A7C0E */
1.19360783792111533330e+03, /* 0x4092A66E, 0x6D1061D6 */
1.12799679856907414432e+03, /* 0x40919FFC, 0xB8C39B7E */
1.73580930813335754692e+02, /* 0x4065B296, 0xFC379081 */
};

static const double pR2[6] = {/* for x in [2.8570,2]=1/[0.3499,0.5] */
-8.87534333032526411254e-08, /* 0xBE77D316, 0xE927026D */
-7.03030995483624743247e-02, /* 0xBFB1FF62, 0x495E1E42 */
-1.45073846780952986357e+00, /* 0xBFF73639, 0x8A24A843 */
-7.63569613823527770791e+00, /* 0xC01E8AF3, 0xEDAFA7F3 */
-1.11931668860356747786e+01, /* 0xC02662E6, 0xC5246303 */
-3.23364579351335335033e+00, /* 0xC009DE81, 0xAF8FE70F */
};
static const double pS2[5] = {
2.22202997532088808441e+01, /* 0x40363865, 0x908B5959 */
1.36206794218215208048e+02, /* 0x4061069E, 0x0EE8878F */
2.70470278658083486789e+02, /* 0x4070E786, 0x42EA079B */
1.53875394208320329881e+02, /* 0x40633C03, 0x3AB6FAFF */
1.46576176948256193810e+01, /* 0x402D50B3, 0x44391809 */
};

static double pzero(double x)
{
const double *p,*q;
double z,r,s;
int ix;
ix = 0x7fffffff&__HI(x);
if(ix>=0x40200000)     {p = pR8; q= pS8;}
else if(ix>=0x40122E8B){p = pR5; q= pS5;}
else if(ix>=0x4006DB6D){p = pR3; q= pS3;}
else if(ix>=0x40000000){p = pR2; q= pS2;}
z = one/(x*x);
r = p[0]+z*(p[1]+z*(p[2]+z*(p[3]+z*(p[4]+z*p[5]))));
s = one+z*(q[0]+z*(q[1]+z*(q[2]+z*(q[3]+z*q[4]))));
return one+ r/s;
}

/* For x >= 8, the asymptotic expansions of qzero is
*	-1/8 s + 75/1024 s^3 - ..., where s = 1/x.
* We approximate pzero by
* 	qzero(x) = s*(-1.25 + (R/S))
* where  R = qR0 + qR1*s^2 + qR2*s^4 + ... + qR5*s^10
* 	  S = 1 + qS0*s^2 + ... + qS5*s^12
* and
*	| qzero(x)/s +1.25-R/S | <= 2  ** ( -61.22)
*/
static const double qR8[6] = { /* for x in [inf, 8]=1/[0,0.125] */
0.00000000000000000000e+00, /* 0x00000000, 0x00000000 */
7.32421874999935051953e-02, /* 0x3FB2BFFF, 0xFFFFFE2C */
1.17682064682252693899e+01, /* 0x40278952, 0x5BB334D6 */
5.57673380256401856059e+02, /* 0x40816D63, 0x15301825 */
8.85919720756468632317e+03, /* 0x40C14D99, 0x3E18F46D */
3.70146267776887834771e+04, /* 0x40E212D4, 0x0E901566 */
};
static const double qS8[6] = {
1.63776026895689824414e+02, /* 0x406478D5, 0x365B39BC */
8.09834494656449805916e+03, /* 0x40BFA258, 0x4E6B0563 */
1.42538291419120476348e+05, /* 0x41016652, 0x54D38C3F */
8.03309257119514397345e+05, /* 0x412883DA, 0x83A52B43 */
8.40501579819060512818e+05, /* 0x4129A66B, 0x28DE0B3D */
-3.43899293537866615225e+05, /* 0xC114FD6D, 0x2C9530C5 */
};

static const double qR5[6] = { /* for x in [8,4.5454]=1/[0.125,0.22001] */
1.84085963594515531381e-11, /* 0x3DB43D8F, 0x29CC8CD9 */
7.32421766612684765896e-02, /* 0x3FB2BFFF, 0xD172B04C */
5.83563508962056953777e+00, /* 0x401757B0, 0xB9953DD3 */
1.35111577286449829671e+02, /* 0x4060E392, 0x0A8788E9 */
1.02724376596164097464e+03, /* 0x40900CF9, 0x9DC8C481 */
1.98997785864605384631e+03, /* 0x409F17E9, 0x53C6E3A6 */
};
static const double qS5[6] = {
8.27766102236537761883e+01, /* 0x4054B1B3, 0xFB5E1543 */
2.07781416421392987104e+03, /* 0x40A03BA0, 0xDA21C0CE */
1.88472887785718085070e+04, /* 0x40D267D2, 0x7B591E6D */
5.67511122894947329769e+04, /* 0x40EBB5E3, 0x97E02372 */
3.59767538425114471465e+04, /* 0x40E19118, 0x1F7A54A0 */
-5.35434275601944773371e+03, /* 0xC0B4EA57, 0xBEDBC609 */
};

static const double qR3[6] = {/* for x in [4.547,2.8571]=1/[0.2199,0.35001] */
4.37741014089738620906e-09, /* 0x3E32CD03, 0x6ADECB82 */
7.32411180042911447163e-02, /* 0x3FB2BFEE, 0x0E8D0842 */
3.34423137516170720929e+00, /* 0x400AC0FC, 0x61149CF5 */
4.26218440745412650017e+01, /* 0x40454F98, 0x962DAEDD */
1.70808091340565596283e+02, /* 0x406559DB, 0xE25EFD1F */
1.66733948696651168575e+02, /* 0x4064D77C, 0x81FA21E0 */
};
static const double qS3[6] = {
4.87588729724587182091e+01, /* 0x40486122, 0xBFE343A6 */
7.09689221056606015736e+02, /* 0x40862D83, 0x86544EB3 */
3.70414822620111362994e+03, /* 0x40ACF04B, 0xE44DFC63 */
6.46042516752568917582e+03, /* 0x40B93C6C, 0xD7C76A28 */
2.51633368920368957333e+03, /* 0x40A3A8AA, 0xD94FB1C0 */
-1.49247451836156386662e+02, /* 0xC062A7EB, 0x201CF40F */
};

static const double qR2[6] = {/* for x in [2.8570,2]=1/[0.3499,0.5] */
1.50444444886983272379e-07, /* 0x3E84313B, 0x54F76BDB */
7.32234265963079278272e-02, /* 0x3FB2BEC5, 0x3E883E34 */
1.99819174093815998816e+00, /* 0x3FFFF897, 0xE727779C */
1.44956029347885735348e+01, /* 0x402CFDBF, 0xAAF96FE5 */
3.16662317504781540833e+01, /* 0x403FAA8E, 0x29FBDC4A */
1.62527075710929267416e+01, /* 0x403040B1, 0x71814BB4 */
};
static const double qS2[6] = {
3.03655848355219184498e+01, /* 0x403E5D96, 0xF7C07AED */
2.69348118608049844624e+02, /* 0x4070D591, 0xE4D14B40 */
8.44783757595320139444e+02, /* 0x408A6645, 0x22B3BF22 */
8.82935845112488550512e+02, /* 0x408B977C, 0x9C5CC214 */
2.12666388511798828631e+02, /* 0x406A9553, 0x0E001365 */
-5.31095493882666946917e+00, /* 0xC0153E6A, 0xF8B32931 */
};

static double qzero(double x)
{
const double *p,*q;
double s,r,z;
int ix;
ix = 0x7fffffff&__HI(x);
if(ix>=0x40200000)     {p = qR8; q= qS8;}
else if(ix>=0x40122E8B){p = qR5; q= qS5;}
else if(ix>=0x4006DB6D){p = qR3; q= qS3;}
else if(ix>=0x40000000){p = qR2; q= qS2;}
z = one/(x*x);
r = p[0]+z*(p[1]+z*(p[2]+z*(p[3]+z*(p[4]+z*p[5]))));
s = one+z*(q[0]+z*(q[1]+z*(q[2]+z*(q[3]+z*(q[4]+z*q[5])))));
return (-.125 + r/s)/x;
}
```