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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package math

/*
        Bessel function of the first and second kinds of order one.
*/

// The original C code and the long comment below are
// from FreeBSD's /usr/src/lib/msun/src/e_j1.c and
// came with this notice.  The go code is a simplified
// version of the original C.
//
// ====================================================
// Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
//
// Developed at SunPro, a Sun Microsystems, Inc. business.
// Permission to use, copy, modify, and distribute this
// software is freely granted, provided that this notice
// is preserved.
// ====================================================
//
// __ieee754_j1(x), __ieee754_y1(x)
// Bessel function of the first and second kinds of order one.
// Method -- j1(x):
//      1. For tiny x, we use j1(x) = x/2 - x**3/16 + x**5/384 - ...
//      2. Reduce x to |x| since j1(x)=-j1(-x),  and
//         for x in (0,2)
//              j1(x) = x/2 + x*z*R0/S0,  where z = x*x;
//         (precision:  |j1/x - 1/2 - R0/S0 |<2**-61.51 )
//         for x in (2,inf)
//              j1(x) = sqrt(2/(pi*x))*(p1(x)*cos(x1)-q1(x)*sin(x1))
//              y1(x) = sqrt(2/(pi*x))*(p1(x)*sin(x1)+q1(x)*cos(x1))
//         where x1 = x-3*pi/4. It is better to compute sin(x1),cos(x1)
//         as follow:
//              cos(x1) =  cos(x)cos(3pi/4)+sin(x)sin(3pi/4)
//                      =  1/sqrt(2) * (sin(x) - cos(x))
//              sin(x1) =  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.)
//
//      3 Special cases
//              j1(nan)= nan
//              j1(0) = 0
//              j1(inf) = 0
//
// Method -- y1(x):
//      1. screen out x<=0 cases: y1(0)=-inf, y1(x<0)=NaN
//      2. For x<2.
//         Since
//              y1(x) = 2/pi*(j1(x)*(ln(x/2)+Euler)-1/x-x/2+5/64*x**3-...)
//         therefore y1(x)-2/pi*j1(x)*ln(x)-1/x is an odd function.
//         We use the following function to approximate y1,
//              y1(x) = x*U(z)/V(z) + (2/pi)*(j1(x)*ln(x)-1/x), z= x**2
//         where for x in [0,2] (abs err less than 2**-65.89)
//              U(z) = U0[0] + U0[1]*z + ... + U0[4]*z**4
//              V(z) = 1  + v0[0]*z + ... + v0[4]*z**5
//         Note: For tiny x, 1/x dominate y1 and hence
//              y1(tiny) = -2/pi/tiny, (choose tiny<2**-54)
//      3. For x>=2.
//               y1(x) = sqrt(2/(pi*x))*(p1(x)*sin(x1)+q1(x)*cos(x1))
//         where x1 = x-3*pi/4. It is better to compute sin(x1),cos(x1)
//         by method mentioned above.

// J1 returns the order-one Bessel function of the first kind.
//
// Special cases are:
//      J1(±Inf) = 0
//      J1(NaN) = NaN
func J1(x float64) float64 {
        const (
                TwoM27 = 1.0 / (1 << 27) // 2**-27 0x3e40000000000000
                Two129 = 1 << 129        // 2**129 0x4800000000000000
                // R0/S0 on [0, 2]
                R00 = -6.25000000000000000000e-02 // 0xBFB0000000000000
                R01 = 1.40705666955189706048e-03  // 0x3F570D9F98472C61
                R02 = -1.59955631084035597520e-05 // 0xBEF0C5C6BA169668
                R03 = 4.96727999609584448412e-08  // 0x3E6AAAFA46CA0BD9
                S01 = 1.91537599538363460805e-02  // 0x3F939D0B12637E53
                S02 = 1.85946785588630915560e-04  // 0x3F285F56B9CDF664
                S03 = 1.17718464042623683263e-06  // 0x3EB3BFF8333F8498
                S04 = 5.04636257076217042715e-09  // 0x3E35AC88C97DFF2C
                S05 = 1.23542274426137913908e-11  // 0x3DAB2ACFCFB97ED8
        )
        // special cases
        switch {
        case IsNaN(x):
                return x
        case IsInf(x, 0) || x == 0:
                return 0
        }

        sign := false
        if x < 0 {
                x = -x
                sign = true
        }
        if x >= 2 {
                s, c := Sincos(x)
                ss := -s - c
                cc := s - c

                // make sure x+x does not overflow
                if x < MaxFloat64/2 {
                        z := Cos(x + x)
                        if s*c > 0 {
                                cc = z / ss
                        } else {
                                ss = z / cc
                        }
                }

                // j1(x) = 1/sqrt(pi) * (P(1,x)*cc - Q(1,x)*ss) / sqrt(x)
                // y1(x) = 1/sqrt(pi) * (P(1,x)*ss + Q(1,x)*cc) / sqrt(x)

                var z float64
                if x > Two129 {
                        z = (1 / SqrtPi) * cc / Sqrt(x)
                } else {
                        u := pone(x)
                        v := qone(x)
                        z = (1 / SqrtPi) * (u*cc - v*ss) / Sqrt(x)
                }
                if sign {
                        return -z
                }
                return z
        }
        if x < TwoM27 { // |x|<2**-27
                return 0.5 * x // inexact if x!=0 necessary
        }
        z := x * x
        r := z * (R00 + z*(R01+z*(R02+z*R03)))
        s := 1.0 + z*(S01+z*(S02+z*(S03+z*(S04+z*S05))))
        r *= x
        z = 0.5*x + r/s
        if sign {
                return -z
        }
        return z
}

// Y1 returns the order-one Bessel function of the second kind.
//
// Special cases are:
//      Y1(+Inf) = 0
//      Y1(0) = -Inf
//      Y1(x < 0) = NaN
//      Y1(NaN) = NaN
func Y1(x float64) float64 {
        const (
                TwoM54 = 1.0 / (1 << 54)             // 2**-54 0x3c90000000000000
                Two129 = 1 << 129                    // 2**129 0x4800000000000000
                U00    = -1.96057090646238940668e-01 // 0xBFC91866143CBC8A
                U01    = 5.04438716639811282616e-02  // 0x3FA9D3C776292CD1
                U02    = -1.91256895875763547298e-03 // 0xBF5F55E54844F50F
                U03    = 2.35252600561610495928e-05  // 0x3EF8AB038FA6B88E
                U04    = -9.19099158039878874504e-08 // 0xBE78AC00569105B8
                V00    = 1.99167318236649903973e-02  // 0x3F94650D3F4DA9F0
                V01    = 2.02552581025135171496e-04  // 0x3F2A8C896C257764
                V02    = 1.35608801097516229404e-06  // 0x3EB6C05A894E8CA6
                V03    = 6.22741452364621501295e-09  // 0x3E3ABF1D5BA69A86
                V04    = 1.66559246207992079114e-11  // 0x3DB25039DACA772A
        )
        // special cases
        switch {
        case x < 0 || IsNaN(x):
                return NaN()
        case IsInf(x, 1):
                return 0
        case x == 0:
                return Inf(-1)
        }

        if x >= 2 {
                s, c := Sincos(x)
                ss := -s - c
                cc := s - c

                // make sure x+x does not overflow
                if x < MaxFloat64/2 {
                        z := Cos(x + x)
                        if s*c > 0 {
                                cc = z / ss
                        } else {
                                ss = z / cc
                        }
                }
                // y1(x) = sqrt(2/(pi*x))*(p1(x)*sin(x0)+q1(x)*cos(x0))
                // where x0 = x-3pi/4
                //     Better formula:
                //         cos(x0) = cos(x)cos(3pi/4)+sin(x)sin(3pi/4)
                //                 =  1/sqrt(2) * (sin(x) - cos(x))
                //         sin(x0) = sin(x)cos(3pi/4)-cos(x)sin(3pi/4)
                //                 = -1/sqrt(2) * (cos(x) + sin(x))
                // To avoid cancellation, use
                //     sin(x) +- cos(x) = -cos(2x)/(sin(x) -+ cos(x))
                // to compute the worse one.

                var z float64
                if x > Two129 {
                        z = (1 / SqrtPi) * ss / Sqrt(x)
                } else {
                        u := pone(x)
                        v := qone(x)
                        z = (1 / SqrtPi) * (u*ss + v*cc) / Sqrt(x)
                }
                return z
        }
        if x <= TwoM54 { // x < 2**-54
                return -(2 / Pi) / x
        }
        z := x * x
        u := U00 + z*(U01+z*(U02+z*(U03+z*U04)))
        v := 1 + z*(V00+z*(V01+z*(V02+z*(V03+z*V04))))
        return x*(u/v) + (2/Pi)*(J1(x)*Log(x)-1/x)
}

// For x >= 8, the asymptotic expansions of pone is
//      1 + 15/128 s**2 - 4725/2**15 s**4 - ..., where s = 1/x.
// We approximate pone by
//      pone(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
//      | pone(x)-1-R/S | <= 2**(-60.06)

// for x in [inf, 8]=1/[0,0.125]
var p1R8 = [6]float64{
        0.00000000000000000000e+00, // 0x0000000000000000
        1.17187499999988647970e-01, // 0x3FBDFFFFFFFFFCCE
        1.32394806593073575129e+01, // 0x402A7A9D357F7FCE
        4.12051854307378562225e+02, // 0x4079C0D4652EA590
        3.87474538913960532227e+03, // 0x40AE457DA3A532CC
        7.91447954031891731574e+03, // 0x40BEEA7AC32782DD
}
var p1S8 = [5]float64{
        1.14207370375678408436e+02, // 0x405C8D458E656CAC
        3.65093083420853463394e+03, // 0x40AC85DC964D274F
        3.69562060269033463555e+04, // 0x40E20B8697C5BB7F
        9.76027935934950801311e+04, // 0x40F7D42CB28F17BB
        3.08042720627888811578e+04, // 0x40DE1511697A0B2D
}

// for x in [8,4.5454] = 1/[0.125,0.22001]
var p1R5 = [6]float64{
        1.31990519556243522749e-11, // 0x3DAD0667DAE1CA7D
        1.17187493190614097638e-01, // 0x3FBDFFFFE2C10043
        6.80275127868432871736e+00, // 0x401B36046E6315E3
        1.08308182990189109773e+02, // 0x405B13B9452602ED
        5.17636139533199752805e+02, // 0x40802D16D052D649
        5.28715201363337541807e+02, // 0x408085B8BB7E0CB7
}
var p1S5 = [5]float64{
        5.92805987221131331921e+01, // 0x404DA3EAA8AF633D
        9.91401418733614377743e+02, // 0x408EFB361B066701
        5.35326695291487976647e+03, // 0x40B4E9445706B6FB
        7.84469031749551231769e+03, // 0x40BEA4B0B8A5BB15
        1.50404688810361062679e+03, // 0x40978030036F5E51
}

// for x in[4.5453,2.8571] = 1/[0.2199,0.35001]
var p1R3 = [6]float64{
        3.02503916137373618024e-09, // 0x3E29FC21A7AD9EDD
        1.17186865567253592491e-01, // 0x3FBDFFF55B21D17B
        3.93297750033315640650e+00, // 0x400F76BCE85EAD8A
        3.51194035591636932736e+01, // 0x40418F489DA6D129
        9.10550110750781271918e+01, // 0x4056C3854D2C1837
        4.85590685197364919645e+01, // 0x4048478F8EA83EE5
}
var p1S3 = [5]float64{
        3.47913095001251519989e+01, // 0x40416549A134069C
        3.36762458747825746741e+02, // 0x40750C3307F1A75F
        1.04687139975775130551e+03, // 0x40905B7C5037D523
        8.90811346398256432622e+02, // 0x408BD67DA32E31E9
        1.03787932439639277504e+02, // 0x4059F26D7C2EED53
}

// for x in [2.8570,2] = 1/[0.3499,0.5]
var p1R2 = [6]float64{
        1.07710830106873743082e-07, // 0x3E7CE9D4F65544F4
        1.17176219462683348094e-01, // 0x3FBDFF42BE760D83
        2.36851496667608785174e+00, // 0x4002F2B7F98FAEC0
        1.22426109148261232917e+01, // 0x40287C377F71A964
        1.76939711271687727390e+01, // 0x4031B1A8177F8EE2
        5.07352312588818499250e+00, // 0x40144B49A574C1FE
}
var p1S2 = [5]float64{
        2.14364859363821409488e+01, // 0x40356FBD8AD5ECDC
        1.25290227168402751090e+02, // 0x405F529314F92CD5
        2.32276469057162813669e+02, // 0x406D08D8D5A2DBD9
        1.17679373287147100768e+02, // 0x405D6B7ADA1884A9
        8.36463893371618283368e+00, // 0x4020BAB1F44E5192
}

func pone(x float64) float64 {
        var p [6]float64
        var q [5]float64
        if x >= 8 {
                p = p1R8
                q = p1S8
        } else if x >= 4.5454 {
                p = p1R5
                q = p1S5
        } else if x >= 2.8571 {
                p = p1R3
                q = p1S3
        } else if x >= 2 {
                p = p1R2
                q = p1S2
        }
        z := 1 / (x * x)
        r := p[0] + z*(p[1]+z*(p[2]+z*(p[3]+z*(p[4]+z*p[5]))))
        s := 1.0 + z*(q[0]+z*(q[1]+z*(q[2]+z*(q[3]+z*q[4]))))
        return 1 + r/s
}

// For x >= 8, the asymptotic expansions of qone is
//      3/8 s - 105/1024 s**3 - ..., where s = 1/x.
// We approximate qone by
//      qone(x) = s*(0.375 + (R/S))
// where R = qr1*s**2 + qr2*s**4 + ... + qr5*s**10
//       S = 1 + qs1*s**2 + ... + qs6*s**12
// and
//      | qone(x)/s -0.375-R/S | <= 2**(-61.13)

// for x in [inf, 8] = 1/[0,0.125]
var q1R8 = [6]float64{
        0.00000000000000000000e+00,  // 0x0000000000000000
        -1.02539062499992714161e-01, // 0xBFBA3FFFFFFFFDF3
        -1.62717534544589987888e+01, // 0xC0304591A26779F7
        -7.59601722513950107896e+02, // 0xC087BCD053E4B576
        -1.18498066702429587167e+04, // 0xC0C724E740F87415
        -4.84385124285750353010e+04, // 0xC0E7A6D065D09C6A
}
var q1S8 = [6]float64{
        1.61395369700722909556e+02,  // 0x40642CA6DE5BCDE5
        7.82538599923348465381e+03,  // 0x40BE9162D0D88419
        1.33875336287249578163e+05,  // 0x4100579AB0B75E98
        7.19657723683240939863e+05,  // 0x4125F65372869C19
        6.66601232617776375264e+05,  // 0x412457D27719AD5C
        -2.94490264303834643215e+05, // 0xC111F9690EA5AA18
}

// for x in [8,4.5454] = 1/[0.125,0.22001]
var q1R5 = [6]float64{
        -2.08979931141764104297e-11, // 0xBDB6FA431AA1A098
        -1.02539050241375426231e-01, // 0xBFBA3FFFCB597FEF
        -8.05644828123936029840e+00, // 0xC0201CE6CA03AD4B
        -1.83669607474888380239e+02, // 0xC066F56D6CA7B9B0
        -1.37319376065508163265e+03, // 0xC09574C66931734F
        -2.61244440453215656817e+03, // 0xC0A468E388FDA79D
}
var q1S5 = [6]float64{
        8.12765501384335777857e+01,  // 0x405451B2FF5A11B2
        1.99179873460485964642e+03,  // 0x409F1F31E77BF839
        1.74684851924908907677e+04,  // 0x40D10F1F0D64CE29
        4.98514270910352279316e+04,  // 0x40E8576DAABAD197
        2.79480751638918118260e+04,  // 0x40DB4B04CF7C364B
        -4.71918354795128470869e+03, // 0xC0B26F2EFCFFA004
}

// for x in [4.5454,2.8571] = 1/[0.2199,0.35001] ???
var q1R3 = [6]float64{
        -5.07831226461766561369e-09, // 0xBE35CFA9D38FC84F
        -1.02537829820837089745e-01, // 0xBFBA3FEB51AEED54
        -4.61011581139473403113e+00, // 0xC01270C23302D9FF
        -5.78472216562783643212e+01, // 0xC04CEC71C25D16DA
        -2.28244540737631695038e+02, // 0xC06C87D34718D55F
        -2.19210128478909325622e+02, // 0xC06B66B95F5C1BF6
}
var q1S3 = [6]float64{
        4.76651550323729509273e+01,  // 0x4047D523CCD367E4
        6.73865112676699709482e+02,  // 0x40850EEBC031EE3E
        3.38015286679526343505e+03,  // 0x40AA684E448E7C9A
        5.54772909720722782367e+03,  // 0x40B5ABBAA61D54A6
        1.90311919338810798763e+03,  // 0x409DBC7A0DD4DF4B
        -1.35201191444307340817e+02, // 0xC060E670290A311F
}

// for x in [2.8570,2] = 1/[0.3499,0.5]
var q1R2 = [6]float64{
        -1.78381727510958865572e-07, // 0xBE87F12644C626D2
        -1.02517042607985553460e-01, // 0xBFBA3E8E9148B010
        -2.75220568278187460720e+00, // 0xC006048469BB4EDA
        -1.96636162643703720221e+01, // 0xC033A9E2C168907F
        -4.23253133372830490089e+01, // 0xC04529A3DE104AAA
        -2.13719211703704061733e+01, // 0xC0355F3639CF6E52
}
var q1S2 = [6]float64{
        2.95333629060523854548e+01,  // 0x403D888A78AE64FF
        2.52981549982190529136e+02,  // 0x406F9F68DB821CBA
        7.57502834868645436472e+02,  // 0x4087AC05CE49A0F7
        7.39393205320467245656e+02,  // 0x40871B2548D4C029
        1.55949003336666123687e+02,  // 0x40637E5E3C3ED8D4
        -4.95949898822628210127e+00, // 0xC013D686E71BE86B
}

func qone(x float64) float64 {
        var p, q [6]float64
        if x >= 8 {
                p = q1R8
                q = q1S8
        } else if x >= 4.5454 {
                p = q1R5
                q = q1S5
        } else if x >= 2.8571 {
                p = q1R3
                q = q1S3
        } else if x >= 2 {
                p = q1R2
                q = q1S2
        }
        z := 1 / (x * x)
        r := p[0] + z*(p[1]+z*(p[2]+z*(p[3]+z*(p[4]+z*p[5]))))
        s := 1 + z*(q[0]+z*(q[1]+z*(q[2]+z*(q[3]+z*(q[4]+z*q[5])))))
        return (0.375 + r/s) / x
}