#!/usr/bin/env python3
import mpmath as mp


# APPLIED OPTICS / Vol. 53, No. 31 / 1 November 2014, eq(13)
def LeRu_cutoff(z):
    x = mp.fabs(z)
    return int(x + 11 * x**(1/3) + 1)


# Wu, Wang, Radio Science, Volume 26, Number 6, Pages 1393-1401, November-December 1991,
# after eq 13f.
# Riccati-Bessel z*j_n(z)
def psi(n,z):
    return mp.sqrt( (mp.pi * z)/2 ) * mp.autoprec(mp.besselj)(n+1/2,z)
# Riccati-Bessel -z*y_n(z)
def xi(n,z):
    return -mp.sqrt( (mp.pi * z)/2 ) * mp.autoprec(mp.bessely)(n+1/2,z)
# Riccati-Bessel psi - i* xi
def ksi(n,z):
    return psi(n,z) - 1.j * xi(n,z)

def psi_div_ksi(n,z):
    return psi(n,z)/ksi(n,z)

def psi_div_xi(n,z):
    return psi(n,z)/xi(n,z)

# to compare r(n,z) with Wolfram Alpha
# n=49, z=1.3-2.1i,  SphericalBesselJ[n-1,z]/SphericalBesselJ[n,z]
def r(n,z):
    if n > 0:
        return psi(n-1,z)/psi(n,z)
    return mp.cos(z)/mp.sin(z)


def D(n, z, f):
    return f(n-1,z)/f(n,z) - n/z

def D1(n,z):
    if n == 0: return mp.cos(z)/mp.sin(z)
    return D(n, z, psi)

# Wolfram Alpha example D2(10, 10-10j): SphericalBesselY[9, 10-10i]/SphericalBesselY[10,10-10i]-10/(10-10i)
def D2(n,z):
    if n == 0: return -mp.sin(z)/mp.cos(z)
    return D(n, z, xi)

def D3(n,z):
    if n == 0: return 1j
    return D(n, z, ksi)