///|
let i1_arrA : ReadOnlyArray[Double] = [
  2.77791411276104639959E-18, -2.11142121435816608115E-17, 1.55363195773620046921E-16,
  -1.10559694773538630805E-15, 7.60068429473540693410E-15, -5.04218550472791168711E-14,
  3.22379336594557470981E-13, -1.98397439776494371520E-12, 1.17361862988909016308E-11,
  -6.66348972350202774223E-11, 3.62559028155211703701E-10, -1.88724975172282928790E-9,
  9.38153738649577178388E-9, -4.44505912879632808065E-8, 2.00329475355213526229E-7,
  -8.56872026469545474066E-7, 3.47025130813767847674E-6, -1.32731636560394358279E-5,
  4.78156510755005422638E-5, -1.61760815825896745588E-4, 5.12285956168575772895E-4,
  -1.51357245063125314899E-3, 4.15642294431288815669E-3, -1.05640848946261981558E-2,
  2.47264490306265168283E-2, -5.29459812080949914269E-2, 1.02643658689847095384E-1,
  -1.76416518357834055153E-1, 2.52587186443633654823E-1,
]

///|
let i1_arrB : ReadOnlyArray[Double] = [
  7.51729631084210481353E-18, 4.41434832307170791151E-18, -4.65030536848935832153E-17,
  -3.20952592199342395980E-17, 2.96262899764595013876E-16, 3.30820231092092828324E-16,
  -1.88035477551078244854E-15, -3.81440307243700780478E-15, 1.04202769841288027642E-14,
  4.27244001671195135429E-14, -2.10154184277266431302E-14, -4.08355111109219731823E-13,
  -7.19855177624590851209E-13, 2.03562854414708950722E-12, 1.41258074366137813316E-11,
  3.25260358301548823856E-11, -1.89749581235054123450E-11, -5.58974346219658380687E-10,
  -3.83538038596423702205E-9, -2.63146884688951950684E-8, -2.51223623787020892529E-7,
  -3.88256480887769039346E-6, -1.10588938762623716291E-4, -9.76109749136146840777E-3,
  7.78576235018280120474E-1,
]

///|
/// Computes the modified Bessel function of the first kind of order one.
pub fn bessel_i1(x : Double) -> Double {
  let z = x.abs()
  let z = if z <= 8.0 {
    let y = z / 2.0 - 2.0
    chbevl(y, i1_arrA) * z * exp(z)
  } else {
    exp(z) * chbevl(32.0 / z - 2.0, i1_arrB) / sqrt(z)
  }
  return if x < 0.0 { -z } else { z }
}

///|
/// `i1` is an alias for `bessel_i1`.
pub let i1 : (Double) -> Double = bessel_i1