poliastro/poliastro

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src/poliastro/core/stumpff.py

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from math import gamma

import numpy as np

from ._jit import jit


@jit
def c2(psi):
    r"""Second Stumpff function.

    For positive arguments:

    .. math::

        c_2(\psi) = \frac{1 - \cos{\sqrt{\psi}}}{\psi}

    """
    eps = 1.0
    if psi > eps:
        res = (1 - np.cos(np.sqrt(psi))) / psi
    elif psi < -eps:
        res = (np.cosh(np.sqrt(-psi)) - 1) / (-psi)
    else:
        res = 1.0 / 2.0
        delta = (-psi) / gamma(2 + 2 + 1)
        k = 1
        while res + delta != res:
            res = res + delta
            k += 1
            delta = (-psi) ** k / gamma(2 * k + 2 + 1)

    return res


@jit
def c3(psi):
    r"""Third Stumpff function.

    For positive arguments:

    .. math::

        c_3(\psi) = \frac{\sqrt{\psi} - \sin{\sqrt{\psi}}}{\sqrt{\psi^3}}

    """
    eps = 1.0
    if psi > eps:
        res = (np.sqrt(psi) - np.sin(np.sqrt(psi))) / (psi * np.sqrt(psi))
    elif psi < -eps:
        res = (np.sinh(np.sqrt(-psi)) - np.sqrt(-psi)) / (-psi * np.sqrt(-psi))
    else:
        res = 1.0 / 6.0
        delta = (-psi) / gamma(2 + 3 + 1)
        k = 1
        while res + delta != res:
            res = res + delta
            k += 1
            delta = (-psi) ** k / gamma(2 * k + 3 + 1)

    return res