import numpy as np


def phi_single_source(
        r: float,
        activity: float | int,
        branching_ratio: float,
        mu_mass_air: float,
        air_density: float,
        ) -> float:
    """Compute the contribution of a single point source to the
    primary photon fluence rate phi at position (x,y,z).

    Args:
        r (float): [m] Distance to the point source.
        activity (float | int): [Bq] Activity of the point source.
        branching_ratio (float): Branching ratio for the photon energy E_gamma.
        mu_mass_air (float): [cm^2/g] Mass attenuation coefficient for air.
        air_density (float): [kg/m^3] Air density.

    Returns:
        phi (float): [s^-1 m^-2] Primary photon fluence rate at distance r from
        a point source.
    """

    # Linear photon attenuation coefficient in m^-1.
    mu_mass_air *= 0.1
    mu_air = 0.1 * mu_mass_air * air_density

    phi = (
        activity
        * branching_ratio
        * np.exp(-mu_air * r)
        / (4 * np.pi * r**2)
    )

    return phi