import numpy as np

def hli(irradiance : float, air_temperature : float, RH : float, wind_speed : float):
    """
    Compute Heat Load Index (HLI), a thermal stress indicator for animals.

    Parameters
    ----------
    irradiance : float
        Solar radiation [W.m-2]

    air_temperature : float
        Air temperature [°C]

    RH : float
        Relative humidity [%]
        
    wind_speed : float
        Wind speed [m.s-1]

    Returns
    -------
    float
        Heat Load Index value.
    """

    BGT = 0.01498*irradiance + 1.184*air_temperature - 0.0789*RH - 2.739
    HLI = np.where(BGT >= 25, 1.55*BGT- 0.5*wind_speed + np.exp(2.4 - wind_speed)+8.62 + 0.38*RH, 1.30*BGT - wind_speed+10.66 + 0.28*RH)
    
    return HLI