import math


def pmv(ta, tr, va, rh, met, clo, wmet=0):
    """
    Get PMV value based on the 2017 ASHRAE Handbook—Fundamentals, Chapter 9:
    Thermal Comfort, Equations 63 - 68.

    Parameters
    ----------
    ta : float, optional
        Air temperature [oC]
    tr : float, optional
        Mean radiant temperature [oC]
    va : float, optional
        Air velocity [m/s]
    rh : float, optional
        Relative humidity [%]
    met : float, optional
        Metabolic rate [met]
    clo : float, optional
        Clothing insulation [clo]
    wmet : float, optional
        External work [met], optional. The default is 0.

    Returns
    -------
    PMV value
    """

    met *= 58.15  # chage unit [met] to [W/m2]
    wmet *= 58.15  # chage unit [met] to [W/m2]
    mw = met - wmet  # heat production [W/m2]

    if clo < 0.5: fcl = 1 + 0.2*clo  # clothing area factor [-]
    else: fcl = 1.05 + 0.1*clo

    antoine = lambda x: math.e**(16.6536-(4030.183/(x+235)))  # antoine's formula
    pa = antoine(ta) * rh/100  # vapor pressure [kPa]
    rcl = 0.155 * clo  # clothing thermal resistance [K.m2/W]
    hcf = 12.1 * va**0.5  # forced convective heat transfer coefficience

    hc = hcf  # initial convective heat transfer coefficience
    tcl = (34 + ta) / 2  # initial clothing temp.

    # Cal. clothing temp. by iterative calculation method
    for i in range(100):
        # clothing temp. [oC]
        tcliter = 35.7 - 0.028 * mw \
            - rcl * (39.6 * 10**(-9) * fcl * ((tcl+273)**4 - (tr+273)**4) \
                     + fcl * hc * (tcl - ta))  # Eq.68
        # new clothin temp. [oC]
        tcl = (tcliter + tcl) / 2

        hcn = 2.38 * abs(tcl - ta)**0.25  # natural convective heat transfer coefficience

        # select forced or natural convection
        if  hcn > hcf: hc = hcf
        else: hc = hcf

        # terminate iterative calculation
        if abs(tcliter - tcl) < 0.0001:
            break

    # tcl = 35.7 - 0.0275 * mw \
    #     - rcl * (mw - 3.05 * (5.73 - 0.007 * mw - pa) \
    #     - 0.42 * (mw - 58.15) - 0.0173 * met * (5.87 - pa) \
    #     + 0.0014 * met * (34 - ta))  # Eq.64

    # Heat loss of human body
    rad = 3.96 * (10**(-8)) * fcl * ((tcl+273)**4 - (tr+273)**4)  # by radiation
    conv = fcl * hc * (tcl - ta)  # by convction
    diff = 3.05 * (5.73 - 0.007 * mw - pa)  # by insensive perspiration
    sweat = max(0, 0.42 * (mw - 58.15))  # by sweating
    res = 0.0173 * met * (5.87 - pa) + 0.0014 * met * (34 - ta)  # by repiration
    load = mw - rad - conv - diff - sweat - res

    pmv_value = (0.303 * math.exp(-0.036 * met) + 0.028) * load  # Eq.63

    return pmv_value