"""
    Computing apparent magnitude from difference magnitudes for ZTF
    Adapted by Roy Williams from Eric Bellm's notebook
    https://github.com/ZwickyTransientFacility/ztf-avro-alert/blob/master/notebooks/Variable_star_lightcurves.ipynb
"""
import math

def dc_mag_dict(fid, magpsf,sigmapsf, magnr,sigmagnr, magzpsci, isdiffpos):
    """dc_mag_dict.
    Compute apparent magnitude from difference magnitude supplied by ZTF
    Args:
        fid: filter, 1 for green and 2 for red
        magpsf:
        x,sigmapsf: magnitude from PSF-fit photometry, and 1-sigma error
    magnr:
    sigmagnr: magnitude of nearest source in reference image PSF-catalog within 30 arcsec
        and 1-sigma error
    magzpsci: Magnitude zero point for photometry estimates
    isdiffpos:
        t => candidate is from positive (sci minus ref) subtraction; 
        f => candidate is from negative (ref minus sci) subtraction
    """

    # zero points. Looks like they are fixed.
    ref_zps = {1:26.325, 2:26.275, 3:25.660}
    magzpref = ref_zps[fid]

    # reference flux and its error
    magdiff = magzpref - magnr
    if magdiff > 12.0:
        magdiff = 12.0
    ref_flux = 10**( 0.4* ( magdiff) )
    ref_sigflux = (sigmagnr/1.0857)*ref_flux

    # difference flux and its error
    if magzpsci == 0.0: magzpsci = magzpref
    magdiff = magzpsci - magpsf
    if magdiff > 12.0:
        magdiff = 12.0
    difference_flux = 10**( 0.4* ( magdiff) )
    difference_sigflux = (sigmapsf/1.0857)*difference_flux

    # add or subract difference flux based on isdiffpos
    if isdiffpos == 't': 
        dc_flux = ref_flux + difference_flux
    elif isdiffpos == 'f':
        dc_flux = ref_flux - difference_flux
    else:
        print('Unkown isdiffpos=%s' % isdiffpos)

    # assumes errors are independent. Maybe too conservative.
    dc_sigflux =  math.sqrt( difference_sigflux**2 + ref_sigflux**2 )

    # apparent mag and its error from fluxes
    if dc_flux > 0.0:
        dc_mag = magzpsci - 2.5 * math.log10(dc_flux)
        dc_sigmag = dc_sigflux/dc_flux*1.0857
    else:
        dc_mag = magzpsci
        dc_sigmag = sigmapsf

    return {'dc_mag':dc_mag, 'dc_sigmag':dc_sigmag}

if __name__ == "__main__":
    fid = 1
    magpsf    = 17.7439
    sigmapsf  =  0.1057
    magnr     = 14.7309
    sigmagnr  =  0.0189
    magzpsci  = 26.1389

    isdiffpos = 't'
    d = dc_mag_dict(fid, magpsf,sigmapsf, magnr,sigmagnr, magzpsci, isdiffpos)
    print('As a positive difference', d)

    isdiffpos = 'f'
    d = dc_mag_dict(fid, magpsf,sigmapsf, magnr,sigmagnr, magzpsci, isdiffpos)
    print('As a negative difference', d)