"""How deep did the luminance master actually go?

Calibrates instrumental magnitudes against Gaia G, then reports the faintest
star still detected at 5 sigma. That number decides which follow-up analyses
are worth attempting on this data and which are wishful thinking.
"""
import os

import numpy as np
import sep
from astropy import units as u
from astropy.coordinates import SkyCoord
from astropy.io import fits
from astropy.wcs import WCS

import layout

OUT = layout.SESSION
CACHE = layout.path("_gaia_deep.npz")

with fits.open(layout.path("master-Luminance.fit")) as hd:
    img = hd[0].data.astype(np.float32)
    hdr = hd[0].header
wcs = WCS(hdr, naxis=2)
ny, nx = img.shape

bkg = sep.Background(img, bw=64, bh=64, fw=3, fh=3)
sub = img - bkg.back()
objs = sep.extract(sub, 3.0, err=bkg.globalrms, minarea=6, deblend_cont=0.005)
objs = objs[(objs["flag"] == 0) & (objs["npix"] > 6)]
flux, fluxerr, _ = sep.sum_circle(sub, objs["x"], objs["y"], 5.0,
                                  err=bkg.globalrms, subpix=5)
snr = flux / np.maximum(fluxerr, 1e-9)
keep = (flux > 0) & (snr > 3)
objs, flux, snr = objs[keep], flux[keep], snr[keep]
print(f"{len(objs)} sources detected at SNR > 3 (rms {bkg.globalrms:.2f} ADU)")

sky = wcs.pixel_to_world(objs["x"], objs["y"])
centre = wcs.pixel_to_world(nx / 2, ny / 2)

if os.path.exists(CACHE):
    z = np.load(CACHE)
    gra, gdec, gmag = z["ra"], z["dec"], z["g"]
else:
    from astroquery.gaia import Gaia
    Gaia.ROW_LIMIT = 60000
    job = Gaia.launch_job_async(f"""
        SELECT ra, dec, phot_g_mean_mag FROM gaiadr3.gaia_source
        WHERE 1 = CONTAINS(POINT('ICRS', ra, dec),
                           CIRCLE('ICRS', {centre.ra.deg}, {centre.dec.deg}, 0.42))
          AND phot_g_mean_mag IS NOT NULL AND phot_g_mean_mag < 20.5
    """)
    t = job.get_results()
    gra = np.asarray(t["ra"], float)
    gdec = np.asarray(t["dec"], float)
    gmag = np.asarray(t["phot_g_mean_mag"], float)
    np.savez_compressed(CACHE, ra=gra, dec=gdec, g=gmag)
print(f"{len(gmag)} Gaia sources in the field, G down to {gmag.max():.2f}")

gcoord = SkyCoord(gra * u.deg, gdec * u.deg)
idx, sep2d, _ = sky.match_to_catalog_sky(gcoord)
matched = sep2d.arcsec < 1.5
print(f"{matched.sum()} detections matched to Gaia within 1.5\"")

inst = -2.5 * np.log10(flux[matched])
gm = gmag[idx[matched]]
# Fit the zero point on well exposed, unsaturated stars only.
fit = (gm > 12) & (gm < 17) & (snr[matched] > 20)
zp = float(np.median(gm[fit] - inst[fit]))
scatter = float(np.std(gm[fit] - inst[fit] - 0.0))
print(f"zero point {zp:.3f} (G = inst + zp) from {fit.sum()} stars, "
      f"scatter {scatter:.3f} mag")

mag_all = -2.5 * np.log10(flux) + zp
# SNR falls monotonically with magnitude, so read the SNR=5 crossing off a
# running median rather than requiring sources to land in a narrow SNR bin.
order = np.argsort(mag_all)
ms, ss = mag_all[order], snr[order]
win = max(11, len(ms) // 60)
run_m = np.array([np.median(ms[i:i + win]) for i in range(0, len(ms) - win, win // 2)])
run_s = np.array([np.median(ss[i:i + win]) for i in range(0, len(ss) - win, win // 2)])
below = np.where(run_s < 5.0)[0]
lim5 = float(run_m[below[0]]) if len(below) else float(run_m[-1])
print(f"limiting magnitude at SNR 5: G ~ {lim5:.2f}  "
      f"(SNR range {snr.min():.1f}-{snr.max():.0f})")
print(f"faintest detection: G ~ {mag_all.max():.2f} (SNR "
      f"{snr[np.argmax(mag_all)]:.1f})")

unmatched = ~matched
print(f"{unmatched.sum()} detections with NO Gaia counterpart "
      f"({100 * unmatched.mean():.1f}% of sources)")
r_gal = np.hypot(objs["x"] - nx / 2, objs["y"] - ny / 2) * 0.5376 / 60.0
near = unmatched & (r_gal < 12.0) & (mag_all > 18.0) & (mag_all < 22.0)
print(f"  of those, {near.sum()} lie within 12' of the galaxy at "
      f"G 18-22: the magnitude and radius range of Centaurus A's "
      f"globular cluster system")

# Surface brightness of the sky, a fair summary of how much the moon cost.
pixarea = 0.5376 ** 2
sky_adu = float(np.median(bkg.back()))
print(f"sky background {sky_adu:.1f} ADU/px -> "
      f"{zp - 2.5 * np.log10(max(sky_adu, 1e-6) / pixarea):.2f} mag/arcsec^2")