#!/usr/bin/env python3
#
# create_metar_dat.py
#
# Script to create metar.dat.gz by checking tgftp.nws.noaa.gov for stations
# that have reported recently.
#
# Usage: create_metar_dat.py [OUTPUT_FILENAME]
#
# If OUTPUT_FILENAME is omitted, creates metar.dat.gz in working directory.
#
# Copyright (c) 2016 Richard Senior
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
# MA 02110-1301, USA.

import datetime
import dateutil.parser
import gzip
import os
import re
import sys
import urllib.request

filename = "metar.dat.gz" if len(sys.argv) <= 1 else sys.argv[1]
max_age_days = 7
url = "http://tgftp.nws.noaa.gov/data/observations/metar/stations/"

now = datetime.datetime.now()
script = os.path.basename(__file__)

class ParseException(Exception):
    pass

def stations():
    for line in urllib.request.urlopen(url).readlines():
        if line.startswith(b"<img"):
            yield line

def active_stations():
    cutoff = now - datetime.timedelta(max_age_days)
    icao_pattern = re.compile("[A-Z][A-Z0-9]{3}")
    for station in stations():
        tokens = station.split()
        last_modified = tokens[5]
        if dateutil.parser.parse(last_modified) > cutoff:
            icao = tokens[4][16:20:].decode()
            # Sanity check on parsed ICAO code.
            if not icao_pattern.match(icao):
                raise ParseException("Dubious ICAO code: " + icao)
            yield icao

# Run through the active stations before opening the file so that a file
# is not created (or overwritten) if an exception is thrown.
active = [icao + "\n" for icao in active_stations()]

# Sanity check on the number of stations.
if len(active) < 5000:
    raise ParseException("Expected more than " + str(len(active)) + " stations")

with gzip.open(filename, "wt") as f:
    print("# List of airports known to have metar data available", file=f)
    print("# Generated by", script, "on", now.strftime("%Y-%m-%d"), file=f)
    print("#", url, file=f)
    f.writelines(active)

print(script + ": Wrote", len(active), "stations to", filename)