
CRC = (
0x00, 0x31, 0x62, 0x53, 0xc4, 0xf5, 0xa6, 0x97,
0xb9, 0x88, 0xdb, 0xea, 0x7d, 0x4c, 0x1f, 0x2e,
0x43, 0x72, 0x21, 0x10, 0x87, 0xb6, 0xe5, 0xd4,
0xfa, 0xcb, 0x98, 0xa9, 0x3e, 0x0f, 0x5c, 0x6d,
0x86, 0xb7, 0xe4, 0xd5, 0x42, 0x73, 0x20, 0x11,
0x3f, 0x0e, 0x5d, 0x6c, 0xfb, 0xca, 0x99, 0xa8,
0xc5, 0xf4, 0xa7, 0x96, 0x01, 0x30, 0x63, 0x52,
0x7c, 0x4d, 0x1e, 0x2f, 0xb8, 0x89, 0xda, 0xeb,
0x3d, 0x0c, 0x5f, 0x6e, 0xf9, 0xc8, 0x9b, 0xaa,
0x84, 0xb5, 0xe6, 0xd7, 0x40, 0x71, 0x22, 0x13,
0x7e, 0x4f, 0x1c, 0x2d, 0xba, 0x8b, 0xd8, 0xe9,
0xc7, 0xf6, 0xa5, 0x94, 0x03, 0x32, 0x61, 0x50,
0xbb, 0x8a, 0xd9, 0xe8, 0x7f, 0x4e, 0x1d, 0x2c,
0x02, 0x33, 0x60, 0x51, 0xc6, 0xf7, 0xa4, 0x95,
0xf8, 0xc9, 0x9a, 0xab, 0x3c, 0x0d, 0x5e, 0x6f,
0x41, 0x70, 0x23, 0x12, 0x85, 0xb4, 0xe7, 0xd6,
0x7a, 0x4b, 0x18, 0x29, 0xbe, 0x8f, 0xdc, 0xed,
0xc3, 0xf2, 0xa1, 0x90, 0x07, 0x36, 0x65, 0x54,
0x39, 0x08, 0x5b, 0x6a, 0xfd, 0xcc, 0x9f, 0xae,
0x80, 0xb1, 0xe2, 0xd3, 0x44, 0x75, 0x26, 0x17,
0xfc, 0xcd, 0x9e, 0xaf, 0x38, 0x09, 0x5a, 0x6b,
0x45, 0x74, 0x27, 0x16, 0x81, 0xb0, 0xe3, 0xd2,
0xbf, 0x8e, 0xdd, 0xec, 0x7b, 0x4a, 0x19, 0x28,
0x06, 0x37, 0x64, 0x55, 0xc2, 0xf3, 0xa0, 0x91,
0x47, 0x76, 0x25, 0x14, 0x83, 0xb2, 0xe1, 0xd0,
0xfe, 0xcf, 0x9c, 0xad, 0x3a, 0x0b, 0x58, 0x69,
0x04, 0x35, 0x66, 0x57, 0xc0, 0xf1, 0xa2, 0x93,
0xbd, 0x8c, 0xdf, 0xee, 0x79, 0x48, 0x1b, 0x2a,
0xc1, 0xf0, 0xa3, 0x92, 0x05, 0x34, 0x67, 0x56,
0x78, 0x49, 0x1a, 0x2b, 0xbc, 0x8d, 0xde, 0xef,
0x82, 0xb3, 0xe0, 0xd1, 0x46, 0x77, 0x24, 0x15,
0x3b, 0x0a, 0x59, 0x68, 0xff, 0xce, 0x9d, 0xac,
)

def calc_crc(d):
    crc = 0
    for v in d:
        crc  = CRC[(v ^ crc)&0xff];
    return crc

"""
11 byte packets. Always seem to begin with a 12-bit station ID, that
changes on a battery change, and end with byte 10=0,
byte 9 is a checksum (CRC-8 Dallas/Maxim w/ polynomial 0x131)

Fields present: (counting from byte 0/ bit 0)
Temperature (deg C): bits 12(?)-24 = temp * 10. 12 bits = max temp 409.6 deg C
Humidity (%): byte 3. Max observed humidity 94%
avg wind speed (km/h): byte 4 times 0.34
peak wind speed (km/h): byte 5 times 0.34
rainfall (mm):  bytes 6 and 7, counter 0.3mm/count (wraps @ 19660.5mm ?)
wind dir (0-15): byte 8. Only needs lower nibble, but higher always seems to be zero
"""

def ew_check_and_decode(data):
    if (data[10] != 0x00):
        return None # Not a valid packet
    crc = calc_crc(data[0:10])
    if crc != 0:
        return None # Failed CRC

    temperature = 0.1 * (((data[1] << 8) & 0xf00) | data[2])
    if temperature > 204.8:
      temperature = 204.8 - temperature;
    humidity = data[3]
    orig_avg_wind = data[4]
    orig_peak_wind = data[5]
    avg_wind = orig_avg_wind * 0.36
    peak_wind = orig_peak_wind * 0.36
    rain_cnt = ((data[6] << 8) | data[7]) * 0.3
    wind_dir = data[8]
    return (temperature, humidity, avg_wind, peak_wind, rain_cnt, wind_dir, orig_avg_wind, orig_peak_wind)
