Imitate the file naming of Garmin devices

garmin_encode_file_name.py

import datetime
import string


def name_file(dt):
    """
    Return a name according to the given `datetime.datetime` object.

    This is how Garmin devices seem to name their activity files, as explained
    `here <https://forums.garmin.com/forum/into-sports/running/forerunner-220-aa/53766->`_.

    Quote::

        I think i have worked out the activity naming for the .FIT files.

        I did a run on 16 Feb 2014 at 7:40PM (OZ DST Time).

        The .FIT file is "42GJ4011.FIT"

        The first 4 characters are easy. 1 to 9, A towards Z (A=10, B=11...,
        G=16).

        The last 4 digits are the last 4 digits (with in a few 10s of seconds)
        of the number of seconds since 00:00 31 Dec 1989.

        Its not exactly the time the watch was put into GPS mode or the time the
        START button was pushed but its close.

        File create time = 761474409
        START time = 761474411
        End time = 761474969

        4      2     G    J    4 0 1 1
        Year   Month Date Hour << last 4 digits of number of seconds since 00:00 Dec 31 1989 UTC >>
        4=2014 2=Feb G=16 7=7

        * Year
          1=2011
          2=2012
          3=2013
          4=2014
          5=2015
          ...
        * Month:
            1=Jan  7=Ju1
            2=Feb  8=Aug
            3=Mar  9=Sep
            4=Apr  A=Oct
            5=May  B=Nov
            6=Jun  C=Dec
        * Date
            1=1  9=9   H=17  P=25
            2=2  A=10  I=18  Q=26
            3=3  B=11  J=19  R=27
            4=4  C=12  K=20  S=28
            5=5  D=13  L=21  T=29
            6=6  E=14  M=22  U=30
            7=7  F=15  N=23  V=31
            8=8  G=16  O=24
        * Hour
            1=1
            2=2
            ...
    """
    table = string.digits + string.ascii_uppercase
    epoch = datetime.datetime(
        year=1989, month=12, day=31,
        tzinfo=datetime.timezone.utc if dt.tzinfo else None)

    name = str(dt.year - 2010)
    name += table[dt.month]
    name += table[dt.day]
    name += table[dt.hour]
    name += str(int((dt - epoch).total_seconds()))[-4:]

    return name