DESCRIPTION

This executable will create /var/statelog if it doesn't exist, or expand it to
contain the state for the current minute. The current state is determined by
checking whether the file /tmp/space-is-open exists; if it does not exist, the
space is considered closed. The contents of the file are ignored.

The file /tmp/space-is-open on 'gateway' is administered by one of the scripts
in /home/spaceloop/revspace_dicht.d/ and revspace_open.d/ and is considered
more canonical than the retained value in the mqtt topic "revspace/state".


FILE FORMAT

(Copied from https://revspace.nl/Spacestategeschiedenis)

    First 8 bytes (64 bit): unix time_t epoch time of first data point

    Subsequent data: one byte per minute,
        byte offset = 8 + floor((time() - $start_time) / 60)

        ASCII '0': space was closed (or mostly closed)
        ASCII '1': space was open (or mostly open)
        ASCII '?': script did run but could not determine space state (not
                   currently in use)
        ASCII '\0' (NUL): space state was not recorded
        Any other value: treat as '?'


CAVEATS

If it's executed multiple times within the same time slot, the state is
overwritten with the current value.

A "minute" is defined as a 60-second period since the start_time as defined in
the header of the file, and will probably not be aligned with the MM in
HH:MM:SS. When used with cron(1), this will probably result in some data being
skipped because two subsequent runs fall in the same time slot. In the worst
case, half of the samples will be invalid, which is still considered good
enough for the purpose of this project. (Originally, we considered 10 samples
per hour, now we have 60, or in the worst case, 30.)