#!/usr/bin/perl -T

use strict;
use warnings;
use utf8;
use open qw(:std :utf8);
use Encode qw(encode decode);

# Δt = 24h/day / $samples/day

# ∫ lux(x)Δt
# → ∑ lux₁*Δt + lux₂ * Δt + lux₃ * Δt + lux_n * Δt + …
# → Δt ∑ lux_n

sub usage() {
	print "usage: $0 filename\n";
	exit;
}

my ($num, $lux, $lux_sum) = (0, 0, 0);

unless ($#ARGV == 0) {
	usage();
}

@ARGV = map { decode("UTF-8", $_) } @ARGV;

my $read_f = $ARGV[0] ;

unless (-e $read_f) {
	usage;
}

open(IN, "<", $read_f) or die "omg! can't open input file: $read_f";

while (<IN>) {
	chomp;
	my ($x, $a, $lux, $z) = split;
	if ($lux ne "??" && $lux ne "nan") {
		$lux_sum += $lux; 
		$num++;
	}
}

if ($lux_sum && $num && $num > 0) {
	printf "Δt = %.12f h\n", 24.0 / $num;
	printf "∑ lux = %.2f lx\n", $lux_sum;
	printf "lux hours: %.2f lxh\n", $lux_sum * 24 / $num;
}

close(IN);