use warnings;
use strict;

use constant {
    PII     => 3.1416,
    DIVISOR => 61.02374409, # cubic inches to litres divisor
    RADIUS  => 2.5,
    DEPTH   => 6,
};

my $cubic_inches = (RADIUS * RADIUS) * PII * DEPTH; # total cubes
my $litres = $cubic_inches / DIVISOR; # total litres

my $empty_depth_inches = 2.80; # inches from top of tank to current liquid level

printf("total cubic inches: %f\n", $cubic_inches);

printf("total litres: %f\n", $litres);

printf("current empty litres: %f\n", empty_litres($empty_depth_inches));

printf("current litres: %f\n", current_litres($empty_depth_inches));

printf(
    "percent litres remaining: %f\n",
    current_litres($empty_depth_inches) / $litres * 100
);

printf(
    "percent litres missing: %f\n",
    empty_litres($empty_depth_inches) / $litres * 100
);

printf(
    "percent cubes remaining: %f\n",
    current_cubic_inches($empty_depth_inches) / $cubic_inches * 100
);

sub current_cubic_inches {
    my ($empty_inches) = @_;
    return (RADIUS * RADIUS) * PII * (DEPTH - $empty_inches);
}

sub empty_litres {
    my ($empty_inches) = @_;
    return $litres - (current_cubic_inches($empty_inches) / DIVISOR);
}

sub current_litres {
    my ($empty_inches) = @_;
    return $litres - empty_litres($empty_inches);
}

##</code><code>##

total cubic inches: 117.810000
total litres: 1.930560
current empty litres: 0.900928
current litres: 1.029632
percent litres remaining: 53.333333
percent litres missing: 46.666667
percent cubes remaining: 53.333333