use POSIX qw/strftime/;
use strict;
$|++;

use constant Intervals => 20;

my ($prev_time, $prev_val, $tmin, $tmax, $last_val, @curve);
open my $data, "weightdata.txt" or die "weightdata.txt: $!\n";
while (<$data>){
    chomp;
    next unless m{^\s*([\d/]+)\s+([\d.]+)};
    my($m, $d, $y) = split /\//, $1;
    my $time_point = strftime("%s", 0, 0, 0, $d, $m - 1, $y - 1900);
    if (defined $prev_time){
        my $slope = ($2 - $prev_val) / ($time_point - $prev_time);
        push @curve, [ $prev_time, $time_point, $prev_val, $slope ];
    }
    ($prev_time, $prev_val) = ($time_point, $2);
    ($tmax, $last_val) = ($time_point, $2);
}
close $data;

$tmin = $curve[0][0];
my $interval = ($tmax - $tmin) / (Intervals - 1);

my ($prev_t, $val, $slope);
for my $t (0 .. Intervals - 2){
    my $calc_t = $t * $interval + $tmin;
    shift @curve unless $calc_t >= $curve[0][0] && $calc_t <= $curve[0][1];
    ($prev_t, $val, $slope) = @{$curve[0]}[0, 2, 3];
    printf "%2d: Weight on %s was %.1f\n", $t + 1, strftime("%F",localtime($calc_t)),
        ($calc_t - $prev_t) * $slope + $val;
}
printf "%2d: Weight on %s was %.1f\n", Intervals, strftime("%F",localtime($tmax)), $last_val;