#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use List::Util qw{ shuffle };
use Function::Parameters;
use Tk;

use constant {
    SEPARATOR => ',',
    OK        => 'OK',
    TITLE     => 'SI Units Conversions',
    DONE      => 'Done',
    QUIT      => 'Quit',
    ERRORS    => 'Errors',
};

push @ARGV, -font => 'sans 15';


fun prefixes (@prefixes) {
    +{ map +($_ => undef), "", @prefixes }
}


fun do_format ($x) {
    my $f = reverse $x;
    $f =~ s/(...)(?=.)/$1${\SEPARATOR}/g;
    return reverse $f
}


fun validate ($new_value, @) {
    $new_value =~ /^[0-9${\SEPARATOR}]*$/
}


my %known = (l => prefixes(qw( m c d h )),
             m => prefixes(qw( m c d k )));

my %times = (m  => 0.001,
             c  => 0.01,
             d  => 0.1,
             "" => 1,
             h  => 100,
             k  => 1000);

my @exercises;
for (1 .. shift) {
    my $unit = (keys %known)[rand keys %known];
    my @prefixes = sort { $times{$a} <=> $times{$b} }
                        (shuffle(keys %{ $known{$unit} }))[0, 1];
    push @exercises, [ $unit, @prefixes, 1 + int rand 100 ];
    $exercises[-1][-1] *= 10 for 1 .. rand 2;
    if (int rand 2) {
        my $times = $times{ $prefixes[1] } / $times{ $prefixes[0] };
        $exercises[-1]
            = [ $unit, @prefixes[1, 0], $exercises[-1][-1] * $times ];
    }
}

my (@solutions, @entries);
my $errors = 0;

my $mw = 'MainWindow'->new(title => TITLE);
for my $i (0 .. $#exercises) {
    my $exercise = $exercises[$i];
    my $f = $mw->Frame->pack;
    my $ratio = $times{ $exercise->[2] } / $times{ $exercise->[1] };
    $f->Label(-text => do_format($exercise->[3] * $ratio)
                       . " $exercise->[1]$exercise->[0] = ")
        ->pack(-side => 'left');
    push @entries, $f->Entry(-textvariable    => \$solutions[$i],
                             -validate        => 'all',
                             -validatecommand => \&validate,
                            )->pack(-side => 'left');
    $f->Label(-text => " $exercise->[2]$exercise->[0]")
        ->pack(-side => 'left');
}

my $f = $mw->Frame->pack;

$f->Button(-text    => DONE,
           -command => fun () {
               for my $i (0 .. $#solutions) {
                   next if $entries[$i]->cget('-state') eq 'readonly';

                   my $value = ($solutions[$i] // "") =~ s/${\SEPARATO
+R}//gr;
                   if (($value || 0) == $exercises[$i][3]) {
                       $entries[$i]->configure(-state    => 'readonly'
+,
                                               -validate => 'none');
                       $solutions[$i] = OK;
                   } else {
                       ++$errors;
                   }
               }
           })->pack(-side => 'left');
$f->Button(-text    => QUIT,
           -command => [$mw => 'destroy'])->pack(-side => 'left');

MainLoop();
say ERRORS, ": $errors";
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