#!/usr/bin/env perl

use strict;
use warnings;
use constant DEBUG => 0;
use constant FUZZINESS => 1e-15;

use Scalar::Util 'looks_like_number';

my %vals = map { $_ => rand } 'a' .. 'z';

my @equations = (
    'x + y',
    'x - y',
    'x * y',
    'x / y',
    'x % y',
    'x ^ y',
    'x * (y + z)',
    'x / (y + z)',
    '(x + y) * (x + y)',
    '(x + y) * (x - y)',
);

EQUATION:
for my $equation (@equations) {
    TRY:
    while (1) {
        print '-' x 40, "\n";
        print "Base equation: $equation\n";
        print 'Try/Skip/Quit (t/s/q) [t]: ';
        my $choice = <>;
        next EQUATION if $choice =~ /^s/i;
        last EQUATION if $choice =~ /^q/i;
        print 'Equivalent equation: ';
        my $try = <>;
        chomp $try;
        if (equivalent($equation, $try)) {
            print "'$equation' and '$try' are equivalent.\n";
            print 'Try again (y/n) [n]: ';
            my $choice = <>;
            next TRY if $choice =~ /^y/i;
            last TRY;
        }
        else {
            print "'$equation' and '$try' are not equivalent.\n";
            next TRY;
        }
    }
}

sub equivalent {
    my ($equation, $try) = @_;

    my $eqn_val = get_value($equation);
    return 0 unless defined $eqn_val;

    my $try_val = get_value($try);
    return 0 unless defined $try_val;

    return abs($eqn_val - $try_val) < FUZZINESS ? 1 : 0;
}

sub get_value {
    my $in_equation = shift;

    print "get_value($in_equation)\n" if DEBUG;
    (my $equation = $in_equation) =~ s/([a-z])/$vals{$1}/g;
    $equation =~ s/\^/**/g;
    print "SUBS: $equation\n" if DEBUG;

    my $value = eval {
        eval $equation;
    };

    if ($@) {
        warn $@;
        return undef;
    }

    if (not defined $value && length $value && looks_like_number $value) {
        warn "!!! BAD EQUATION: '$in_equation'\n";
        return undef;
    }

    print "VALUE: $value\n" if DEBUG;

    return $value;
}