#!/usr/bin/perl -w
use strict;

use constant pi => 3.14159265358979;

use Tk;
use Tk::WorldCanvas;
my $top = new MainWindow( -width    => 800, -height => 600 );
my $cvs = $top->WorldCanvas( -width => 800, -height => 550 );
$cvs->center( 0, 0 );

my $frm = $top->Canvas( -width => 800, -height => 20 );

my $txt = $frm->Entry( -width => 40 );
my $min = $frm->Entry( -width => 5 );
my $max = $frm->Entry( -width => 5 );
my $stp = $frm->Entry( -width => 5 );
my $clr = $frm->Button( -text => 'Clear' );

$clr->bind(
    '<Button-1>' => [ sub { $cvs->delete($_) for $cvs->find('all') } ] );
$txt->bind( '<KeyPress-Return>', [ \&evaluate ] );

$frm->pack( -side => 'bottom' );
$min->pack( -side => 'left' );
$max->pack( -side => 'left' );
$stp->pack( -side => 'left' );
$clr->pack( -side => 'right' );
$txt->pack( -side => 'right' );
$cvs->pack( -side => 'top' );

$min->insert( 0, "0" );
$max->insert( 0, "100" );
$stp->insert( 0, "1" );

MainLoop;

sub evaluate {
    my $text = $txt->get();
    my $func = lc $text;
    $func =~ s/(\d+)(pi|x)/$1 * $2/g;
    $func =~ s/([xy])/\$$1/g;
    $func =~ s/\^/**/g;
    my $mini = $min->get();
    my $maxi = $max->get();
    my $step = $stp->get();

    #my $size =   1;
    my @points = ();
    for ( my $i = $mini ; $i <= $maxi ; $i += $step ) {
        my $y = 0;
        my $x = $i;
        eval "$func";

        # warn "$func: $x, $y\n" if !($x % 50);
        push @points, $x, $y;

        #   $cvs->createRectangle(
        #      800 * $x/$maxx,         300 - $y,
        #      800 * $x/$maxx + $size, 300 - $y + $size,
        #      -width => 1);
    }
    $cvs->createLine( @points, -smooth => 1 );
    $cvs->viewFit('all');
}
__END__