package Circle;
use strict;
use warnings;

use constant ATR_RADIUS => 0;
use constant Pi         => 3.14159;

sub new {
	my $self = bless [], shift;
	$self->redefine(@_);
	return $self;
}

sub redefine {
	my $self = shift;
	my ($prop, $value) = @_;
	my $radius = 0;
	
	if    ($prop eq 'radius') {
		$radius = $value;
	}
	elsif ($prop eq 'area') {
		$radius = sqrt( $value / Pi );
	}
	elsif ($prop eq 'circumference' or $prop eq 'circum') {
		$radius = $value / ( 2 * Pi );
	}
	elsif ($prop eq 'diameter') {
		$radius = $value / 2;
	}
	else {
		die "Property '$prop' cannot be defined";
	}
	
	$self->[ATR_RADIUS] = $radius;
}

sub learn {
	my $self = shift;
	my $prop = shift;
	my $radius = $self->[ATR_RADIUS];
	
	my $value = 0;
	
	if    ($prop eq 'radius') {
		$value = $radius;
	}
	elsif ($prop eq 'area') {
		$value = Pi * ($radius ** 2);
	}
	elsif ($prop eq 'circumference' or $prop eq 'circum') {
		$value = 2 * Pi * $radius;
	}
	elsif ($prop eq 'diameter') {
		$value = $radius * 2;
	}
	else {
		die "'$prop' is not a property of a Circle";
	}
	
	return $value;
}

sub grow {
	my $self = shift;
	my ($prop, $value) = @_;
	
	my $old_value = $self->learn($prop);
	$value += $old_value;
	
	$self->redefine($prop => $value);
}

sub shrink {
	my $self = shift;
	$self->grow(shift, 0-shift); # shrink is neg. growth
}

1;