3d6        # simplest one
3d6+3      # with a result modifier

3d8r1      # reroll and discard any 1
3d8rlt3    # reroll and discard any lesser than 3
3d8rgt6    # reroll and discard any greater than 6
3d8rgt6+2  # reroll and discard any greater than 6 and add +2 to the final result


##</code><code>##

# 3d8r1
modifier_r received: ({}, { die_type => "1d8", rolls => [8, 1, 7] }, "r", 1)

# 3d8rgt1
modifier_r received: ({}, { die_type => "1d8", rolls => [4, 8, 1] }, "r", "gt", 1)

##</code><code>##

use Marpa::R2;
use Data::Dump;

# resurces (along with ones on cpan):

# http://marpa-guide.github.io/
# http://savage.net.au/Perl-modules/html/marpa.faq/faq.html
# http://savage.net.au/Perl-modules/html/marpa.papers/

# https://github.com/choroba/marpa-enhanced-calculator

# https://perlmaven.com/marpa-for-building-parsers
# https://perlmaven.com/marpa-debugging

my $dsl = <<'END_OF_DSL'; #:default ::= action => [values]

:default ::= action => [name,values]
lexeme default = latm => 1
 
Dice_Expression ::= Simple_Dice
					|Dice_with_modifier_x
					|Dice_with_modifier_r
					
Dice_with_modifier_x ::= Simple_Dice 'x' Die_Modifier_Val action => modifier_x

Dice_with_modifier_r ::= Simple_Dice 'r' Die_Modifier_Val action => modifier_r
							|Simple_Dice 'r' <Die_Modifier_Comp> Die_Modifier_Val action => modifier_r


Simple_Dice ::= Rolls 'd' Sides action => do_simple_roll


Die_Modifier_Val ~ digits
Die_Modifier_Comp ~ 'gt' | 'lt' 

Rolls ~ digits
Sides ~ digits

digits ~ [\d]+

:discard ~ whitespace
whitespace ~ [\s]+
END_OF_DSL
 
my $grammar = Marpa::R2::Scanless::G->new( { source => \$dsl } );
my $input = $ARGV[0] // '6d4x1';
my $value_ref = $grammar->parse( \$input, 'My_Actions' );

print "\n\nFinal result: ";dd $value_ref;


sub My_Actions::modifier_r{
	print "modifier_r received: ";dd @_;
}

 
sub My_Actions::do_simple_roll {
    my ( undef, $rolls, undef, $sides ) = @_;
	print "do_simple_roll received: "; dd @_;
	my $res = [];
	map{ 	$die = 1+int(rand($sides)); 
			print "\tRolled : $die\n"; 
			push @$res, $die}	1..$rolls;
    my $return = { die_type => "1d$sides", rolls => $res};
	print "do_simple_roll returning: "; dd $return;
	return $return;
}

sub My_Actions::modifier_x {
    my ( undef, $rolls_ref, $modifier, $modifier_val ) = @_;
	print "modifier_x received: "; dd @_; #dd ($rolls_ref,$modifier, $modifier_val );
	
	my @descr = @{$rolls_ref->{rolls}};
	# some roll need to be exploded
	while ( 0 < grep{ $_ =~ /^$modifier_val$/ }@descr  ){
			foreach my $roll( @descr ){
				print "\tanalyzing: $roll\n";
				if ( $roll == $modifier_val ){
					$roll = $roll."x";
					print "\t\texploding a die..\n";
					my $new = $grammar->parse( \$rolls_ref->{die_type}, 'My_Actions' );
					print "\tdo_simple_roll returned: ";dd $new;
					push @descr, $$$new[1]->{rolls}->[0];
				}
				
			}
	}
		my @numbers = map{ $_=~/(\d+)/; $1 }@descr;
	my $sum = 0;
	$sum += $_ for @numbers;
	
	my $return = { result => $sum, description => join ' ',@descr};
	print "do_roll_with_die_modifier_x returning: "; dd $return;
	return $return;
}