#!/usr/bin/env perl
# Time-stamp: <2020-07-31 12:00:03 phi>
# generate a sentence using a context free grammar

use strict;
use warnings;

my %grammar =			# "<Non-Terminal>", "Terminal"
  (
   "S"		=> [["<Int>", "<NP>", "<VP>"],
		    ["<NP>", "<VP>"]],

   "Int"	=> [["Hi,"], ["Hey,"]],

   "NP"		=> [["<Pro>"], ["<Art>", "<Noun>"]],
   "Pro"	=> [["Du"]],
   "Art"	=> [["Diese"]],
   "Noun"	=> [["Aufgabe"], ["Abgabe"]],

   "VP"		=> [["<Verb>", "<Adj>"]],
   "Verb"	=> [["ist"], ["war"]],
   "Adj"	=> [["cool"], ["gut"], ["schön"]]
  );

sub produce {
  my $start = shift;
  my $rules = $grammar{$start};
  my $res = "";

  my $rule = $rules->[rand(int($#{$rules}) + 1)];
  for (my $i = 0; $i <= scalar $#{$rule}; $i++){
    my $elem = $rule->[$i];
    if ($elem =~ /<(\w+)>/) {
      $res .= produce($1);
    } else {
      $res .= $elem;
    }
    $res .= " ";
  }

  return $res;
}

# test:
my $result = produce("S");
$result =~ s/ +/ /g;
print "$result\n";