#!/usr/local/bin/perl
# -*- perl -*-

sub usage {
    print <<USAGE;

nfa - run an nondeterministic finite automaton on an input

Usage
  nfa [-h] nfa_file input_string

  nfa_file is a file that describes the NFA.  Each line in the file
  contains a state and a symbol followed by one or more states.  This
  represents a transition from the first state to the other state(s)
  if the symbol is the next input.  The first state on the first line
  is the initial state.  For lambda transitions, use . as the symbol.

  For example, the following lines describe an NFA that accepts
  strings with a 00 in them (assuming qf is the final state).

      q1 0 q1 q2
      q1 1 q1
      q2 0 q3
      q2 1 q1
      q3 0 q3
      q3 1 q3
      q3 . qf

  input_string is the string to be input to the NFA.

  nfa will print the states that the NFA could be in after each input
  symbol.

  The -h option will print this help and exit.

USAGE
    exit;
}



use Getopt::Std;

getopts("h");

usage() if ($opt_h);

sub transition {
    my $symbol = shift;
    my @stateset = @_;
    my @nextset;
    @nextset = @stateset if $symbol eq "."; # for lambda transitions
    my ($i, $state1, $state2, $j);
    for ($i = 0; $i <= $#stateset; $i++) {
	$state1 = $stateset[$i];
	foreach $state2 (@{$next_state{$state1}{$symbol}}) {
	    for ($j = 0; $j <= $#nextset && $state2 ne $nextset[$j]; $j++) {};
	    if ($j > $#nextset) {
		push(@nextset, $state2);
		push(@stateset, $state2) if $symbol eq ".";
	    }
	}
    }
    return @nextset;
}


usage() unless $#ARGV == 0 || $#ARGV == 1;
$file = $ARGV[0];
$string = $ARGV[1];

# input NFA
open(NFA, $file);
while ($line = <NFA>) {
    @states = split(/\s+/,$line);
    $state1 = shift(@states);
    $state1 = shift(@states) if $state1 eq "";
    $symbol = shift(@states);
    foreach $state2 (@states) {
	push(@{$next_state{$state1}{$symbol}}, $state2);
    }
    $initial_state = $state1 unless defined($initial_state);
}
close(NFA);

usage() unless defined($initial_state);

# initialize input and NFA
for ($i = 0; $i < length($string); $i++) {
    $input[$i] = substr($string,$i,1);
}
@stateset = transition(".", $initial_state);

print("Input\tState(s)\n");
print("-----\t--------\n");
printf "\t%s\n", join(", ", @stateset);

for ($position = 0; $position <= $#input; $position++) {

    # perform transition
    @stateset = transition($input[$position], @stateset);
    @stateset = transition(".", @stateset);

    printf "  %s\t%s\n", $input[$position], join(", ", @stateset);
}