# by bliako on 10-09-2021
# for https://perlmonks.org/?node_id=11136582
# and specifically https://perlmonks.org/?node_id=11136632
use strict;
use warnings;

use Data::Dumper;

# easily reproduce results or comment it out
srand 1234;

# how many lines of new data to produce
my $lines_to_produce = 1000;

# this is the actual data we want to discover
# and simulate its statistical properties:
my @actual_data = qw/
	ABBCBCAAAAABBCBCACCCAAAAACAAAAABBBBBAAAAABBAAAAAAAABBCCCACCAABC
	BCCCBCAACAABBBCAAACCAAAAACAAAAABBBBBAAAAABBAAAAAAAABBCCCACCABBC
	ABCCBBBAAAABBABCACABCCCCCCAAAAABBCBBCCCCAAAAAAAAAAAAACCCACCACCC
/;

my $random_data = random_data(\@actual_data, $lines_to_produce);

# and calculate and print statistics on the data just produced
my ($xdist1, $xdist2) = CumProbDists($random_data);

## end

sub random_data {
	my $actual_data = shift;
        my $lines = shift;

	# this calculates the mono-gram and di-gram
	# prob.dist of the actual data.
	# mono-gram P(A), di-gram P(A|B)
	my ($dist1, $dist2) = CumProbDists($actual_data);

	my $width = length $actual_data->[0];
	my @results;
	while( $lines-- ){
		my @line;
		my $letter = which_letter($dist1);
		push @line, $letter;
		for (2..$width){
			$letter = which_letter($dist2->{$letter});
			push @line, $letter;
		}
		push @results, join('', @line);
	}
	return \@results;
}
#### end main ####

# given some data in the form of an arrayref of strings, it will
# calculate cumulative probability distribution (1st frequency, then p.d.
# and then cumulative p.d.)
sub CumProbDists {
	# make a copy because it destructs $data
	my $data = [ @{$_[0]} ];
	# the results:
	my %dist1; # cpd for each letter A, B, C
	my %dist2; # cpd for each digram, e.g. A->A, A->B, C->A etc.
	for my $aline (@$data){
		###################################
		# I hope this somewhat obsene regex
		# does not violate any CoCs
		###################################
	        while( $aline =~ s/^(.)(.)/$2/g ){
        	        $dist1{$1}++;
                	$dist2{$1}->{$2}++;
	        }
	}
	print "Frequencies:\n";
	print Dumper(\%dist1);
	print Dumper(\%dist2);

	# convert to prob.dist.
	my $sum = 0;
	$sum += $_ for values %dist1;
	$_ /= $sum for values %dist1;
	for my $v1 (keys %dist1){
		$sum = 0;
		$sum += $_ for values %{$dist2{$v1}};
		$_ /= $sum for values %{$dist2{$v1}};
	}
	print "Probability Distribution:\n";
	print Dumper(\%dist1);
	print Dumper(\%dist2);

	# convert to cumulative prob.dist.
	$sum = 0;
	for (sort keys %dist1){
		$dist1{$_} += $sum;
		$sum = $dist1{$_};
	}
	for my $v1 (keys %dist1){
		$sum = 0;
		for (sort keys %{$dist2{$v1}}){
			$dist2{$v1}->{$_} += $sum;
			$sum = $dist2{$v1}->{$_};
		}
	}
	print "Cumulative Probability distribution:\n";
	print Dumper(\%dist1);
	print Dumper(\%dist2);

	return (\%dist1, \%dist2)
}
# given a cum-prob-dist (as a hashref where key is the letter to choose,
# and value is the cum-prob-dist)
# it will return a letter randomly but satisfying the distribution
sub which_letter {
	my $dist = shift;
	my $rand = rand;
	for (sort keys %$dist){
		if( $rand <= $dist->{$_} ){ return $_ }
	}
}