package Data::AsString;

require Carp;

use strict;
use warnings;

use overload q("") => sub { shift->as_string() };

sub new {
    my $class   = shift;
    my $self    = {};

    $self->{struct} = shift;
    $self->{info}   = { depth => 0 };

    return bless $self, $class;
}

sub as_string {
    my $self = shift;

    $self->{'ref'}  = $self->_traverse($self->{struct});
    $self->{string} = $self->_collapse_info();

    return $self->{string};
}

sub _traverse {
    my $self    = shift;
    my $ds      = shift;

    my @values  = ();
    if(ref($ds) eq 'HASH') {
        @values = values %$ds;
    } elsif(ref($ds) eq 'ARRAY') {
        @values = @$ds;
    } else {
        Carp::croak("can't iterate through a ".ref($ds));
    }

    foreach my $el (@values) {
        if(ref($el) eq 'HASH' or ref($el) eq 'ARRAY') {
            my $key = "#".$self->{info}->{depth};
            push @{$self->{info}->{$key}}, ref($el);

            $self->{info}->{depth}++;
            push @{$self->{info}->{$key}}, $self->_traverse($el);
            $self->{info}->{depth}--;
        }
    }

    return ref $ds;
}

sub _collapse_info {
    my $self    = shift;

    my $str     = substr($self->{'ref'}, 0, 1);

    for my $lvl (sort grep /^#\d+/, keys %{$self->{info}}) {
        my %uniq = ();
        $uniq{$_}++ for @{$self->{info}->{$lvl}};

        my @type = keys %uniq;
        if(@type > 1) {
            Carp::carp("data is not homogeneous at depth $lvl");
            $str .= '['.join("o", map { substr($_, 0, 1) } @type).']';
        } else {
            $str .= 'o' . substr($type[0], 0, 1);
        }
    }

    return $str;
}

qq(and I'm spent);

package main;

use Data::AsString;

my $data = {
    foo => {
        one => [qw(x y z)]
    },
    bar => {
        two => [qw(a b c)]
    }
};
my $sig = Data::AsString->new($data);

print "signature of \$data is $sig\n";