How can I create a repeatable signature of a data structure?

Question

I have a situation where I want to create a signature of a data structure:

my $signature = ds_to_sig(
  { foo   => 'bar',
    baz   => 'bundy',
    boing => undef,
    number => 1_234_567,
  }
);

The aim should be that if the data structure changes then so should the signature.

Is there an established way to do this?

Answer 1

I think what you're looking for is a hash function. I would recommend an approach like this:

use Storable;
$Storable::canonical = 1;
sub ds_to_sig {
    my $structure = shift;
    return hash(freeze $structure);
}

The function hash can be any hash function, for example the function md5 from Digest::MD5

Answer 2

The best way to do this is to use a deep-structure serialization system like Storable. Two structures with the same data will produce the same blob of Storable output, so they can be compared.

#!/usr/bin/perl

use strict;
use warnings;

use Storable ('freeze');

$Storable::canonical = 1;

my $one = { foo => 42, bar => [ 1, 2, 3 ] };
my $two = { foo => 42, bar => [ 1, 2, 3 ] };

my $one_s = freeze $one;
my $two_s = freeze $two;

print "match\n" if $one_s eq $two_s;

...And to prove the inverse:

$one = [ 4, 5, 6 ];
$one_s = freeze $one;

print "no match" if $one_s ne $two_s;

Answer 3

Use Storable::nstore to turn it into a binary representation, and then calculate a checksum (for example with the Digest module).

Both modules are core modules.

Answer 4

Digest::MD5->new->add(
  Data::Dumper->new([$structure])
   ->Purity(0)
   ->Terse(1)
   ->Indent(0)
   ->Useqq(1)
   ->Sortkeys(1)
   ->Dump()
)->b64digest();

Answer 5

I think the word you're looking for is "hashing".

Basically, you put your data structure through a function that generates a fairly unique value from it. This value would be your signiture.

Answer 6

Can't you use an object instead of a struct? That way you could see if an object is an instance of a type without having to compare hashes, etc.