Decimal and binary conversion with big numbers

Question

I have to convert big numbers in Perl from decimal to binary and the other way around.

An example number of that length:

Dec: 76982379919017706648824420266
Bin: 111110001011111001010101000010011001000010101111001110000000000000000000000000000000000000000000

I found two functions:

sub dec2bin {
    my $str = unpack("B32", pack("N", shift));
    $str =~ s/^0+(?=\d)//;   # otherwise you'll get leading zeros
    return $str;
}
sub bin2dec {
    return unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}

But, both of them seem to stop working with big numbers.

Output of
bin2dec(111110001011111001010101000010011001000010101111001110000000000000000000000000000000000000000000)
is 1543163 
and output of
dec2bin(76982379919017706422040262422)
is 11111111111111111111111111111111

Is there a proper way of doing it with such big numbers?

Answer 1

You can use Math::BigInt. Please note, that input to these functions should be strings.

use Math::BigInt;

sub bin2dec {
  my $bin = shift;
  return Math::BigInt->new("0b$bin");
}

sub dec2bin {
  my $dec = shift;
  my $i = Math::BigInt->new($dec);
  return substr($i->as_bin(), 2);
}

print "Dec: " . bin2dec("111110001011111001010101000010011001000010101111001110000000000000000000000000000000000000000000") . "\n";
print "Bin: " . dec2bin("76982379919017706648824420266") . "\n";

Output is:

Dec: 76982379919017710405206147072
Bin: 111110001011111001010101000010011001000010101111001101001001010101100110001100111001011110101010

Answer 2

Perl provides built-in bignum facilities. Turn them on with use bignum;. Your conversion functions would look like this:

use bignum;
my ($b_orig, $d_orig, $b, $d);

$d_orig = 76982379919017706648824420266;
$b_orig = '111110001011111001010101000010011001000010101111001110000000000000000000000000000000000000000000';

print ("dec($b_orig) [orig] = $d_orig;\n");
print ("dec($b_orig) [comp] = " . Math::BigInt->from_bin($b_orig) . ";\n");
print ("bin($d_orig) [orig] = $b_orig;\n");
print ("bin($d_orig) [comp] = ".substr(Math::BigInt->new($d_orig)->as_bin(), 2).";\n");

Caveat

There is no correspondence between the binary and the decimal number that you provide. I have not checked whether this is a flawof the bigint library or not.