use strict;
use warnings;
use integer;

my $string = 'Hello World!';
my $x = Checksum($string);

printf("\n\nChecksum = %0.8X\n", $x);
exit;

#
# This function returns a 32-bit integer.
#
#   Usage: LONG = Checksum(STRING)
#
sub Checksum
{
  my $C = 0x55555555;  # Starting value.

  # Make sure we got a valid string argument.
  @_ or return $C;
  my $S = shift;
  defined $S or return $C;

  # Start the loop with the
  # last character of the string:

  my $i = length($S);
  my $V;
  my $H;
  my $L;
  while ($i--)
  {
    # $V contains the character code shifted
    # to the left by either 0, 8, 16, or 24 bits:

    $V = ((vec($S, $i, 8) + 1) << (($i & 3) << 3));

    # We add this value to the checksum $C
    $C += $V;

    # Then we rotate this 32-bit "checksum" to
    # the left by 1. There is no ROL operator
    # in Perl, so we take the 32nd bit ($H) and
    # then we shift the rest to the left, and
    # finally we OR the low and high bits
    # together to achieve the desired rotation.
    # So, it would look something like this:
    # 01110101011111111101000001110000 <<<
    # 11101010111111111010000011100000 <<<
    # 11010101111111110100000111000001 <<<
    # 10101011111111101000001110000011 <<<
    # 01010111111111010000011100000111 <<<
    # 10101111111110100000111000001110 <<<
    # 01011111111101000001110000011101 <<<
    # 10111111111010000011100000111010 <<<
    # 01111111110100000111000001110101 <<<

    $H = (($C >> 31) & 1);
    $L = (($C << 1) & 0xFFFFFFFE);
    $C = $H | $L;

    # Now, here we print a snapshot of $V
    # and we print the high bit and the low
    # portion of the integer:
    printf("\nV=%0.8X \tH=$H L=%0.32b", $V, $L);
  }

  # Make sure we return a 32-bit integer
  return $C & 0xFFFFFFFF;
}
[download]