#!/usr/bin/perl

# version 1.03	10/28/2013 brianb@paladingrp.com
# fluke 9010a ROM checksum algorithm
# reverse engineered from QuarterArcades "calcsig" program

$dir = $ARGV[0] || ".";	  				# directory to scan is first arg, or current dir 
my $str="";		  				# initialize empty string

opendir($dh,$dir) || die "cannot open $dir: $!\n";
while($f=readdir($dh)) {				# for each directory entry
  next if (-d "$dir/$f"); 				# skip directories
  $str=getFile("$dir/$f");				# read in file
  printf("%s/%s: %04x\n",$dir,$f,doChecksum($str));	# compute and print checksum
}

# getFile takes a pathname, and reads the file and resturns a string
# which is the contents of the file
sub getFile() {
  my $str="";
  my $file=shift(@_);

  open(F,"$dir/$f") || die "cannot open $dir/$f: $!\n";
    while(<F>) { $str = $str .= "$_"; }
  close(F);
  return($str);
}
  

sub doChecksum() {
  my @numArray; $numArray[15]=0;			# initialize an array with 16 elements
  my @numArray2=(6,8,11,15,0);				# initialize second array with start values

  my $sig=0;			
  my $str=shift(@_);					# retrieve the rom data passed in
  my @data=unpack("C*",$str); 				# unpack each char into a list of bytes
  
  for(my $i=0; $i <= $#data; $i++) {			# for each byte element i in the data
    my $x=$data[$i];					# x is the data
    for(my $j=0; $j <= $#numArray2; $j++) {
       my $index=$numArray2[$j];
        if($j == $#numArray2) {				# if j is the last element in numArray2
          $numArray[$index]=$x;
        } else {			
          $x = $numArray[$index] ^ $x
        }

       if(--$index < 0) {  $index = $#numArray; }
       $numArray2[$j]=$index;
       } 
    }

    $sig=0;
    for my $i (@numArray) {
    my $x=$i;
    for(my $k=0; $k < 8; $k++) {
      $sig=($sig << 1) | ( ($x ^ &shiftbits($sig)) & 1);
      $x=$x >> 1;
      }
  }
  $sig=$sig % (256*256);				# we want only the first 16 bits
							# so do a modulus of the scalar mod 65536
  return($sig);
}

sub shiftbits() {
  my $a=shift(@_);
  return( ($a >> 6) ^ ($a >> 8) ^ ($a >> 11) ^ ($a >> 15));
  }