What is the CUSIP Checksum Algorithm?

The CUSIP Checksum Algorithm is called the Luhn algorithm with "Modulus 10 Double Add Double" technique.

Here is the pseudo code of the CUSIP Checksum Algorithm provided by wikipedia.com:

algorithm Cusip-Check-Digit is
  input: cusip, an 8-character CUSIP.

  sum := 0
  for 0 ≤ i < 8 do
    c := the ith character of cusip
    if c is a digit then
      v := numeric value of the digit c
    else if c is a letter then
      p := ordinal position of c in the alphabet (A=1, B=2...)
      v := p + 9
    else if c = "*" then
      v := 36
    else if c = "@" then
      v := 37
    else if c = "#" then
      v := 38
    end if
    if i is not even then
      v := v × 2
    end if

    sum := sum + int(v div 10) + v mod 10
  repeat

  return (10 - (sum mod 10)) mod 10

Here is a PHP version of the above pseudo code published on stackoverflow.com.

/**
* function to return the check digit value of a cusip
* @param $cusip
*       the cusip for processing.
* @return Int
*       cusip check digit.
*/
function cusip_checksum($cusip){
    $sum = 0;
    $rebuiltcusip = '';
    for($i = 1; $i <= 8; $i++){
        $c = substr($cusip, ($i - 1), 1); //$i needs to be 0, so as we start at 1, take 1 off.
        $rebuiltcusip .= $c;
        switch(true){
            case $c == '0': // ctype_digit(0) returns false, so checking for 0 here.
                $v = $c;
                watchdog("case 0: ", $v);
            break;
            case ctype_digit($c): //check if numeric
                $v = $c;
                watchdog("case ctype_digit: ", $v);
            break;
            case $c == '*':
                $v = 36;
                watchdog("case *: ", $v);
            break;
            case $c == '@':
                $v = 37;
                watchdog("case @: ", $v);
            break;
            case $c == '#':
                $v = 38;
                watchdog("case #: ", $v);
            break;
            case !ctype_digit($c): //check letter last as this check would pass with * @ or # so allow them to be checked first
                $v = (ord($c) - 64) + 9; //set ordinal number, -64 as this returns ASKII value, then add 9.
                watchdog("case not ctype_digit: ", $v);
            break;
        }
        if(($i % 2) == 0){ //check if odd
            $v = $v * 2;
            watchdog("case odd: ", $v);
        }

        $sum = $sum + floor($v / 10) + ($v % 10);
        watchdog("sum end loop: ", $sum);
    }

    $ncd = (10 - ($sum % 10)) % 10;
    $rebuiltcusip .= $ncd;
    watchdog("rebuilt cusip: ", "Cusip: ".$cusip." Rebuilt: ".$rebuiltcusip);
    return $ncd;
}

⇒ CUSIP vs. ISIN

⇐ What Is CUSIP

⇑ Introduction on CUSIP

⇑⇑ CUSIP - North American Financial Security ID