Function name:

    CardChek()

  Purpose:

    Verifies proper entry of credit card account numbers.

  Written by:

    Mike Coticchio
          Compuserve: 74740,2611
          Internet  : 74740.2611@compuserve.com

  Notice:

    Copyright (c) Michael Coticchio, 1993.  All rights reserved.

    Donated to the CA-Clipper programming community for its free use.

    Author does not warrant the fitness of this function for any
    application and assumes no liability for damages resulting from the
    use or misuse of this function.  Use of this function is solely at
    your own risk.

  Compiler version:

    CA-Clipper 5.2b

  Syntax:

    CardChek(cCardNum) --> nCardType

  Arguments:

    cCardNum - a credit card number to be tested

  Returns:

    a numeric value as follows:

       -1 - number is invalid
        0 - unknown card type (account number valid)
        1 - American Express
        2 - Visa
        3 - MasterCard
        4 - Discover
        5 - Carte Blanche
        6 - Diners Club
        7 - JCB

  Description:

     CardChek() is a function useful in POS applications which handle
     credit-card processing.  The function serves two purposes:

     1. Perform check digit calculation on credit card numbers.  This
        will help trap keying errors that frequently occur when a credit
        card number is manually entered at the POS terminal.

        NOTE: the function DOES NOT and CANNOT indicate whether an
        account number is valid - only whether the account number itself
        passes an internal check digit test.  Do NOT assume that a
        credit card is legitimate just because CardChek() tells you the
        number is OK. Accounts can be invalid for a number of reasons;
        it is quite possible for invalid account numbers to generate
        correct check digit calculations.

     2. Determine the type of credit card being used. This is especially
        useful if the credit card is being read by a magnetic stripe
        reader, since your POS application will be able to auto-detect
        credit cards rather than having the sales clerk select
        (sometimes incorrectly) the type of card. CardChek() recognizes
        7 major credit cards.

  Example:

        function test()

        local cCard, nResult

        local aCards := {"AmEx", "Visa", "MasterCard", "Discover",;
                         "Carte Blanche", "Diners", "JCB"}

        cCard := space(16)
        @ 5, 10 say "Card number:" get cCard
        read

        if !empty(cCard)
          nResult := CardChek(cCard)
          if nResult < 0
            @ 7,10 say "Card is invalid"
          elseif nResult == 0
            @ 7,10 say "Unknown card type"
          else
            @ 7, 10 say "Card type: " + aCards[nResult]
          endif
        endif

        return nil