#! /bin/ksh

#
# Modulus-10 in Korn-shell / sed.
#
# Tested on: Tru64 5.1[ab], Solaris 9, Debian GNU/Linux.
#
# If using the 'bash'-shell, the calls to sed could be 
# replaced with bash's builtin variable substring 
# functionality; ${3:0:1} and ${3:1} respectivley.
#
# (Modulus-10 is used for Credit Card number validation,
#  in Swedish social security numbers, bank account numbers,
#  organization numbers and more...)
#
# 
# Usage: ./modulus10.ksh <digits to calculate checksum for>
#
#                                                28th of August, 2004
#                                           http://netilium.org/~mad/
#                                 Martin Adolfsson <mad@netilium.org>
#

function modulus10 {
  if [ $# -ne 3 ] ; then
    modulus10 1 0 "$1"
  else
    if [ -z "$3" ] ; then
      ACCUM="$2"
      echo $(((($ACCUM+(10-($ACCUM%10)))-$ACCUM)%10))
    else
      NUM="$1"
      VAL="`echo $3 | sed 's/^\(.\).*/\1/'`"
      CALC="$(($VAL * ($NUM+1)))"
      [ "${CALC}" -gt 9 ] && CALC="$((CALC-9))"
      NUM="$(($NUM + 1))"
      modulus10 $(($NUM % 2)) $(($CALC + $2)) "`echo $3|sed 's/^.//'`"
    fi
  fi
}

modulus10 "$1"