#@@ luhn - check a number for validity using the Luhn formula
#@  USAGE: luhn [-n|-v] NUMBER
#@  NOTE: Non-digits are allowed in NUMBER and will be skipped over
#@  OUTPUT: none
#@  EXIT_CODE: 0 if NUMBER is valid, otherwise 1
#@  OPTIONS: -n Generate checksum digit for number supplied and print
#@           -v Print OK or Not valid; with -n, -v has no effect

case $1 in
    --help) printf "%s\n" \
"luhn - check a number for validity using the Luhn formula
  USAGE: luhn [-n|-v] NUMBER
  NOTE: Non-digits are allowed in NUMBER and will be skipped over
  OUTPUT: none unless -v is used
  EXIT_CODE: 0 if NUMBER is valid, otherwise 1
  OPTIONS: -n Generate checksum digit for number supplied and print
           -v Print OK or Not valid; with -n, -v has no effect
"
   exit
esac

_ld() #@@ Double a [single-digit] number and, if >10, add both digits
{     #@ USAGE: _ld N
      #@ RESULT: stored in $_LD
      #@ OUTPUT: none
      #@ RETURN: 0 if $1 is a single digit, otherwise 5
    case $1 in
        0) _LD=0 ;;
        1) _LD=2 ;;
        2) _LD=4 ;;
        3) _LD=6 ;;
        4) _LD=8 ;;
        5) _LD=1 ;;
        6) _LD=3 ;;
        7) _LD=5 ;;
        8) _LD=7 ;;
        9) _LD=9 ;;
        *) _LD=; return 5 ;;
    esac
}

n=1
gen=
verbose=0

while getopts vn opt
do
  case $opt in
      n) gen=0 ;;
      v) verbose=$(( verbose + 1 )) ;;
  esac
done
shift $(( $OPTIND - 1 ))

sum=0

#: If check digit generation is requested, tack on dummy digit, 0
number=$1$gen

while [ -n "$number" ]
do
  #: Split $number into last digit and everything else
  left=${number%?}        #~ Everything but the last character
  digit=${number#"$left"} #~ The last character
  number=$left            #~ Discard last character

  #: Skip non-digits
  case $digit in
      [0-9]) ;;
      *) continue ;;
  esac

  #: Calculate checksum
  case $n in
      *[24680]) _ld "${digit:-0}" || exit
                sum=$(( $sum + $_LD ))
                ;;
      *) sum=$(( $sum + ${digit:-0} )) ;;
  esac
  n=$(( $n + 1 ))
done

if [ -z "$gen" ]
then
  case $sum in
      *0) [ "$verbose" -gt 0 ] && echo OK ; true ;;
      *) [ "$verbose" -gt 0 ] && echo Not valid ; false;;
  esac
else
  printf "%s\n" $1$(( 10 - ( $sum % 10 ) ))
fi