cksum16 - CRC-16 checksum of a string

#!/bin/bash
#
# Calculates the CRC-16 checksum of a string
# like cksum does for CRC32
#
# Copyright (C) 2011 Rodrigo Silva (MestreLion) <linux@rodrigosilva.com>
# License: GPLv3 or later, at your choice. See <http://www.gnu.org/licenses/gpl>
#
# References:
# 0x10 blog: <http://zeroxten.blogspot.com/search/label/crc16>
# Wine's winemenubuilder: <http://source.winehq.org/git/wine.git>
# The wise, friendly and helpful #bash gurus: <irc://irc.freenet.org/#bash>
#
# NOTE: Newlines are properly handled, but last newline is stripped off
# NOTE: NUL chars are stripped off before computing
# TODO: Test how non-ASCI chars are handled
# FIXME: Decent usage explaining options and behaviour

self="${0##*/}"

error() {
    local msg="$1"
    printf "%s: %s\nTry '%s --help' for more information\n" \
           "$self" "$msg" "$self" >&2
    exit 1
}
usage() {
    printf "Print CRC-16 checksum (and optionally the byte count) of STRING\n\n"
    printf "Usage: %s [--hex|--dec|--format FORMAT] [--count] [STRING]\n\n" "$self"
    printf "Default format is decimal. If no string or -, reads from stdin\n"
    printf "NULs and last newline are ignored. Use for texts, not binary data.\n"
    exit
}
crc16()
(
    export LC_ALL=C

    local string="$1"
    local format="${2:-"%d"}"
    local crc=0
    local i j xor_poly

    for ((i=0; i<${#string}; i++)); do

        c=$(printf "%d" "'${string:i:1}")
        for ((j=0; j<8; c>>=1, j++)); do

            (( xor_poly = (c ^ crc) & 1 ))
            (( crc >>= 1 ))
            (( xor_poly )) && (( crc ^= 0xA001 ))

        done
    done
    printf "$format" "$crc"
)

while [[ $# -gt 0 ]]; do
    arg="$1"; shift
    case "$arg" in
    -h|--help  ) usage                              ;;
    -x|--hex   ) format="%04X"                      ;;
    -d|--dec   ) format="%d"                        ;;
    -f|--format) format="$1"; shift                 ;;
    -c|--count ) count=1                            ;;
    --         ) string="$1"                ; break ;;
    -          ) string=$( cat )   ; done=1 ; break ;;
    -*         ) error "unrecognized option '$arg'" ;;
    *          ) string="$arg"     ; done=1 ; break ;;
    esac
done

[[ "$done" || "$string" ]] || string=$( cat )

# Expected command-line behaviour table:
# <none>    => str = stdin
# -         => str = stdin
# something => str = something
# -- <none> => str = stdin
# -- someth => str = something

[[ "$string" ]] || error "missing string"

crc16 "$string" "$format"
[[ "$count" ]] && printf " %d" "${#string}"
printf "\n"

#Original function from winemenubuilder.c (for reference)
#static unsigned short crc16(const char* string)
#{
#    unsigned short crc = 0;
#    int i, j, xor_poly;
#
#    for (i = 0; string[i] != 0; i++)
#    {
#        char c = string[i];
#        for (j = 0; j < 8; c >>= 1, j++)
#        {
#            xor_poly = (c ^ crc) & 1;
#            crc >>= 1;
#            if (xor_poly)
#                crc ^= 0xa001;
#        }
#    }
#    return crc;
#}