#!/usr/bin/env bash

# For testing purposes, an input of "123456789" should produce a crc of 0xBB3D

function calc_crc16( )
{

	poly=0x8005

	declare -a crc_16_table=(     
	  0x0000 0xC0C1 0xC181 0x0140 0xC301 0x03C0 0x0280 0xC241
	  0xC601 0x06C0 0x0780 0xC741 0x0500 0xC5C1 0xC481 0x0440
	  0xCC01 0x0CC0 0x0D80 0xCD41 0x0F00 0xCFC1 0xCE81 0x0E40
	  0x0A00 0xCAC1 0xCB81 0x0B40 0xC901 0x09C0 0x0880 0xC841
	  0xD801 0x18C0 0x1980 0xD941 0x1B00 0xDBC1 0xDA81 0x1A40
	  0x1E00 0xDEC1 0xDF81 0x1F40 0xDD01 0x1DC0 0x1C80 0xDC41
	  0x1400 0xD4C1 0xD581 0x1540 0xD701 0x17C0 0x1680 0xD641
	  0xD201 0x12C0 0x1380 0xD341 0x1100 0xD1C1 0xD081 0x1040
	  0xF001 0x30C0 0x3180 0xF141 0x3300 0xF3C1 0xF281 0x3240
	  0x3600 0xF6C1 0xF781 0x3740 0xF501 0x35C0 0x3480 0xF441
	  0x3C00 0xFCC1 0xFD81 0x3D40 0xFF01 0x3FC0 0x3E80 0xFE41
	  0xFA01 0x3AC0 0x3B80 0xFB41 0x3900 0xF9C1 0xF881 0x3840
	  0x2800 0xE8C1 0xE981 0x2940 0xEB01 0x2BC0 0x2A80 0xEA41
	  0xEE01 0x2EC0 0x2F80 0xEF41 0x2D00 0xEDC1 0xEC81 0x2C40
	  0xE401 0x24C0 0x2580 0xE541 0x2700 0xE7C1 0xE681 0x2640
	  0x2200 0xE2C1 0xE381 0x2340 0xE101 0x21C0 0x2080 0xE041
	  0xA001 0x60C0 0x6180 0xA141 0x6300 0xA3C1 0xA281 0x6240
	  0x6600 0xA6C1 0xA781 0x6740 0xA501 0x65C0 0x6480 0xA441
	  0x6C00 0xACC1 0xAD81 0x6D40 0xAF01 0x6FC0 0x6E80 0xAE41
	  0xAA01 0x6AC0 0x6B80 0xAB41 0x6900 0xA9C1 0xA881 0x6840
	  0x7800 0xB8C1 0xB981 0x7940 0xBB01 0x7BC0 0x7A80 0xBA41
	  0xBE01 0x7EC0 0x7F80 0xBF41 0x7D00 0xBDC1 0xBC81 0x7C40
	  0xB401 0x74C0 0x7580 0xB541 0x7700 0xB7C1 0xB681 0x7640
	  0x7200 0xB2C1 0xB381 0x7340 0xB101 0x71C0 0x7080 0xB041
	  0x5000 0x90C1 0x9181 0x5140 0x9301 0x53C0 0x5280 0x9241
	  0x9601 0x56C0 0x5780 0x9741 0x5500 0x95C1 0x9481 0x5440
	  0x9C01 0x5CC0 0x5D80 0x9D41 0x5F00 0x9FC1 0x9E81 0x5E40
	  0x5A00 0x9AC1 0x9B81 0x5B40 0x9901 0x59C0 0x5880 0x9841
	  0x8801 0x48C0 0x4980 0x8941 0x4B00 0x8BC1 0x8A81 0x4A40
	  0x4E00 0x8EC1 0x8F81 0x4F40 0x8D01 0x4DC0 0x4C80 0x8C41
	  0x4400 0x84C1 0x8581 0x4540 0x8701 0x47C0 0x4680 0x8641
	  0x8201 0x42C0 0x4380 0x8341 0x4100 0x81C1 0x8081 0x4040 
	)

	input="$1"
	crc=0

	len=${#input}
	for (( i = 0; i<len; i++ ))
	do
		char=${input:$i:1}
		j=$( printf "%02x" "'$char" )
		declare -i buf=0x${j}
		crc=$(( crc_16_table[ ( crc ^ buf ) & 0xff ] ^ ( crc >> 8 ) ))
	done

	printf "%04x" $crc
}

crc16=$( calc_crc16 "$1" )
echo "$crc16"