crc16
Wikipedia: A cyclic redundancy check (CRC) is a type of function that takes as input a data stream of any length, and produces as output a value of a certain space, commonly a 32-bit integer. The term CRC is often used to denote either the function or the function's output. A CRC can be used as a checksum to detect alteration of data during transmission or storage. CRCs are popular because they are simple to implement in binary hardware, are easy to analyze mathematically, and are particularly good at detecting common errors caused by noise in transmission channels. The CRC was invented by W. Wesley Peterson, and published in his 1961 paper.
NOTE: This a the common 16-bit CRC16-CCITT algorithm (with two modes of operation) used in X.25, V.41, CDMA, Bluetooth, XMODEM, PPP, IrDA, BACnet, and other protocols.
crc16(str,mode) Returns a CRC16-CCITT hash value for the given string.
/*
** Usage:
** crc16(str,mode)
**
** Arguments:
** str string from which to compute a CRC16-CCITT hash value
** mode (optional) skip or set to FALSE to use the common "bad"
** mode, set to TRUE to use the pedantic "good" mode.
**
** Returns:
** a CRC16-CCITT hash value for the given string
**
** Notes:
** For speed, this creates a lookup table in the form of a global array
** called CRC16tab[]. It features two modes of operation. The "bad" mode
** is the common implementation found most places. The "good" mode is an
** implemetation that is closer to the actual CRC16-CCITT specification.
**
** GMLscripts.com
*/
{
var str,crc,len,pos,byte;
str = argument0;
if (argument1) crc = $1D0F else crc = $FFFF;
len = string_length(str);
pos = 1;
if (!variable_global_exists("CRC16tab")) {
var tab,i;
tab = "00000041290825812387165162064524774289033303237161412904541949548536775780661935";
tab += "04657005281291508786211731704429431253023768933560459474181854205500766246358334";
tab += "09314133790105605121258302989517572216374234646411340883815358862629275060454669";
tab += "13907098420564901584304232635822165181004693942874386813461663455593905519751132";
tab += "18628227572675830887021120624110242143715166055789597906391935144392734327447403";
tab += "23285191563141527286067690264014899107705631752188644476031839801356724793143802";
tab += "27814318791968423749112981536303168072336084664911527165678144330483953620040265";
tab += "32407283422427720212158911182607761036966543961374573095324448923448584079336728";
tab += "37256331934551441451535164945361774577110422400161124820841920484164212874224679";
tab += "33721377844197946042499815404458239623020068904752089471301016949210122520729270";
tab += "46570424433831234185628305870354572504451353809411052800115329798256712154017413";
tab += "42971470983471338840592316335850973551000993914066016810580826199303261794122068";
tab += "55628515656375859695393683530547498434352259618533307262666306336022731446610403";
tab += "52093561566022364286358333989643963480261906123124271913125402801068641093114994";
tab += "64814606875668452557485544442740424362973178227655236521952515522113950739203265";
tab += "61215653425308557212449554908236825409522818332310200532418011923160500379307920";
for(i=0;i<256;i+=1) {
global.CRC16tab[i] = real(string_copy(tab,i*5+1,5));
}
}
repeat(len) {
byte = ord(string_char_at(str,pos));
crc = ((crc<<8)^global.CRC16tab[((crc>>8)^byte)&$FF])&$FFFF;
pos += 1;
}
return crc;
}
** Usage:
** crc16(str,mode)
**
** Arguments:
** str string from which to compute a CRC16-CCITT hash value
** mode (optional) skip or set to FALSE to use the common "bad"
** mode, set to TRUE to use the pedantic "good" mode.
**
** Returns:
** a CRC16-CCITT hash value for the given string
**
** Notes:
** For speed, this creates a lookup table in the form of a global array
** called CRC16tab[]. It features two modes of operation. The "bad" mode
** is the common implementation found most places. The "good" mode is an
** implemetation that is closer to the actual CRC16-CCITT specification.
**
** GMLscripts.com
*/
{
var str,crc,len,pos,byte;
str = argument0;
if (argument1) crc = $1D0F else crc = $FFFF;
len = string_length(str);
pos = 1;
if (!variable_global_exists("CRC16tab")) {
var tab,i;
tab = "00000041290825812387165162064524774289033303237161412904541949548536775780661935";
tab += "04657005281291508786211731704429431253023768933560459474181854205500766246358334";
tab += "09314133790105605121258302989517572216374234646411340883815358862629275060454669";
tab += "13907098420564901584304232635822165181004693942874386813461663455593905519751132";
tab += "18628227572675830887021120624110242143715166055789597906391935144392734327447403";
tab += "23285191563141527286067690264014899107705631752188644476031839801356724793143802";
tab += "27814318791968423749112981536303168072336084664911527165678144330483953620040265";
tab += "32407283422427720212158911182607761036966543961374573095324448923448584079336728";
tab += "37256331934551441451535164945361774577110422400161124820841920484164212874224679";
tab += "33721377844197946042499815404458239623020068904752089471301016949210122520729270";
tab += "46570424433831234185628305870354572504451353809411052800115329798256712154017413";
tab += "42971470983471338840592316335850973551000993914066016810580826199303261794122068";
tab += "55628515656375859695393683530547498434352259618533307262666306336022731446610403";
tab += "52093561566022364286358333989643963480261906123124271913125402801068641093114994";
tab += "64814606875668452557485544442740424362973178227655236521952515522113950739203265";
tab += "61215653425308557212449554908236825409522818332310200532418011923160500379307920";
for(i=0;i<256;i+=1) {
global.CRC16tab[i] = real(string_copy(tab,i*5+1,5));
}
}
repeat(len) {
byte = ord(string_char_at(str,pos));
crc = ((crc<<8)^global.CRC16tab[((crc>>8)^byte)&$FF])&$FFFF;
pos += 1;
}
return crc;
}
Comments? 
[Please Login]
Projects: 2
Related: b64_to_bytes, base_convert, bin_to_bytes, bin_to_dec, bin_to_fraction, bin_to_hex, bytes_to_b64, bytes_to_bin, bytes_to_hex, crc16, dec_to_bin, dec_to_hex, dec_to_oct, dec_to_roman, decode_real_double, encode_real_double, fraction_to_bin, hex_to_bin, hex_to_bytes, hex_to_dec, md2, md5, oct_to_dec, roman_to_dec, string_soundex, value_min_bits, word_max_value
Contributors: Eyas Sharaiha, xot
History:
Jun 17, 2007 - xot submits original script
Jul 12, 2008 - Eyas Sharaiha spots a misnamed variable
