<?php
function crc16($data)
 {
   $crc = 0xFFFF;
   for ($i = 0; $i < strlen($data); $i++)
   {
     $x = (($crc >> 8) ^ ord($data[$i])) & 0xFF;
     $x ^= $x >> 4;
     $crc = (($crc << 8) ^ ($x << 12) ^ ($x << 5) ^ $x) & 0xFFFF;
   }
   return $crc;
 }

function CRC16CCITT(bytes: TBytes): Word;
const
  polynomial = $1021;   // 0001 0000 0010 0001  (0, 5, 12)
var
  crc: Word;
  I, J: Integer;
  b: Byte;
  bit, c15: Boolean;
begin
  crc := $FFFF; // initial value
  for I := 0 to High(bytes) do
  begin
    b := bytes[I];
    for J := 0 to 7 do
    begin
      bit := (((b shr (7-J)) and 1) = 1);
      c15 := (((crc shr 15) and 1) = 1);
      crc := crc shl 1;
      if ((c15 xor bit) <> 0) then crc := crc xor polynomial;
    end;
  end;
  Result := crc and $ffff;
end;

function crc16(Buffer:String;Polynom,Initial:Cardinal):Cardinal;
var
  i,j: Integer;
begin
Result:=Initial;
for i:=1 to Length(Buffer) do begin
  Result:=Result xor (ord(buffer[i]) shl 8);
  for j:=0 to 7 do begin
    if (Result and $8000)<>0 then Result:=(Result shl 1) xor Polynom
    else Result:=Result shl 1;
    end;
  end;
Result:=Result and $ffff;
end;

unit CRC16CCITT;

interface

  function ComputeCRC16CCITT(crc: word; const data: PByte; len:integer) : word;

implementation

const
  crc16_table: array [0..$FF] of word = (0,4489,8978,12955,17956,22445,25910,29887,35912,40385,44890,48851,51820,56293,59774,
    63735,4225,264,13203,8730,22181,18220,30135,25662,40137,36160,49115,44626,56045,52068,63999,
    59510,8450,12427,528,5017,26406,30383,17460,21949,44362,48323,36440,40913,60270,64231,51324,
    55797,12675,8202,4753,792,30631,26158,21685,17724,48587,44098,40665,36688,64495,60006,55549,
    51572,16900,21389,24854,28831,1056,5545,10034,14011,52812,57285,60766,64727,34920,39393,43898,
    47859,21125,17164,29079,24606,5281,1320,14259,9786,57037,53060,64991,60502,39145,35168,48123,
    43634,25350,29327,16404,20893,9506,13483,1584,6073,61262,65223,52316,56789,43370,47331,35448,
    39921,29575,25102,20629,16668,13731,9258,5809,1848,65487,60998,56541,52564,47595,43106,39673,
    35696,33800,38273,42778,46739,49708,54181,57662,61623,2112,6601,11090,15067,20068,24557,28022,
    31999,38025,34048,47003,42514,53933,49956,61887,57398,6337,2376,15315,10842,24293,20332,32247,
    27774,42250,46211,34328,38801,58158,62119,49212,53685,10562,14539,2640,7129,28518,32495,19572,
    24061,46475,41986,38553,34576,62383,57894,53437,49460,14787,10314,6865,2904,32743,28270,23797,
    19836,50700,55173,58654,62615,32808,37281,41786,45747,19012,23501,26966,30943,3168,7657,12146,
    16123,54925,50948,62879,58390,37033,33056,46011,41522,23237,19276,31191,26718,7393,3432,16371,
    11898,59150,63111,50204,54677,41258,45219,33336,37809,27462,31439,18516,23005,11618,15595,3696,
    8185,63375,58886,54429,50452,45483,40994,37561,33584,31687,27214,22741,18780,15843,11370,7921,
    3960);

  function ComputeCRC16CCITT(crc: word; const data: PByte; len:integer) : word;
  var
    i : integer;
  begin
    for i := 0 to len-1 do
      crc := (crc shr 8) xor crc16_table[(crc xor data[i]) and $ff];
    result := crc;
  end;

end.