CRC16校验，通信协议中经常会用到

纯计算方式:

procedure ByteCrc(data:byte;var crc:word);
VAR i:BYTE;
BEGIN
 FOR i:=0 TO 7 DO
  BEGIN
   IF ((data and $01)XOR(crc AND $0001)<>0) THEN
    BEGIN
     crc:=crc shr 1;
     crc:= crc XOR $A001;
    END
   ELSE crc:=crc shr 1;
   data:=data shr 1; // this line is not ELSE and executed anyway.
  END;
END;

function StringCrc(s:shortstring):word; 
var len,i:integer;
begin
 result:=0;
 len:=length(s);
 for i:=1 to len do bytecrc(ord(s[i]),result);
end;

crcstring:shortstring:=chr(data[1])+..+chr(data[N]);

MyCrc:word:=StringCrc(crcstring);

var
   crc16tab:array[0..255]   of   word=
    ($0000, $1189, $2312, $329b, $4624, $57ad, $6536, $74bf,
    $8c48, $9dc1, $af5a, $bed3, $ca6c, $dbe5, $e97e, $f8f7,
    $1081, $0108, $3393, $221a, $56a5, $472c, $75b7, $643e,
    $9cc9, $8d40, $bfdb, $ae52, $daed, $cb64, $f9ff, $e876,
    $2102, $308b, $0210, $1399, $6726, $76af, $4434, $55bd,
    $ad4a, $bcc3, $8e58, $9fd1, $eb6e, $fae7, $c87c, $d9f5,
    $3183, $200a, $1291, $0318, $77a7, $662e, $54b5, $453c,
    $bdcb, $ac42, $9ed9, $8f50, $fbef, $ea66, $d8fd, $c974,
    $4204, $538d, $6116, $709f, $0420, $15a9, $2732, $36bb,
    $ce4c, $dfc5, $ed5e, $fcd7, $8868, $99e1, $ab7a, $baf3,
    $5285, $430c, $7197, $601e, $14a1, $0528, $37b3, $263a,
    $decd, $cf44, $fddf, $ec56, $98e9, $8960, $bbfb, $aa72,
    $6306, $728f, $4014, $519d, $2522, $34ab, $0630, $17b9,
    $ef4e, $fec7, $cc5c, $ddd5, $a96a, $b8e3, $8a78, $9bf1,
    $7387, $620e, $5095, $411c, $35a3, $242a, $16b1, $0738,
    $ffcf, $ee46, $dcdd, $cd54, $b9eb, $a862, $9af9, $8b70,
    $8408, $9581, $a71a, $b693, $c22c, $d3a5, $e13e, $f0b7,
    $0840, $19c9, $2b52, $3adb, $4e64, $5fed, $6d76, $7cff,
    $9489, $8500, $b79b, $a612, $d2ad, $c324, $f1bf, $e036,
    $18c1, $0948, $3bd3, $2a5a, $5ee5, $4f6c, $7df7, $6c7e,
    $a50a, $b483, $8618, $9791, $e32e, $f2a7, $c03c, $d1b5,
    $2942, $38cb, $0a50, $1bd9, $6f66, $7eef, $4c74, $5dfd,
    $b58b, $a402, $9699, $8710, $f3af, $e226, $d0bd, $c134,
    $39c3, $284a, $1ad1, $0b58, $7fe7, $6e6e, $5cf5, $4d7c,
    $c60c, $d785, $e51e, $f497, $8028, $91a1, $a33a, $b2b3,
    $4a44, $5bcd, $6956, $78df, $0c60, $1de9, $2f72, $3efb,
    $d68d, $c704, $f59f, $e416, $90a9, $8120, $b3bb, $a232,
    $5ac5, $4b4c, $79d7, $685e, $1ce1, $0d68, $3ff3, $2e7a,
    $e70e, $f687, $c41c, $d595, $a12a, $b0a3, $8238, $93b1,
    $6b46, $7acf, $4854, $59dd, $2d62, $3ceb, $0e70, $1ff9,
    $f78f, $e606, $d49d, $c514, $b1ab, $a022, $92b9, $8330,
    $7bc7, $6a4e, $58d5, $495c, $3de3, $2c6a, $1ef1, $0f78);
    function CalCRC16(buf:pointer;len:integer):Word;
    var
      i:integer;
      crc:word;
      tp:pansichar;
    begin
      crc:=$ffff;
      i:=0;
      tp:=buf;
      while i<len do
      begin
        crc:=(crc shr 8)xor(crc16tab[(crc xor ord(tp[i])) and $ff]);
        inc(i);
      end;
      result:= not crc;
    end;

记得在数学上CRC有个生成多项式的东西，使用不同的生成多项式，纠错能力和生成的CRC码也不相同。所以如果是专门针对已经有的目标代码，还是自己重新写吧。下面是一个单片机上的算法

var
  CRCHi: array[0..255]   of   byte=(
    $00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0,
    $80, $41, $00, $C1, $81, $40, $01, $C0, $80, $41,
    $00, $C1, $81, $40, $00, $C1, $81, $40, $01, $C0,
    $80, $41, $01, $C0, $80, $41, $00, $C1, $81, $40,
    $00, $C1, $81, $40, $01, $C0, $80, $41, $00, $C1,
    $81, $40, $01, $C0, $80, $41, $01, $C0, $80, $41,
    $00, $C1, $81, $40, $01, $C0, $80, $41, $00, $C1,
    $81, $40, $00, $C1, $81, $40, $01, $C0, $80, $41,
    $00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0,
    $80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40,
    $01, $C0, $80, $41, $01, $C0, $80, $41, $00, $C1,
    $81, $40, $01, $C0, $80, $41, $00, $C1, $81, $40,
    $00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0,
    $80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40,
    $01, $C0, $80, $41, $00, $C1, $81, $40, $01, $C0,
    $80, $41, $01, $C0, $80, $41, $00, $C1, $81, $40,
    $00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0,
    $80, $41, $00, $C1, $81, $40, $01, $C0, $80, $41,
    $00, $C1, $81, $40, $00, $C1, $81, $40, $01, $C0,
    $80, $41, $00, $C1, $81, $40, $01, $C0, $80, $41,
    $01, $C0, $80, $41, $00, $C1, $81, $40, $01, $C0,
    $80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40,
    $01, $C0, $80, $41, $01, $C0, $80, $41, $00, $C1,
    $81, $40, $00, $C1, $81, $40, $01, $C0, $80, $41,
    $00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0,
    $80, $41, $00, $C1, $81, $40
);

  CRCLo: array[0..255]   of   byte=(
    $00, $C0, $C1, $01, $C3, $03, $02, $C2, $C6, $06,
    $07, $C7, $05, $C5, $C4, $04, $CC, $0C, $0D, $CD,
    $0F, $CF, $CE, $0E, $0A, $CA, $CB, $0B, $C9, $09,
    $08, $C8, $D8, $18, $19, $D9, $1B, $DB, $DA, $1A,
    $1E, $DE, $DF, $1F, $DD, $1D, $1C, $DC, $14, $D4,
    $D5, $15, $D7, $17, $16, $D6, $D2, $12, $13, $D3,
    $11, $D1, $D0, $10, $F0, $30, $31, $F1, $33, $F3,
    $F2, $32, $36, $F6, $F7, $37, $F5, $35, $34, $F4,
    $3C, $FC, $FD, $3D, $FF, $3F, $3E, $FE, $FA, $3A,
    $3B, $FB, $39, $F9, $F8, $38, $28, $E8, $E9, $29,
    $EB, $2B, $2A, $EA, $EE, $2E, $2F, $EF, $2D, $ED,
    $EC, $2C, $E4, $24, $25, $E5, $27, $E7, $E6, $26,
    $22, $E2, $E3, $23, $E1, $21, $20, $E0, $A0, $60,
    $61, $A1, $63, $A3, $A2, $62, $66, $A6, $A7, $67,
    $A5, $65, $64, $A4, $6C, $AC, $AD, $6D, $AF, $6F,
    $6E, $AE, $AA, $6A, $6B, $AB, $69, $A9, $A8, $68,
    $78, $B8, $B9, $79, $BB, $7B, $7A, $BA, $BE, $7E,
    $7F, $BF, $7D, $BD, $BC, $7C, $B4, $74, $75, $B5,
    $77, $B7, $B6, $76, $72, $B2, $B3, $73, $B1, $71,
    $70, $B0, $50, $90, $91, $51, $93, $53, $52, $92,
    $96, $56, $57, $97, $55, $95, $94, $54, $9C, $5C,
    $5D, $9D, $5F, $9F, $9E, $5E, $5A, $9A, $9B, $5B,
    $99, $59, $58, $98, $88, $48, $49, $89, $4B, $8B,
    $8A, $4A, $4E, $8E, $8F, $4F, $8D, $4D, $4C, $8C,
    $44, $84, $85, $45, $87, $47, $46, $86, $82, $42,
    $43, $83, $41, $81, $80, $40
);

  function CalCRC16(pcData: PAnsiChar; cDataLeng: Byte): WORD;
  var
    cCRCHi, cCRCLo: byte;
    cIndex: Byte;
  begin
    Result:=0;
    cCRCHi:= $FF ; // Initial High Byte
    cCRCLo:= $FF ; // Initial Low Byte

    while (cDataLeng > 0) do
    begin
      cIndex:= cCRCHi XOR Byte(pcData^); // Calculate CRC
      Inc(pcData);

      cCRCHi:= cCRCLo XOR CRCHi[cIndex] ;
      cCRCLo:= CRCLo[cIndex] ;
      Dec(cDataLeng)
    end;
    Inc(Result, cCRCHi);
    Result:= Result shl 8;
    Inc(Result, cCRCLo);
  end;

而实际上，都是CRC16,但是生成的校验码也不相同，猜测可能是生成多项式的原因。网络使人变懒了，不去想了，知道有这么回事就好。