(*
 * Copyright 1987, 1989 Samuel H. Smith;  All rights reserved
 *
 * This is a component of the ProDoor System.
 * Do not distribute modified versions without my permission.
 * Do not remove or alter this notice or any other copyright notice.
 * If you use this in your own program you must distribute source code.
 * Do not use any of this in a commercial product.
 *
 *)

(*
 * crc16.inc - very fast CRC16 calculation
 *
 * S.H.Smith, 27-May-88
 *
 *)

const 
  crc_table : array [0..255] of word =
  ( $0000, $C0C1, $C181, $0140, $C301, $03C0, $0280, $C241,
    $C601, $06C0, $0780, $C741, $0500, $C5C1, $C481, $0440,
    $CC01, $0CC0, $0D80, $CD41, $0F00, $CFC1, $CE81, $0E40,
    $0A00, $CAC1, $CB81, $0B40, $C901, $09C0, $0880, $C841,
    $D801, $18C0, $1980, $D941, $1B00, $DBC1, $DA81, $1A40,
    $1E00, $DEC1, $DF81, $1F40, $DD01, $1DC0, $1C80, $DC41,
    $1400, $D4C1, $D581, $1540, $D701, $17C0, $1680, $D641,
    $D201, $12C0, $1380, $D341, $1100, $D1C1, $D081, $1040,
    $F001, $30C0, $3180, $F141, $3300, $F3C1, $F281, $3240,
    $3600, $F6C1, $F781, $3740, $F501, $35C0, $3480, $F441,
    $3C00, $FCC1, $FD81, $3D40, $FF01, $3FC0, $3E80, $FE41,
    $FA01, $3AC0, $3B80, $FB41, $3900, $F9C1, $F881, $3840,
    $2800, $E8C1, $E981, $2940, $EB01, $2BC0, $2A80, $EA41,
    $EE01, $2EC0, $2F80, $EF41, $2D00, $EDC1, $EC81, $2C40,
    $E401, $24C0, $2580, $E541, $2700, $E7C1, $E681, $2640,
    $2200, $E2C1, $E381, $2340, $E101, $21C0, $2080, $E041,
    $A001, $60C0, $6180, $A141, $6300, $A3C1, $A281, $6240,
    $6600, $A6C1, $A781, $6740, $A501, $65C0, $6480, $A441,
    $6C00, $ACC1, $AD81, $6D40, $AF01, $6FC0, $6E80, $AE41,
    $AA01, $6AC0, $6B80, $AB41, $6900, $A9C1, $A881, $6840,
    $7800, $B8C1, $B981, $7940, $BB01, $7BC0, $7A80, $BA41,
    $BE01, $7EC0, $7F80, $BF41, $7D00, $BDC1, $BC81, $7C40,
    $B401, $74C0, $7580, $B541, $7700, $B7C1, $B681, $7640,
    $7200, $B2C1, $B381, $7340, $B101, $71C0, $7080, $B041,
    $5000, $90C1, $9181, $5140, $9301, $53C0, $5280, $9241,
    $9601, $56C0, $5780, $9741, $5500, $95C1, $9481, $5440,
    $9C01, $5CC0, $5D80, $9D41, $5F00, $9FC1, $9E81, $5E40,
    $5A00, $9AC1, $9B81, $5B40, $9901, $59C0, $5880, $9841,
    $8801, $48C0, $4980, $8941, $4B00, $8BC1, $8A81, $4A40,
    $4E00, $8EC1, $8F81, $4F40, $8D01, $4DC0, $4C80, $8C41,
    $4400, $84C1, $8581, $4540, $8701, $47C0, $4680, $8641,
    $8201, $42C0, $4380, $8341, $4100, $81C1, $8081, $4040 );

var
   crc_out:    word;


procedure crcstr(var s; len: integer);
   {calculate crc-16 of a buffer}
var
   b: array[0..maxint] of byte absolute s;
   i: integer;
begin

(***
   for i := 0 to len-1 do
      crc_out := lo(crc_out shr 8) xor crc_table[lo(crc_out xor b[i])];
***)

Inline(
  $8B/$4E/$04/           {   mov cx,[bp+4]        ;cx <- len}
  $C4/$76/$06/           {   les si,[bp+6]        ;es:si <- b}
  $8B/$16/>CRC_OUT/      {   mov dx,[>crc_out]    ;dx <- crc_out}

                         {next:}
  $26/                   {   es:}
  $8A/$04/               {   mov al,[si]}
  $46/                   {   inc si}
  $30/$E4/               {   xor ah,ah            ;ax <- b[i++]}

  $31/$D0/               {   xor ax,dx}
  $30/$E4/               {   xor ah,ah}
  $89/$C7/               {   mov di,ax}
  $D1/$E7/               {   shl di,1             ;bx <- crctab[crcout xor b[i]]}
  $8B/$9D/>CRC_TABLE/    {   mov bx,[di+>crc_table]}

  $88/$F0/               {   mov al,dh}
  $30/$E4/               {   xor ah,ah}
  $31/$D8/               {   xor ax,bx}
  $89/$C2/               {   mov dx,ax            ;crcout := lo(crcout) xor crctab[..]}

  $49/                   {   dec cx}
  $75/$E3/               {   jnz next}

  $89/$16/>CRC_OUT);     {   mov [>crc_out],dx}
end;