// CRC32 calculates a cyclic redundancy code (CRC), known as CRC-32, using
// a byte-wise algorithm.
//
// (C) Copyright 1989, 1995-1996, 1999 Earl F. Glynn, Overland Park, KS.
// All Rights Reserved.
//
// This UNIT was derived from the CRCT FORTRAN 77 program given in
// "Byte-wise CRC Calculations" by Aram Perez in IEEE Micro, June 1983,
// pp. 40-50.  The constants here are for the CRC-32 generator polynomial,
// as defined in the Microsoft Systems Journal, March 1995, pp. 107-108
//
// This CRC algorithm emphasizes speed at the expense of the 256-element
// lookup table.
//
// Updated for Delphi 4 dynamic arrays and stream I/O.  July 1999.

UNIT CRC32;

INTERFACE

  USES
   Windows;   // DWORD for D3/D4 compatibility

  TYPE
{$IFDEF VER130}    // This is a bit awkward
   // 8-byte integer
   TInteger8 = Int64;    // Delphi 5
{$ELSE}
  {$IFDEF VER120}
   TInteger8 = Int64;    // Delphi 4
  {$ELSE}
   TInteger8 = COMP;    // Delphi  2 or 3
  {$ENDIF}
{$ENDIF}

  PROCEDURE CalcCRC32 (p:  pointer; ByteCount:  DWORD; VAR CRCvalue:  DWORD);

  PROCEDURE CalcFileCRC32 (FromName:  STRING; VAR CRCvalue:  DWORD;
   VAR TotalBytes:  TInteger8;
   VAR error:  WORD);

IMPLEMENTATION

  USES
{$IFDEF StreamIO}
   SysUtils,  // SysErrorMessage
{$ENDIF}
   Dialogs,   // ShowMessage
   Classes;   // TMemoryStream

  CONST
   table:  ARRAY[0..255] OF DWORD =
  ($00000000, $77073096, $EE0E612C, $990951BA,
   $076DC419, $706AF48F, $E963A535, $9E6495A3,
   $0EDB8832, $79DCB8A4, $E0D5E91E, $97D2D988,
   $09B64C2B, $7EB17CBD, $E7B82D07, $90BF1D91,
   $1DB71064, $6AB020F2, $F3B97148, $84BE41DE,
   $1ADAD47D, $6DDDE4EB, $F4D4B551, $83D385C7,
   $136C9856, $646BA8C0, $FD62F97A, $8A65C9EC,
   $14015C4F, $63066CD9, $FA0F3D63, $8D080DF5,
   $3B6E20C8, $4C69105E, $D56041E4, $A2677172,
   $3C03E4D1, $4B04D447, $D20D85FD, $A50AB56B,
   $35B5A8FA, $42B2986C, $DBBBC9D6, $ACBCF940,
   $32D86CE3, $45DF5C75, $DCD60DCF, $ABD13D59,
   $26D930AC, $51DE003A, $C8D75180, $BFD06116,
   $21B4F4B5, $56B3C423, $CFBA9599, $B8BDA50F,
   $2802B89E, $5F058808, $C60CD9B2, $B10BE924,
   $2F6F7C87, $58684C11, $C1611DAB, $B6662D3D,

   $76DC4190, $01DB7106, $98D220BC, $EFD5102A,
   $71B18589, $06B6B51F, $9FBFE4A5, $E8B8D433,
   $7807C9A2, $0F00F934, $9609A88E, $E10E9818,
   $7F6A0DBB, $086D3D2D, $91646C97, $E6635C01,
   $6B6B51F4, $1C6C6162, $856530D8, $F262004E,
   $6C0695ED, $1B01A57B, $8208F4C1, $F50FC457,
   $65B0D9C6, $12B7E950, $8BBEB8EA, $FCB9887C,
   $62DD1DDF, $15DA2D49, $8CD37CF3, $FBD44C65,
   $4DB26158, $3AB551CE, $A3BC0074, $D4BB30E2,
   $4ADFA541, $3DD895D7, $A4D1C46D, $D3D6F4FB,
   $4369E96A, $346ED9FC, $AD678846, $DA60B8D0,
   $44042D73, $33031DE5, $AA0A4C5F, $DD0D7CC9,
   $5005713C, $270241AA, $BE0B1010, $C90C2086,
   $5768B525, $206F85B3, $B966D409, $CE61E49F,
   $5EDEF90E, $29D9C998, $B0D09822, $C7D7A8B4,
   $59B33D17, $2EB40D81, $B7BD5C3B, $C0BA6CAD,

   $EDB88320, $9ABFB3B6, $03B6E20C, $74B1D29A,
   $EAD54739, $9DD277AF, $04DB2615, $73DC1683,
   $E3630B12, $94643B84, $0D6D6A3E, $7A6A5AA8,
   $E40ECF0B, $9309FF9D, $0A00AE27, $7D079EB1,
   $F00F9344, $8708A3D2, $1E01F268, $6906C2FE,
   $F762575D, $806567CB, $196C3671, $6E6B06E7,
   $FED41B76, $89D32BE0, $10DA7A5A, $67DD4ACC,
   $F9B9DF6F, $8EBEEFF9, $17B7BE43, $60B08ED5,
   $D6D6A3E8, $A1D1937E, $38D8C2C4, $4FDFF252,
   $D1BB67F1, $A6BC5767, $3FB506DD, $48B2364B,
   $D80D2BDA, $AF0A1B4C, $36034AF6, $41047A60,
   $DF60EFC3, $A867DF55, $316E8EEF, $4669BE79,
   $CB61B38C, $BC66831A, $256FD2A0, $5268E236,
   $CC0C7795, $BB0B4703, $220216B9, $5505262F,
   $C5BA3BBE, $B2BD0B28, $2BB45A92, $5CB36A04,
   $C2D7FFA7, $B5D0CF31, $2CD99E8B, $5BDEAE1D,

   $9B64C2B0, $EC63F226, $756AA39C, $026D930A,
   $9C0906A9, $EB0E363F, $72076785, $05005713,
   $95BF4A82, $E2B87A14, $7BB12BAE, $0CB61B38,
   $92D28E9B, $E5D5BE0D, $7CDCEFB7, $0BDBDF21,
   $86D3D2D4, $F1D4E242, $68DDB3F8, $1FDA836E,
   $81BE16CD, $F6B9265B, $6FB077E1, $18B74777,
   $88085AE6, $FF0F6A70, $66063BCA, $11010B5C,
   $8F659EFF, $F862AE69, $616BFFD3, $166CCF45,
   $A00AE278, $D70DD2EE, $4E048354, $3903B3C2,
   $A7672661, $D06016F7, $4969474D, $3E6E77DB,
   $AED16A4A, $D9D65ADC, $40DF0B66, $37D83BF0,
   $A9BCAE53, $DEBB9EC5, $47B2CF7F, $30B5FFE9,
   $BDBDF21C, $CABAC28A, $53B39330, $24B4A3A6,
   $BAD03605, $CDD70693, $54DE5729, $23D967BF,
   $B3667A2E, $C4614AB8, $5D681B02, $2A6F2B94,
   $B40BBE37, $C30C8EA1, $5A05DF1B, $2D02EF8D);

  // Use CalcCRC32 as a procedure so CRCValue can be passed in but
  // also returned.  This allows multiple calls to CalcCRC32 for
  // the "same" CRC-32 calculation.
  PROCEDURE CalcCRC32 (p:  pointer; ByteCount:  DWORD; VAR CRCValue:  DWORD);
   // The following is a little cryptic (but executes very quickly).
   // The algorithm is as follows:
   //  1.  exclusive-or the input byte with the low-order byte of
   //    the CRC register to get an INDEX
   //  2.  shift the CRC register eight bits to the right
   //  3.  exclusive-or the CRC register with the contents of
   //    Table[INDEX]
   //  4.  repeat steps 1 through 3 for all bytes

  VAR
   i:  DWORD;
   q:  ^BYTE;
  BEGIN
   q := p;
   FOR   i := 0 TO ByteCount-1 DO BEGIN
   CRCvalue := (CRCvalue SHR 8)  XOR
   Table[ q^ XOR (CRCvalue AND $000000FF) ];
   INC(q)
   END
  END {CalcCRC32};


{$IFDEF StreamIO}    // Contemporary method using TMemoryStream

  // The CRC-32 value calculated here matches the one from the PKZIP program.
  // Use MemoryStream to read file in binary mode.
  PROCEDURE CalcFileCRC32 (FromName:  STRING; VAR CRCvalue:  DWORD;
   VAR TotalBytes:  TInteger8;
   VAR error:  WORD);
   VAR
   Stream:  TMemoryStream;
  BEGIN
   error := 0;
   CRCValue := $FFFFFFFF;
   Stream := TMemoryStream.Create;
   TRY
   TRY
   Stream.LoadFromFile(FromName);
   IF   Stream.Size > 0
   THEN CalcCRC32 (Stream.Memory, Stream.Size, CRCvalue)
   EXCEPT
   ON E: EReadError DO
   error := 1
   END;

   CRCvalue := NOT CRCvalue
   FINALLY
   Stream.Free
   END;
  END {CalcFileCRC32};

{$ELSE}    // "older" BlockRead method

  // The CRC-32 value calculated here matches the one from the PKZIP program.
  // Use BlockRead to read file in binary mode.
  PROCEDURE CalcFileCRC32 (FromName:  STRING; VAR CRCvalue:  DWORD;
   VAR TotalBytes:  TInteger8;
   VAR error:  WORD);
   CONST
   BufferSize = 32768;

   TYPE
   BufferIndex = 0..BufferSize-1;
   TBuffer    = ARRAY[BufferIndex] OF BYTE;
   pBuffer    = ^TBuffer;

   VAR
   BytesRead:  INTEGER;
   FromFile :  FILE;
   IOBuffer :  pBuffer;
  BEGIN
   New(IOBuffer);
   TRY
   FileMode := 0;  {Turbo default is 2 for R/W; 0 is for R/O}
   CRCValue := $FFFFFFFF;
   ASSIGN (FromFile,FromName);
   {$I-} RESET (FromFile,1); {$I+}
   error := IOResult;
   IF   error = 0
   THEN BEGIN
   TotalBytes := 0;

   REPEAT
   {$I-}
   BlockRead (FromFile, IOBuffer^, BufferSize, BytesRead);
   {$I+}
   error := IOResult;
   IF   (error = 0) AND (BytesRead > 0)
   THEN BEGIN
   CalcCRC32 (IOBuffer, BytesRead, CRCvalue);
   TotalBytes := TotalBytes + BytesRead;  // can't use INC with COMP
   END
   UNTIL (BytesRead = 0) OR (error > 0);

   CLOSE (FromFile)
   END;
   CRCvalue := NOT CRCvalue
   FINALLY
   Dispose(IOBuffer)
   END
  END {CalcFileCRC32};

{$ENDIF}

END {CRC32}.