XML Decoders

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XML Decoders

Starting from SVN revision 12582, XML reader is able to process data in any encoding by using external decoders. Following is the brief description of how it works.

Available decoders

Currently, the decoder using libiconv is available. It has two distinct implementations. The first one, in xmliconv.pas unit, uses the existing iconvenc package and supports Linux, FreeBSD and Darwin targets. The second one, in xmliconv_windows.pas unit, is for Windows targets. It links to the native-build iconv.dll that you should distribute with the application.

Decoder structure

Interfacing with the external decoders is done in a plain procedural style. Writing the decoder is essentially implementing the following three procedures:

  1. GetDecoder
  2. Decode
  3. Cleanup (optional)

Here is the brief desription of external decoder operation:

GetDecoder

function GetDecoder(const AEncoding: string; out Decoder: TDecoder): Boolean; stdcall;

At the program initialization time, decoder registers itself by calling XMLRead.RegisterDecoder procedure, supplying its GetDecoder function as the argument. Whenever the reader encounters the encoding label which it does not handle internally, it calls all registered GetDecoder functions in the same order they were registered, until one of them returns True. The GetDecoder function arguments are the name of encoding and the TDecoder record that the function should fill. The encoding name is restricted to characters in range ['A'..'Z', 'a'..'z', '0'..'9', '.', '-', '_'], and must be compared case-insensitive. If the decoder supports given encoding, the function should set at least the Decode member of the supplied record and return True. Setting other members of Decoder is optional.

Cleanup

procedure Cleanup(Context: Pointer); stdcall;

If GetDecoder sets the Decoder.Cleanup member, it is called by reader once, after processing of the current entity is finished. As the name suggests, the decoder should then free all resources it allocated.

The value of Decoder.Context is passed to Decode and Cleanup procedures each time they are called. The reader does not assign any meaning to this value.

Decode

function Decode(Context: Pointer; InBuf: PChar; var InCnt: Cardinal;
  OutBuf: PWideChar; var OutCnt: Cardinal): Integer; stdcall;

The Decode function does the main job. It should convert the input data pointed by InBuf into UTF-16 in the current platform endianness and place it into OutBuf. The size of input buffer is supplied in InCnt, the space avaliable in output buffer is in OutCnt.

The important difference to note is that InCnt is given in bytes, while OutCnt is in WideChars.

The function must decrement InCnt and OutCnt according to the amount of data it processes. Each processed character decrements OutCnt by one (or by two in case the surrogate pair is written); the amount of InCnt decrement depends on the actual encoding.

No assumptions should be made about initial size of buffers: for example, the reader may call decoder with only a few bytes in input buffer. The decoder function then should return zero indicating nothing is processed, and the reader will fetch more input and call decoder again.

The function should return positive value if it had processed something, zero if it had not (e.g. because no space available in either input or output buffer), and negative value in cause the input data contains illegal sequence. In the future, there may be attempt to categorize the decoding errors, but currently any negative return simply aborts the reader with the 'Decoding error' message.

In case of error in input data the decoder should still decrement OutCnt to reflect the number of successfully processed characters. This will be used by reader to provide location information in the exception error message.

Sample decoder

Following is a sample unit that decodes cp866. This decoder is stateless, so it does not use the Cleanup and Context members. It should be very easy to modify this sample to handle any similar single-byte encoding by just replacing the conversion table.

unit xmlcp866;

interface

implementation

uses
  SysUtils, xmlread;

const
  cp866table: array[#128..#255] of WideChar=(
      #$0410, #$0411, #$0412, #$0413, #$0414, #$0415, #$0416, #$0417,
      #$0418, #$0419, #$041A, #$041B, #$041C, #$041D, #$041E, #$041F,
      #$0420, #$0421, #$0422, #$0423, #$0424, #$0425, #$0426, #$0427,
      #$0428, #$0429, #$042A, #$042B, #$042C, #$042D, #$042E, #$042F,
      #$0430, #$0431, #$0432, #$0433, #$0434, #$0435, #$0436, #$0437,
      #$0438, #$0439, #$043A, #$043B, #$043C, #$043D, #$043E, #$043F,
      #$2591, #$2592, #$2593, #$2502, #$2524, #$2561, #$2562, #$2556,
      #$2555, #$2563, #$2551, #$2557, #$255D, #$255C, #$255B, #$2510,
      #$2514, #$2534, #$252C, #$251C, #$2500, #$253C, #$255E, #$255F,
      #$255A, #$2554, #$2569, #$2566, #$2560, #$2550, #$256C, #$2567,
      #$2568, #$2564, #$2565, #$2559, #$2558, #$2552, #$2553, #$256B,
      #$256A, #$2518, #$250C, #$2588, #$2584, #$258C, #$2590, #$2580,
      #$0440, #$0441, #$0442, #$0443, #$0444, #$0445, #$0446, #$0447,
      #$0448, #$0449, #$044A, #$044B, #$044C, #$044D, #$044E, #$044F,
      #$0401, #$0451, #$0404, #$0454, #$0407, #$0457, #$040E, #$045E,
      #$00B0, #$2219, #$00B7, #$221A, #$2116, #$00A4, #$25A0, #$00A0);

function cp866Decode(Context: Pointer; InBuf: PChar; var InCnt: Cardinal; OutBuf: PWideChar;
                     var OutCnt: Cardinal): Integer; stdcall;
var
  I: Integer;
  cnt: Cardinal;
begin
  cnt := OutCnt;         // num of widechars
  if cnt > InCnt then
    cnt := InCnt;
  for I := 0 to cnt-1 do
  begin
    if InBuf[I] < #128 then
      OutBuf[I] := WideChar(ord(InBuf[I]))
    else
      OutBuf[I] := cp866table[InBuf[I]];
  end;
  Dec(InCnt, cnt);
  Dec(OutCnt, cnt);
  Result := cnt;
end;

function GetCP866Decoder(const AEncoding: string; out Decoder: TDecoder): Boolean; stdcall;
begin
// Most encodings typically have one or more alias names.
  if SameText(AEncoding, 'IBM866') or
     SameText(AEncoding, 'cp866') or
     SameText(AEncoding, '866') or
     SameText(AEncoding, 'csIBM866') then
  begin
    Decoder.Decode := @cp866Decode;
    Decoder.Cleanup := nil;
    Decoder.Context := nil;
    Result := True;
  end
  else
    Result := False;
end;

initialization
  RegisterDecoder(@GetCP866Decoder);
end.

See also