Someone please detect if there's a BOM before the plane takes off!

wait...which plane? BMP? SMP? SIP? TIP? SSP?

Sorry, I have nothing else useful to contribute here, though I'm a little surprised this problem isn't solved already...?!?

See http://recycledknowledge.blogspot.com/2005/07/hello-i-am-xml-encoding-sniffer.html for a formal English description of what you have to do to play in the Appendix F leagues.

Mike, you're the only person I know of who pronounces "bee-oh-em" as a word. Cakemakers and codebreakers have every right to say "bomb/bombe" but not we I18Ners.

Here's my approach:

enum BOM {

BOM_NONE,

BOM_UTF8,

BOM_UTF16LE,

BOM_UTF16BE,

BOM_UTF32BE,

BOM_UTF32LE,

};

HRESULT BOMFromStream(Byte pbBytes[], UINT cbLength, BOM *pBOM) {

if (NULL == pbBytes || NULL == pBOM) {

return E_POINTER;

}

// need at least two bytes for UTF16 BOMs

if (cbLength >= 2) {

if (0xFE == pbBytes[0] && 0xFF == pbBytes[1]) {

*pBOM = BOM_UTF16BE;

return S_OK;

}

if (0xFF == pbBytes[0] && 0xFE == pbBytes[1]) {

*pBOM = BOM_UTF16LE;

return S_OK;

}

}

// need at least three bytes for UTF8 BOM

if (

cbLength >= 3 &&

0xEF == pbBytes[0] &&

0xBB == pbBytes[1] &&

0xBF == pbBytes[2]

) {

*pBOM = BOM_UTF8;

return S_OK;

}

// need at least four bytes for UTF32 BOMs

if (cbLength >= 4) {

if (

0 == pbBytes[0] &&

0 == pbBytes[1] &&

0xFE == pbBytes[2] &&

0xFF == pbBytes[3]

) {

*pBOM = BOM_UTF32BE;

return S_OK;

}

if (

0xFF == pbBytes[0] &&

0xFE == pbBytes[1] &&

0 == pbBytes[2] &&

0 == pbBytes[3]

) {

*pBOM = BOM_UTF32LE;

return S_OK;

}

}

// if we made it this far there's no recognizable BOM

*pBOM = BOM_NONE;

return S_OK;

}

Possible future additional features: sanity check UTF16 byte stream length is even, UTF32 is divisible by 4; advance byte stream by length of BOM.

Michael Kaplan posted a small challenge on his blog to write some small code to find out the Byte Order Marker or BOM in the start of a file. So I kicked up the C# compiler and wrote this little bit of code:public enum Encoding{ Unknown = 0, BomBigEnd

I've had a quick stab in C# I've put a longer version that does appendix F (also a port to C for byte counting purposes) on my blog here:

http://www.ibbotson.co.uk/peteri/index.php?/archives/120-Finding-the-BOM.html

public enum Encoding

{

   Unknown = 0, BomBigEndianUcs4, BomUcs4, BomUtf8,

   BomUtf16, BomBigEndianUtf16

}

// We use Bit 3 as a end of data marker, true means end

// bit 5 happens to be same value as bit 3

private static byte[] matchData =

   {

       0x00,0x00,0xF6,0xFF,    //  0- 00 00 FE FF Bom UCS4 Big endian

       0xF7,0xF6,0x00,0x08,    //  4- FF FE 00 00 Bom UCS4 Little endian

       0xE7,0xB3,0xBF,         //  8- EF BB BF    Bom UTF8

       0xF7,0xFE,              // 12 - FF FE      Bom UTF16 Little endian

       0xF6,0xFF               // 14 - FE FF      Bom UTF16 Big endian

   };

public static Encoding DetectType(byte[] data)

{

   int i = 0;

   int offset = 0;

   Encoding currentEncoding = Encoding.BomBigEndianUcs4;

   while (i < matchData.Length)

   {

       byte compare = (byte)((matchData[i] & 0xf7) | ((matchData[i] & 0x20) >> 2));

       if ((offset >= data.Length) || (data[offset] != compare))

       {

           offset = 0;

           while ((matchData[i] & 0x08) == 0) i++;

           currentEncoding++;

       }

       else

       {

           if ((matchData[i] & 0x08) == 0x08) return currentEncoding;

           offset++;

       }

       i++;

   }

Ok, everyone seems to be just testing for the bytes in order. I realise that I'm posting late, so may have to give up the points race, but here's my version. Auto-calculates the BOM based on endianness and wchar size and compares the char* against that. Except for UTF-8. I gave up on that (its midnight here & I'm going to bed now).

#include <string>
#include <cstring>

// Same-endian: feff
// Different-endian: fffe
enum endianness {
  be = -1,
  le = 1
};

bool compare_bom_string(int sizeof_wchar, endianness end, const char* data)
{
  std::string bom(sizeof_wchar, 0);
  int pos = (end==be?3:0);

  bom[pos] = '\xFF';
  bom[pos+end] = '\xFE';

  return !memcmp((void*)data, (void*)bom.c_str(), sizeof_wchar);
}

struct bom {
  bom(int i, endianness e) : sizeof_wchar(i), end(e) { }
  int sizeof_wchar;
  endianness end;
};

int wchar_sizes[] = { 4, 2 };
endianness ends[] = { be, le };

struct ex {
  ex(const char*m) : msg(m) { }
  const char * what() { return msg; }

  const char *msg;

};

bom sniff(const char* data)
{
  for (int i = 0; i < sizeof wchar_sizes; ++i)
     for (int j = 0; j < sizeof ends; ++j)
        if (compare_bom_string(wchar_sizes[i], ends[j], data)) return bom(wchar_sizes[i], ends[j]);

  // Just got lazy
  const char* utf_8 = "\xEF\xBB\xBF";

  if (!memcmp((void*)data, (void*)utf_8, 3)) return bom(1,le);

  // Just got lazier
  throw ex("Whoops");;
}

import Maybe
import List

detectBOM s = snd . fromJust $ find ((flip isPrefixOf) s . fst) byteOrderMarks
   where byteOrderMarks = [("\xef\xbb\xbf","UTF-8"),
                           ("\x00\x00\xfe\xff","UTF-32BE"),
                           ("\xff\xfe\x00\x00","UTF-32LE"),
                           ("\xfe\xff","UTF-16BE"),
                           ("\xff\xfe","UTF-16LE"),
                           ("\x2b\x2f\x76\x38","UTF-7"),
                           ("\x2b\x2f\x76\x39","UTF-7"),
                           ("\x2b\x2f\x76\x2b","UTF-7"),
                           ("\x2b\x2f\x76\x2f","UTF-7"),
                           ("\xf7\x64\x4c","UTF-1"),
                           ("\xdd\x73\x66\x73","UTF-EBCDIC"),
                           ("\x0e\xfe\xff","SCSU"),
                           ("\xfb\xee\x28","BOCU-1"),
                           ("\x84\x31\x95\x33","GB18030"),
                           ("","NO BOM")]