/*
* The following was provided by Ken Thompson of AT&T Bell Laboratories,
* <[email protected]>, on Tue, 8 Sep 92 03:22:07 EDT, to the X/Open
* Joint Internationalization Group.  Some minor formatting changes have
* been made by Glenn Adams, <[email protected]>.
*
* -------------------------------------------------------------------------
*  File System Safe Universal Character Set Transformation Format (FSS-UTF)
* -------------------------------------------------------------------------
*
* With the approval of ISO/IEC 10646 (Unicode) as an international
* standard and the anticipated wide spread use of this universal coded
* character set (UCS), it is necessary for historically ASCII based
* operating systems to devise ways to cope with representation and
* handling of the large number of characters that are possible to be
* encoded by this new standard.
*
* There are several challenges presented by UCS which must be dealt with
* by historical operating systems and the C-language programming
* environment.  The most significant of these challenges is the encoding
* scheme used by UCS. More precisely, the challenge is the marrying of
* the UCS standard with existing programming languages and existing
* operating systems and utilities.
*
* The challenges of the programming languages and the UCS standard are
* being dealt with by other activities in the industry.  However, we are
* still faced with the handling of UCS by historical operating systems
* and utilities.  Prominent among the operating system UCS handling
* concerns is the representation of the data within the file system.  An
* underlying assumption is that there is an absolute requirement to
* maintain the existing operating system software investment while at
* the same time taking advantage of the use the large number of
* characters provided by the UCS.
*
* UCS provides the capability to encode multi-lingual text within a
* single coded character set.  However, UCS and its UTF variant do not
* protect null bytes and/or the ASCII slash ("/") making these character
* encodings incompatible with existing Unix implementations.  The
* following proposal provides a Unix compatible transformation format of
* UCS such that Unix systems can support multi-lingual text in a single
* encoding.  This transformation format encoding is intended to be used
* as a file code.  This transformation format encoding of UCS is
* intended as an intermediate step towards full UCS support.  However,
* since nearly all Unix implementations face the same obstacles in
* supporting UCS, this proposal is intended to provide a common and
* compatible encoding during this transition stage.
*
* Goal/Objective
* --------------
*
* With the assumption that most, if not all, of the issues surrounding
* the handling and storing of UCS in historical operating system file
* systems are understood, the objective is to define a UCS
* transformation format which also meets the requirement of being usable
* on a historical operating system file system in a non-disruptive
* manner.  The intent is that UCS will be the process code for the
* transformation format, which is usable as a file code.
*
* Criteria for the Transformation Format
* --------------------------------------
*
* Below are the guidelines that were used in defining the UCS
* transformation format:
*
*      1) Compatibility with historical file systems:
*
*      Historical file systems disallow the null byte and the ASCII
*      slash character as a part of the file name.
*
*      2) Compatibility with existing programs:
*
*      The existing model for multibyte processing is that ASCII does
*      not occur anywhere in a multibyte encoding.  There should be
*      no ASCII code values for any part of a transformation format
*      representation of a character that was not in the ASCII
*      character set in the UCS representation of the character.
*
*      3) Ease of conversion from/to UCS.
*
*      4) The first byte should indicate the number of bytes to
*      follow in a multibyte sequence.
*
*      5) The transformation format should not be extravagant in
*      terms of number of bytes used for encoding.
*
*      6) It should be possible to find the start of a character
*      efficiently starting from an arbitrary location in a byte
*      stream.
*
* Proposed FSS-UTF
* ----------------
*
* The proposed UCS transformation format encodes UCS values in the range
* [0,0x7fffffff] using multibyte characters of lengths 1, 2, 3, 4, 5,
* and 6 bytes.  For all encodings of more than one byte, the initial
* byte determines the number of bytes used and the high-order bit in
* each byte is set.  Every byte that does not start 10xxxxxx is the
* start of a UCS character sequence.
*
* An easy way to remember this transformation format is to note that the
* number of high-order 1's in the first byte signifies the number of
* bytes in the multibyte character:
*
* Bits  Hex Min  Hex Max  Byte Sequence in Binary
*   7  00000000 0000007f 0vvvvvvv
*  11  00000080 000007FF 110vvvvv 10vvvvvv
*  16  00000800 0000FFFF 1110vvvv 10vvvvvv 10vvvvvv
*  21  00010000 001FFFFF 11110vvv 10vvvvvv 10vvvvvv 10vvvvvv
*  26  00200000 03FFFFFF 111110vv 10vvvvvv 10vvvvvv 10vvvvvv 10vvvvvv
*  31  04000000 7FFFFFFF 1111110v 10vvvvvv 10vvvvvv 10vvvvvv 10vvvvvv 10vvvvvv
*
* The UCS value is just the concatenation of the v bits in the multibyte
* encoding.  When there are multiple ways to encode a value, for example
* UCS 0, only the shortest encoding is legal.
*
* Below are sample implementations of the C standard wctomb() and
* mbtowc() functions which demonstrate the algorithms for converting
* from UCS to the transformation format and converting from the
* transformation format to UCS. The sample implementations include error
* checks, some of which may not be necessary for conformance:
*
*/

typedef struct
{
 int     cmask;
 int     cval;
 int     shift;
 long    lmask;
 long    lval;
} Tab;

static Tab tab[] =
{
 0x80,  0x00,   0*6,    0x7F,           0,              /* 1 byte sequence */
 0xE0,  0xC0,   1*6,    0x7FF,          0x80,           /* 2 byte sequence */
 0xF0,  0xE0,   2*6,    0xFFFF,         0x800,          /* 3 byte sequence */
 0xF8,  0xF0,   3*6,    0x1FFFFF,       0x10000,        /* 4 byte sequence */
 0xFC,  0xF8,   4*6,    0x3FFFFFF,      0x200000,       /* 5 byte sequence */
 0xFE,  0xFC,   5*6,    0x7FFFFFFF,     0x4000000,      /* 6 byte sequence */
 0,                                                     /* end of table    */
};

int
 mbtowc ( wchar_t *p, char *s, size_t n )
{
 long l;
 int c0, c, nc;
 Tab *t;

 if ( s == 0 )
   return 0;

 nc = 0;
 if ( n <= nc )
   return -1;
 c0 = *s & 0xff;
 l = c0;
 for ( t = tab; t->cmask; t++ ) {
   nc++;
   if ( ( c0 & t->cmask ) == t->cval ) {
     l &= t->lmask;
     if ( l < t->lval )
       return -1;
     *p = l;
     return nc;
   }
   if ( n <= nc )
     return -1;
   s++;
   c = ( *s ^ 0x80 ) & 0xFF;
   if ( c & 0xC0 )
     return -1;
   l = ( l << 6 ) | c;
 }
 return -1;
}

int
 wctomb ( char *s, wchar_t wc )
{
 long l;
 int c, nc;
 Tab *t;

 if (s == 0 )
   return 0;

 l = wc;
 nc = 0;
 for ( t=tab; t->cmask; t++ ) {
   nc++;
   if ( l <= t->lmask ) {
     c = t->shift;
     *s = t->cval | ( l >> c );
     while ( c > 0 ) {
       c -= 6;
       s++;
       *s = 0x80 | ( ( l >> c ) & 0x3F );
     }
     return nc;
   }
 }
 return -1;
}

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