The Annoyance Filter.pdf - Fourmilab

§75 ANNOYANCE-FILTER SHIFT-JIS DECODER 67
75. Shift-JIS decoder.
Shift-JIS is used to encode Japanese characters on MS-DOS, Windows, and the Macintosh (which adds
four additional one-byte characters which we support here). The encoding uses code points # 21– # 7E
for ASCII/JIS-Roman single byte characters, code points # A1– # DF for single byte hald width katakana,
plus two-byte characters introduced by first bytes in the ranges # 81– # 9F, # E0– # EF, and, for user-defined
characters, # F0– # FC. The second byte of a valid two-byte character will always be in one of the ranges
#
40– # 7E and # 80– # FC.
〈 Class definitions 10 〉 +≡
class Shift JIS MBCSdecoder : public MBCSdecoder {
protected:
string pending ;
public:
Shift JIS MBCSdecoder( )
: pending ("") { }
virtual ∼Shift JIS MBCSdecoder( )
{ }
virtual string name (void)
{
return "Shift_JIS";
}
virtual int getNextDecodedChar (void); /∗ Get next decoded byte ∗/
};
76. Decode the next logical character. We return −1 when the end of the encoded line is encountered.
An invalid second byte of a two byte character terminates processing of the line, as it’s likely to be
gibberish from then on.
〈 Class implementations 11 〉 +≡
int Shift JIS MBCSdecoder ::getNextDecodedChar (void)
{
〈 Check for pending characters and return if so 78 〉;
}
int c1 = getNextEncodedByte ( );
if (c1 ≥ 0) {
〈 Check for Shift-JIS two byte character and assemble as required 77 〉;
〈 Check for Macintosh-specific single byte characters and translate 79 〉;
}
return c1 ;