The Annoyance Filter.pdf - Fourmilab

§71 ANNOYANCE-FILTER EUC DECODER 65
71. EUC decoder.
This decoder extracts logical characters from byte streams encoded in EUC encoding. In EUC, if a byte
in the input stream is in the range # A1– # FE and the subsequent byte in the range # 80– # FF, then the
variant fields encoded in the two bytes define the character code. A byte not within the range of the first
byte of a two byte character is interpreted as a single byte character with ASCII/ISO-8859 semantics.
〈 Class definitions 10 〉 +≡
class EUC MBCSdecoder : public MBCSdecoder {
public:
virtual string name (void)
{
return "EUC";
}
virtual int getNextDecodedChar (void); /∗ Get next decoded byte ∗/
};
72. Bytes are parsed from the input stream as follows. Any bytes with values within the range
#
A1– # FE denote the first byte of a two byte character, whose second byte must be within the range
#
80– # FF. Any violation of the constraints on the second byte indicates an invalid sequence. Characters
outside the range of initial characters are considered single byte codes. We return −1 when the end of
the encoded line is encountered.
〈 Class implementations 11 〉 +≡
int EUC MBCSdecoder ::getNextDecodedChar (void)
{
int c1 = getNextEncodedByte ( );
if ((c1 ≥ # A1) ∧ (c1 ≤ # FE)) {
int c2 = getNextEncodedByte ( );
if ((c2 ≥ # 80) ∧ (c2 ≤ # FF)) {
return (c1 ≪ 8) | c2 ;
}
if (c2 ≡ −1) {
ostringstream os ;
os ≪ name ( ) ≪ "_MBCSdecoder:␣Premature␣end␣of␣line␣in␣two␣byte␣character.";
reportDecoderDiagnostic(os );
return −1;
} /∗ Odds are that once we’ve encountered an invalid second byte, the balance of the
encoded line will be screwed up as well. To avoid such blithering, discard the line after
such an error. ∗/
discardLine ( );
ostringstream os ;
os ≪ name ( ) ≪ "_MBCSdecoder:␣Invalid␣second␣byte␣in␣two␣byte␣character:␣""0x" ≪
setiosflags (ios ::uppercase ) ≪ hex ≪ c1 ≪ "␣" ≪ "0x" ≪ c2 ≪ ".";
reportDecoderDiagnostic(os );
return c1 ;
}
return c1 ;
}