The Annoyance Filter.pdf - Fourmilab

§147 ANNOYANCE-FILTER MAIL FOLDER 127
147. Message header lines may contain sequences of characters encoded in Quoted−Printable or
Base64 form (since mail headers must not contain 8 bit characters). To better extract words from these
lines, we test for such subsequences and replace them with the encoded text. Due to the fact that, in
the fullness of time, this code will be fed every conceivable kind of nonconforming trash, it must be
completely bulletproof. The flailing around with p4 protects against falling into a loop when decoding
a sequence fails.
〈 Check for encoded header line and decode 147 〉 ≡
if (inHeader ) {
string sc = s;
string ::size type p, p1 , p2 , p3 , p4 ;
char etype ;
unsigned int ndecodes = 0;
string charset ;
stringCanonicalise (sc);
p4 = 0;
while (((p = sc.find ("=?", p4 )) ≠ string ::npos )) {
p4 = p + 2;
if (((p1 = sc.find ("?q?", p4 )) ≠ string ::npos ) ∨ ((p1 = sc.find ("?b?", p4 )) ≠ string ::npos ))
{
charset = sc.substr (p4 , p1 − p4 );
etype = sc[p1 + 1];
p4 = p1 + 3;
if ((p2 = sc.find ("?=", p4 )) ≠ string ::npos ) {
p1 += 3;
p3 = p2 − p1 ;
string drt ;
if (etype ≡ ’q’) {
drt = quotedPrintableMIMEdecoder ::decodeEscapedText (sc.substr (p1 , p3 ), this);
}
else {
assert(etype ≡ ’b’);
drt = base64MIMEdecoder ::decodeEscapedText (sc.substr (p1 , p3 ), this);
}
〈 Interpret header quoted string if character set known 148 〉;
sc.replace (p, (p2 − p) + 2, drt );
p4 = p + drt .length ( );
ndecodes ++;
}
}
}
if (ndecodes > 0) {
s = sc;
}
}
This code is cited in section 256.
This code is used in section 141.