The Annoyance Filter.pdf - Fourmilab

52 BASE64 MIME DECODER ANNOYANCE-FILTER §52
52. Read the encoded input stream and return the next non-white space character. This code does
not verify whether characters it returns are valid within a base64 stream—that’s up to the caller to
determine once the character is returned.
〈 Get next significant character from input stream 52 〉 ≡
while (true ) {
c = −1;
while (ip < inputLine .length ( )) {
if (inputLine [ip] > ’␣’) {
c = inputLine [ip ++];
break;
}
ip ++;
}
if (c ≥ 0) {
break;
}
if (¬getNextEncodedLine ( )) {
break;
}
}
This code is used in section 51.
53. An end of file indication (due to encountering the MIME part separator sentinel) is valid only after
an even number of four character encoded sequences. Validate this and report any errors accordingly.
If an unexpected end of file is encountered, any incomplete encoded sequence is discarded.
〈 Check for end of file in base64 stream 53 〉 ≡
if (c ≡ EOF) {
if (i > 0) {
nDecodeErrors ++;
mf ⃗ reportParserDiagnostic("Unexpected␣end␣of␣file␣in␣Base64␣decoding.");
}
return −1;
}
This code is used in section 51.
54. Once we’ve decoded four characters from the input stream, we have four six-bit fields in the b
array. Now we extract, shift, and ∨ these fields together to form three 8 bit bytes. One subtlety arises
at the end of file. The last one or two characters of an encoded four character field may be replaced by
equal signs to indicate that the final field encodes only one or two source bytes. If this is the case, the
number of bytes placed onto the decodedBytes queue is reduced to the correct value.
〈 Assemble the decoded bits into bytes and place on decoded queue 54 〉 ≡
o[0] = (b[0] ≪ 2) | (b[1] ≫ 4);
o[1] = (b[1] ≪ 4) | (b[2] ≫ 2);
o[2] = (b[2] ≪ 6) | b[3];
j = a[2] ≡ ’=’ ? 1 : (a[3] ≡ ’=’ ? 2 : 3);
for (k = 0; k < j; k++) {
decodedBytes .push back (o[k]);
}
This code is used in section 50.