The Annoyance Filter.pdf - Fourmilab

§50 ANNOYANCE-FILTER BASE64 MIME DECODER 51
50. This is the heart of the base64 decoder. It reads the next four significant (non-white space)
characters from the input stream, extracts the 6 bits encoded by each, and assembles the bits into
three 8 bit bytes which are added to the decodedBytes queue. Although the current decoder always
immediately empties the queue, in principal any sequence of the encoded content up to its entire length
may be decoded by repeated invocations of this code.
〈 Refill decoded bytes queue from input stream 50 〉 ≡
unsigned char a[4], b[4], o[3];
int j, k;
〈 Decode next four characters from input stream 51 〉;
〈 Assemble the decoded bits into bytes and place on decoded queue 54 〉;
This code is used in section 49.
51. Read the next four non-blank bytes from the input stream, checking for end of file, and place
their decoded 6 bit values into the array b. We save the original encoded characters in array a to permit
testing them for the special “=” sentinel which denotes short sequences at the end of file.
〈 Decode next four characters from input stream 51 〉 ≡
for (int i = 0; i < 4; i++) {
int c;
}
〈 Get next significant character from input stream 52 〉;
〈 Check for end of file in base64 stream 53 〉;
if (dtable [c] & # 80) {
nDecodeErrors ++;
ostringstream os ;
os ≪ "Illegal␣character␣’" ≪ c ≪ "’␣in␣Base64␣input␣stream.";
mf ⃗ reportParserDiagnostic(os .str ( )); /∗ Ignoring errors: discard invalid character. ∗/
i−−;
continue;
}
a[i] = (unsigned char) c;
b[i] = dtable [c];
This code is used in section 50.