The Annoyance Filter.pdf - Fourmilab

54 BASE64 MIME DECODER ANNOYANCE-FILTER §56
56. The initialiseDecodingTable method fills the binary encoding table with the characters the 6 bit
values are mapped into. The curious and disparate sequences used to fill this table permit this code to
work both on ASCII and EBCDIC systems.
In EBCDIC systems character codes for letters are not consecutive; the initialisation must be split to
accommodate the EBCDIC consecutive letters:
A–I J–R S–Z a–i j–r s–z
This code works on ASCII as well as EBCDIC systems.
〈 Class implementations 11 〉 +≡
void base64MIMEdecoder ::initialiseDecodingTable (void)
{
int i;
for (i = 0; i < 255; i++) {
dtable [i] = # 80;
}
for (i = ’A’; i ≤ ’I’; i++) {
dtable [i] = 0 + (i − ’A’);
}
for (i = ’J’; i ≤ ’R’; i++) {
dtable [i] = 9 + (i − ’J’);
}
for (i = ’S’; i ≤ ’Z’; i++) {
dtable [i] = 18 + (i − ’S’);
}
for (i = ’a’; i ≤ ’i’; i++) {
dtable [i] = 26 + (i − ’a’);
}
for (i = ’j’; i ≤ ’r’; i++) {
dtable [i] = 35 + (i − ’j’);
}
for (i = ’s’; i ≤ ’z’; i++) {
dtable [i] = 44 + (i − ’s’);
}
for (i = ’0’; i ≤ ’9’; i++) {
dtable [i] = 52 + (i − ’0’);
}
#define CI(x)static cast〈int〉 (x)
dtable [CI(’+’)] = 62;
dtable [CI(’/’)] = 63;
dtable [CI(’=’)] = 0;
#undef CI
}