The Annoyance Filter.pdf - Fourmilab

84 FLASH STREAM DECODER ANNOYANCE-FILTER §103
103. Read the header for the next tag. Each tag begins with a 16 bit field which contains 10 bits
of tag identifier and a 6 bit field specifying the number of argument bytes which follow. For tags with
arguments of 0 to 62 bytes, the 6 bit field is the data length. For longer tags, the 6 bit length field is set
of # 3F and a 32 bit quantity giving the tag data length immediately follows. Regardless of the format
of the tag header, we store the tag type in tType and the number of data bytes in tDataLen .
〈 Class implementations 11 〉 +≡
bool flashStream ::nextTag (void)
{
unsigned short s = get16 ( );
unsigned long l;
}
if (isOK ( )) {
tType = static cast〈tagType 〉(s ≫ 6);
l = s & # 3F;
if (l ≡ # 3F) {
l = get32 ( ); /∗ Long tag; read 32 bit length ∗/
}
if (isOK ( )) {
tDataLen = l;
return tType ≠ stagEnd ;
}
} /∗ In case of error dummy up end tag for sloppy callers ∗/
tType = stagEnd ;
tDataLen = 0;
return false ;
104. Having read the tag header, if we decide we aren’t interested in the tag, we can simply skip
past tDataLen argument bytes to advance to the next tag header; ignoreTag performs this. If you’ve
read into the tag data before deciding you wish to skip the tag, call ignoreTag with the lookedAhead
argument specifying how many bytes of the tag data you’ve already read.
〈 Class implementations 11 〉 +≡
void flashStream ::ignoreTag (unsigned int lookedAhead )
{
if (isOK ( )) {
/∗ assert(lookedAhead ¡= tDataLen); // (This assertion will fail if −−bsdfolder is set) ∗/
for (unsigned int i = lookedAhead ; isOK ( ) ∧ (i < tDataLen ); i++) {
get8 ( );
}
}
}