The Annoyance Filter.pdf - Fourmilab

88 FLASH STREAM DECODER ANNOYANCE-FILTER §109
109. Flash files include quantities packed into bit fields, the width of some of which are specified
by other fields in the file. The following methods decode these packed fields. Call initBits to initialise
decoding of a bit field which begins in the next (as yet unread) byte. Then call getBits or getSignedBits
to return an n bit field without or with sign extension respectively.
〈 Class implementations 11 〉 +≡
void flashStream ::initBits (void)
{ /∗ Reset the bit position and buffer. ∗/
bitPos = 0;
bitBuf = 0;
} /∗ Get n bits from the stream. ∗/
unsigned int flashStream ::getBits (int n)
{
unsigned int v = 0;
while (true ) {
int s = n − bitPos ;
if (s > 0) { /∗ Consume the entire buffer ∗/
v |= bitBuf ≪ s;
n −= bitPos ; /∗ Get the next buffer ∗/
bitBuf = get8 ( );
bitPos = 8;
}
else { /∗ Consume a portion of the buffer ∗/
v |= bitBuf ≫ −s;
bitPos −= n;
bitBuf &= # FF ≫ (8 − bitPos ); /∗ mask off the consumed bits ∗/
return v;
}
}
} /∗ Get n bits from the string with sign extension. ∗/
int flashStream ::getSignedBits (const int n)
{
signed int v = static cast〈int〉(getBits (n)); /∗ Is the number negative? ∗/
}
if (v & (1 ≪ (n − 1))) { /∗ Yes. Extend the sign. ∗/
v |= −1 ≪ n;
}
return v;