Hi,
Jon Burgess wrote:
>> +bool cVideoRepacker::ScanDataForStartCodeFast(const uchar *&Data,
>> const uchar *Limit)
>> +{
>> + Limit--;
>> +
>> + while (Data < Limit) {
>> + if (*Data > 0x01)
>> + Data += 3;
>> + else if (*Data == 0x00)
>> + Data++;
>> + else if (Data[-2] != 0x00 || Data[-1] != 0x00)
>> + Data += 3;
>> + else {
>> + scanner = 0x00000100 | *++Data;
>> + return true;
>> + }
>> + }
>
> Did you consider using memchr()? e.g. something like
> ...
> while (Data < Limit) {
> Data = memchr(Data, 0x01, Limit - Data);
> if (Data == NULL)
> break;
> if (Data[-2] != 0x00 || Data[-1] != 0x00)
> Data += 3;
> ...
>
> It makes no noticeable difference on my AMD64 machine (<1%), but maybe
> it is worth trying on your EPIA?
I don't think that it is worth a try as it tests every byte while the
above code tests most of the time only every third byte.
Consider the following data:
hh ii jj kk [ 00 00 01 01 ] mm nn oo 01 01 pp 01
^^^^^^^^^^^^^^^^^^^ ++++++++++^^^ ^ ^^
6 4 0 1
which will give you the above distance between consecutive 01 which are
not part of a startcode (i. e. first slice start code [ 00 00 01 01 ])
or immediately following a startcode (the area marked with +++++). ++ or
^^ indicate bytes which have to be considered for the distance.
For the above case, a statistical analysis will give this numbers:
distance: 0, count: 1
distance: 1, count: 1
distance: 2, count: 0
distance: 3, count: 0
distance: 4, count: 1
distance: 5, count: 0
distance: 6, count: 1
distance: >, count: 0
Now consider real data, like 001.vdr of "Wetten, dass ..?" which was
broadcast last Saturday on ZDF:
distance: 0, count: 75248
distance: 1, count: 519949
distance: 2, count: 316632
distance: 3, count: 331874
distance: 4, count: 381855
distance: 5, count: 367280
distance: 6, count: 370620
distance: 7, count: 369649
distance: 8, count: 405861
distance: 9, count: 360555
distance: 10, count: 332593
distance: 11, count: 366192
distance: 12, count: 319692
distance: 13, count: 313795
distance: 14, count: 309567
distance: 15, count: 305986
distance: 16, count: 297510
distance: 17, count: 291607
distance: 18, count: 286045
distance: >, count: 18794252
As you can easily see, it's a waste of time to test every byte (=
distance 0) for 01 as it is very unlikely to find one.
Bye.
--
Dipl.-Inform. (FH) Reinhard Nissl
mailto:rnissl@xxxxxx
