[FFmpeg-devel] [PATCH 4/8] avformat/flvenc: Use array instead of linked list for index

[Andreas Rheinhardt]

Tomas Härdin:
> tis 2022-07-05 klockan 22:26 +0200 skrev Andreas Rheinhardt:
>> Using a linked list had very much overhead (the pointer to the next
>> entry increased the size of the index entry struct from 16 to 24
>> bytes,
>> not to mention the overhead of having separate allocations), so it is
>> better to (re)allocate a continuous array for the index.
>> av_fast_realloc_array() is used for this purpose, in order not to
>> reallocate the array for each entry. It's feature of allowing to
>> restrict the number of elements of the array is put to good use here:
>> Each entry will lead to 18 bytes being written and the array is
>> contained in an element whose size field has a length of three bytes,
>> so that more than (2^24 - 1) / 18 entries make no sense.
>>
> 
>> +    /* The filepositions array is part of a metadata element whose
>> size field
>> +     * is three bytes long; so bound the number of filepositions in
>> order not
>> +     * to allocate more than could ever be written. */
>> +    int ret = av_fast_realloc_array(&flv->filepositions,
>> +                                    &flv->filepositions_allocated,
>> +                                    flv->filepositions_count + 1,
>> +                                    (((1 << 24) - 1) - 10) /
> 
> Why -10? I see it bumps max_nb down from 932067 to 932066, but it make
> no difference keeping below 1<<24
> 

This was designed to make the result of the following computation
(performed in shift_data) to always fit into three bytes:

metadata_size = flv->filepositions_count * 9 * 2 + 10

But now upon rereading I see that there is some change added to this
unconditionally after that. I will also account for that.
(Notice that it is not intended to account for flv->metadata_totalsize
which is added later.)

- Andreas