>>I am trying to understand why ZLIB is being used that way. Here is what
>>gives better results on a continuous reliable stream of data:
>>
>>1) You create a z_stream for sending, and another z_stream for
>>receiving.
>>
>>2) You call deflateInit() and inflateInit() on them, respectively,
>>when the communication is established.
>>
>>3) For each data block you send, you call deflate() on it. For
>>each data block you receive, you call inflate() on it.
>
>You then die from the latency in the inflation/deflation routines. You
>have to flush the deflater for each block, and depending on how you do
>it your performance is the same as deflating each block separately.
I think you're totally missing his point. Yes, you have to flush (sync
really) the compressor at the end of each block to ensure that all the
compressed data comes out the other side when that block is received. But
this doesn't mean you have to purge the dictionary!
>>But by far, the main advantage is that you can achieve good compression
>>even for very small blocks of data. The "dictionary" window stays open
>>for the whole communication stream, making it possible to compress a
>>message by reference to a number of previously sent messages.
>If you do a Z_SYNC_FLUSH (iirc), it blows the dictionary. This is
>intentional, since you can restart the inflater at every SYNC mark.
No, this is not true. A Z_SYNC_FLUSH does not permit you to restart the
inflater at every SYNC mark. That is not its purpose.
>I thought there was also a mode to flush the buffer (including any
>necessary padding for partial bytes) but not blowing the dictionary, but
>I'm not sure how portable it is.
That is what Z_SYNC_FLUSH does. The usual strategy when processing a block
is to use Z_SYNC_FLUSH if the input queue is empty. Otherwise, don't bother
to flush because you know more data is coming shortly.
DS
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