zlib, deflate: How much memory to allocate?

Question

I am using zlib to compress a stream of text data. The text data comes in chunks, and for each chunk, deflate() is called, with flush set to Z_NO_FLUSH

Answer 1

While looking at the sources for a hint, I fell over

/* =========================================================================
 * Flush as much pending output as possible. All deflate() output goes
 * through this function so some applications may wish to modify it
 * to avoid allocating a large strm->next_out buffer and copying into it.
 * (See also read_buf()).
 */
local void flush_pending(strm)
    z_streamp strm;
{
    unsigned len = strm->state->pending;
...

tracing the use of void flush_pending() throughout deflate() shows, that an upper bound on the needed output buffer in the middle of the stream is

strm->state->pending + deflateBound(strm, strm->avail_in)

the first part accounts for data still in the pipe from previous calls to deflate(), the second part accounts for the not-yet processed data of length avail_in.