Concatenate multiple zlib compressed data streams into a single stream efficiently

Since you don't mind venturing into C, you can start by looking at the code for gzjoin.

Note, the gzjoin code has to decompress to find the parts that have to change when merged, but it doesn't have to recompress. That's not too bad because decompression is typically faster than compression.

In addition to gzjoin which requires decompression of the first deflate stream, you can take a look at gzlog.h and gzlog.c, which efficiently appends short strings to a gzip file without having to decompress the deflate stream each time. (It can be easily modified to operate on zlib-wrapped deflate data instead of gzip-wrapped deflate data.) You would use this approach if you are in control of the creation of the first deflate stream. If you are not creating the first deflate stream, then you would have to use the approach of gzjoin which requires decompression.

None of the approaches require recompression.