gzip partial modification and re-compression

Question

I am unfamiliar with compression algorithms. Is it possible with zlib or some other library to decompress, modify and recompress only the beginning of a gzip stream and then concatenate it with the compressed remainder of the stream? This would be done in a case where, for example, I need to modify the first bytes of user data (not headers) of a 10GB gzip file so as to avoid decompressing and recompressing the entire file.

Answer 1

No. Compression will generally make use of the preceding data in compressing the subsequent data. So you can't change the preceding data without recompressing the remaining data.

An exception would be if there were breakpoints put in the compressed data originally that reset the history at each breakpoint. In zlib this is accomplished with Z_FULL_FLUSH during compression.