Problem:

A common pattern is:

GUARD(s2n_stuffer_skip_write(stuffer, bytes_to_write));
uint8_t* ptr = suffer->blob.data + stuffer->write_cursor - bytes_to_write;

which could be simplified.

Solution:

*ptr could be an *out parameter to s2n_stuffer_skip_write

• Does this change what S2N sends over the wire? No.
• Does this change any public APIs? No.

• Move transparency testing for grayscale images out of the decoding loop, move it to trns_row()
• Move format conversion code out of the decoding loop so it can be reused for encoding
• Add more zlib-specific error codes - compare z_stream.msg against known strings