How does .7z compression work?

[molotovwars]

Some rom sites host the complete set for each game, meaning that in one compression all sorts of dumps are included, such as the same game, but in differenet languages, or with a translation, or different version of the game. For example a sonic package would include the english version, japanese version, and additional hacks like metal sonic or mighty the armadillo.

The compressed file is still only about 1MB, and when I unpack it sometimes as many as 18 2MB files, all from a 1MB compression.

So what I'm curious is how does the .7z compression work, does it only work for files vary similar in nature? Why isn't it more widely used? I've only really seen it associated with these rom sets.

[SenoreWoodrow]

I didn't know http://en.wikipedia.org/wiki/7-Zip, but now I do. =)

[dgm08]

Would be incredable if this was implemented into the emulators. Seems easy to as 7zip is open source.

[Enclave]

Originally Posted by dgm08

Would be incredable if this was implemented into the emulators. Seems easy to as 7zip is open source.

I think there are PC emulators which support this (I am almost certain ZSNES does) so it isn't far fetched to think this might be supported in PSP emulators in the future.

[AZImmortal]

molotovwars: you must've acquired a "good merged" set, which typically uses 7zip compression and groups roms together in the way you described. 7zip compression itself typically has higher compression ratios than other compression methods, but as you correctly pointed out, most of the compression benefits from a good merged set comes from the fact that similar roms are grouped together. redundant data compresses well, so good merged sets try to achieve the highest compression possible while still making it easy to find the roms you want (by grouping similar roms together).