Re: Fw: Curiosity

> On 2021-12-18 at 00:15:59, João Victor Bonfim wrote:
>
> > > I suspect that for most algorithms and their implementations, this would
> > >
> > > not result in repeatable "recompressed" results. Thus the checked-out
> > >
> > > files might be different every time you checked them out. :(
> >
> > How or why?
> >
> > Sincere question.
>
> A lossless compression algorithm has to produce an encoded value that,
>
> when decoded, must produce the original input. Ideally, it will also
>
> reduce the file size of the original input. Beyond that, there's a
>
> great deal of freedom to implement that.
>
> Just taking Deflate, which is used in zlib and gzip, as an example,
>
> there are different compression settings that control the size of the
>
> window to use that affect compression speed, quality of compression
>
> (resulting size), and memory usage. One might prefer using gzip -1 to
>
> get better performance or use less memory, or gzip -9 to reduce the file
>
> size as much as possible.
>
> Even when the same settings are used, the technique used can vary
>
> between versions of the software. For example, GitHub effectively uses
>
> git archive to generate archives, and one time when they upgraded their
>
> servers, the compression changed in the tarballs and zip files, and
>
> everybody who was relying on the archives being bit-for-bit identical[0]
>
> had a problem.
>
> So it would be nearly impossible to produce bit-for-bit repeatable
>
> results without specifying a specific, hard-coded implementation, and
>
> even in that case, the behavior might need to change for security
>
> reasons, so it would end up being difficult to achieve.
>
> [0] Neither Git nor GitHub provides this guarantee, so please do not
>
> make this mistake. If you need a fixed bit-for-bit tarball, save it as
>
> a release artifact.
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>
> brian m. carlson (he/him or they/them)
>
> Toronto, Ontario, CA