[pacman-dev] Version comparison algorithm.

Dave Reisner d at falconindy.com
Sat Mar 16 15:49:32 UTC 2019

On Sat, Mar 16, 2019, 10:02 Richard Dodd <richard.o.dodd at gmail.com> wrote:

> I'm writing to ask about the version comparison function `rpmvercmp` in
> `libalpm/version.cL83`. The algorithm is complex and I'm trying to
> understand its behavior. I want to hash using the package name and version
> as a key, and so I need a hash function where `i1 == i2 => hash(i1) ==
> hash(i2)` according to the version comparison operation. I'll describe how
> the algorithm behaves and then ask my questions.

Could you explain what your actual goal is? Having ordering in a hash
function sounds extremely odd. Generally, you care about associativity *or*
ordering, not both.

The algorithm works on a byte string and uses ascii comparison rules (no
> unicode).
>  - First, split the input up into blocks of *alpha*, *digit* or
> *non-alphanum*.
>  - For each pair of blocks
>     - If the types are different, then *non-alphanum* is newer than
> *numeric*, which is newer than *alpha*
>     - If the types are the same, then the rules are
>       - For *non-alphanum*, compare lengths, longer is newer, equal lengths
> are equal segments (so *--* and *::* are the same)
>       - For *alpha* just do a lexicographic comparison (so *b* is newer
> than *a* etc.)
>       - For *numeric*, do a numeric comparison. (this can be done by
> skipping leading zeros, then comparing length, then comparing
> lexicographically, to avoid overflow on integer conversion)
>   - If one input is longer than the other, and all sections so far have
> been equal, then if the next section of the longer is *alpha*, it is older,
> and if it is *numeric* it is newer. (so "1a" is older than "1", "a1" is
> newer than "a").
>   - If the inputs have the same number of sections that are all equal, then
> they are equal.
> Questions:
>  1. Is the algorithm correctly described here.
>  2. This should mean that if I hash length for *non-numeric*, the string
> with stripped zeros for *numeric*, and just the string for *alpha*, the has
> law should be upheld. Is this correct?
> Thanks
> Rich

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