Re: [hash2] Formal Review Begins

From: Ruben Perez (rubenperez038_at_[hidden])
Date: 2024-12-13 14:31:16

On Fri, 13 Dec 2024, 11:22 Peter Dimov via Boost, <boost_at_[hidden]>
wrote:

> Samuel Neves wrote:
> > One of the things that puzzles me about this library is its intended
> purpose.
> > "Types don't know #" was clearly about hash types for use in hash tables,
> > which explained why keying the hashes was mandatory. But it is unclear
> to me
> > why MD5, SHA-1, and other cryptographic hashes are present here; they are
> > way too inefficient to be of much use in hash tables, so there is little
> reason to
> > include them. If, on the other hand, this is meant as a more general
> hashing
> > library, I have strong objections as detailed below.
>
> There exist intermediate use cases between "key in a hash table" and
> "cryptographically secure hashing of a binary blob" that are also served by
> the library. Some of them are:
>
> - if you have a program with some program state S, represented by a C++
> object,
>   which the user can save to disk by means of e.g. Ctrl+S, you often want
> to know
>   whether the current state has been saved or not (to display an
> indicator, or to
>   decide whether to autosave on exit.) This is implemented by keeping a
> hash of
>   the last save, and comparing the current hash to the last saved one.
>
> - if you have something like Boost.Compute, which needs to compile shaders,
>   you can keep a cache of already compiled shader binaries, along with the
> hashes
>   of the source used to create them, and then skip the compilation if the
> source
>   matches a hash of an already compiled binary.
>
>
> https://github.com/boostorg/compute/blob/cf7907574d6159cd43e6cf687a7b656278c61dd0/include/boost/compute/detail/meta_kernel.hpp#L364
>
> - if you need to send complex C++ objects over the network, you can first
> ask
>   the remote endpoint "do you already have the object with this hash?" and
> if so,
>   skip sending the object.
>

These three have been pretty clarifying for me regarding some design
decisions in the library, thanks for providing them  I'd encourage you to
mention them in the docs.

Thanks,
Ruben.