Re: [boost] Formal review request for Hash2

Ivan Matek wrote:

I wanted to check the library a bit more with regards to this, but got distracted doing something else so here is unrelated question:

Question probably stems from my lack of understanding why are we hashing size of ranges(tried googling if other languages do this, was unable to find info about that), but maybe example is best: ...

N3980 has a section on that here: https://open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3980.html#hash_append_... In short, if the size isn't hashed, an empty vector would result in an empty message, with the corresponding drawbacks. So I followed N3980 on this when I initially implemented it. I've since "discovered" that this issue is actually present not just for ranges and containers, but for empty tuples such as std::tuple<> and std::array. (It's also present for empty described structs.) I avoid it by imposing the rule that a hash_append call must always result in at least one byte being sent, and make sure that empty tuples and structs send a byte instead of nothing. In principle, we could make containers and ranges also do this, instead of sending the size. On one hand, will remove the need for having hash_append_size and hash_append_sized_range. On the other hand, it makes it trivial to generate collisions pair( "foo", "bar" ) pair( "foob", "ar" ) pair( "fooba", "r" ) so maybe not.