taskflow find_if doesn't work with infinite ranges because std::distance is used

[bradphelan]

In taskflow::find_if doesn't work with infinite ranges.

However an infinite range is a reasonable input to find_if. For example

std::ranges::iota(0)

produces an infinite range starting from 0.

However taskflow::find_if calls std::distance on the two iterators which cannot work if the second iterator is a sentinel std::unreachable_sentinel

In this case taskflow::find_if should find a sensible alternative to chunking. In our case we explicitly pass StaticPartitioner(1) so we don't require chunking.

[tsung-wei-huang]

We are following the C++ standard algorithm (and parallel algorithms as well) that does not check if the given iterators are valid/invalid. Invalid range iterators will result in undefined behavior.

[bradphelan]

An infinite range is not invalid. It is the taskflow code that is problematic because it assumes that only finite ranges can be parallelized. But I see no reason why they cannot. One just needs a constexpr test and then branch to a different chunking strategy that is not based on the number of items in the sequence.

…

On Sat, 16 Dec 2023, 19:40 Tsung-Wei Huang, ***@***.***> wrote: We are following the C++ standard algorithm (and parallel algorithms as well) that does not check if the given iterators are valid/invalid. Invalid range iterators will result in undefined behavior. — Reply to this email directly, view it on GitHub <#534 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAEJ4SCWF25U7HS6JJDWADYJXTIBAVCNFSM6AAAAABACSVEECVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTQNJYHEYDEOJZGQ> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[bradphelan]

Also standard library does support infinite ranges. For example here is how to uses std::ranges::find to implement strlen

https://godbolt.org/z/Efhsjndv6

#include <ranges>
#include <iostream>
   
template<class CharT>
constexpr std::size_t my_strlen(const CharT *s)
{
    return std::ranges::find(s, std::unreachable_sentinel, CharT{}) - s;
}


int main(){
    const char * foo = "hello world";
    std::cout << my_strlen(foo) << std::endl;
}