etl::const_map creation not user friendly

[relativityspace-jsmith]

Hi, I've noticed that creating const_map instances is not particularly easy. This is partly due to the need to repeat types a lot, and partly due to the fact that if the backing array is not sorted already, the map doesn't work. See here for more details: #1306

Ideally, we could get a new etl::make_const_map function that accepts an initializer list, and then would deduce all the needed types and sort the K-V pairs before creating the map. Also, we should probably add a note in the docs that etl::const_map requires a pre-sorted array, and a static assert that verifies this is the case so the user doesn't get unexpected behavior.

Thanks for taking a look at this!

[mike919192]

I decided to investigate what could be done to check for the sort issue at compile time. I'm not an expert at constexpr programming so this was a learning process for me but here is what I have found.

The existing constructor takes elements as function arguments, so its not possible to use the element values for a static assert to check for sorted. In order to use the element values they would need to be template arguments.

So I have created a helper function that takes the elements as template arguments so that they can be checked for is sorted at compile time. This is the code inserted into const_map.h

  namespace private_const_map
  {
    // Base case: if there's only one element, it's sorted
    template <auto first>
    constexpr bool is_sorted() {
        return true;
    }

    // Recursive template function
    template <auto first, auto second, auto... rest>
    constexpr bool is_sorted() {
        return (first.first <= second.first) && is_sorted<second, rest...>();
    }
  }

  template <auto... elements>
  consteval auto make_const_map()
  {
    static_assert(private_const_map::is_sorted<elements...>(), "Elements must be sorted");
    return etl::const_map{elements...};
  }

Then if you try to use elements that are not sorted at least you get a compile error:

using map_value_t = etl::iconst_map<char, int>::value_type;
static constexpr auto data = etl::make_const_map<map_value_t{'B', 0 }, map_value_t{'A', 1 }>();

/config/workspace/etl/test/../include/etl/const_map.h:546:60: error: static assertion failed: Elements must be sorted
  546 |     static_assert(private_const_map::is_sorted<elements...>(), "Elements must be sorted");
      |                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/config/workspace/etl/test/../include/etl/const_map.h:546:60: note: ‘etl::private_const_map::is_sorted<map_value_t{'B', 0}, map_value_t{'A', 1}>()’ evaluates to false

This solution requires C++20 for the auto template parameters so I don't know if there is anything that can be done for older versions.