Enhancement #1089: Replace the implemented binary search by a binary search from the standard library

[Niklas997]

Describe your changes here:

A bidirectional iterator that wraps the serialized content of a t8_element_array_t has been implemented as well as functions providing a begin() and an end() iterator to a t8_element_array_t.
This allows a convenient usage of algorithms from the standard library like std::lower_bound, etc. on t8_element_array_t's.

The implemented binary search in t8_forest_bin_lower has been replaced by a std::upper_bound search utilizing the before mentioned iterators providing the same output as the initial implementation.

The "discussion" and "help wanted" labels result from the fact, that this is more of a pseudo-iterator since the original data is serialized to char-bytes in the internal sc_array_t and I am not sure if this is the best solution. A complete makover of the t8_element_array_t class would be of course a nicer but more complex solution than the presented approch for closing the issue.

Closes #1089

All these boxes must be checked by the reviewers before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually

• The code follows the t8code coding guidelines

• New source/header files are properly added to the Makefiles

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue)

Tests

• The code is covered in an existing or new test case using Google Test

Github action

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

• All tests pass (in various configurations, this should be executed automatically in a github action)

  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
  • If not, does the specific use case compile and all tests pass (check manually)

Scripts and Wiki

• If a new directory with source-files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example/tutorial and a Wiki article.

Licence

• The author added a BSD statement to doc/ (or already has one)

[jmark]

@Niklas997 Always a treat reading your code! I'm learning new quirks of C++ every time from you. I posted some comments/ideas. Let me know what you think.

[Niklas997]

@Niklas997 Always a treat reading your code! I'm learning new quirks of C++ every time from you. I posted some comments/ideas. Let me know what you think.

Thank you! That is very nice to hear! And thanks for the input. Now it seems to be a nice substitution for the previously implemented binary search in t8_forest_bin_lower

[jmark]

I got into the mood for code deletions. Let me know what you think about them.

Moreover, it seems that currently std::upper_bound falls back to linear search with given iterator.

[jmark]

Suggested change

	/* A typedef for the value type of the t8_element_array_t iterator. */
	using element_ptr_t = t8_element_t *;

[jmark]

I think you can save this line by simply using t8_element_array_iterator::value_type. See comment below.

[jmark]

I think you can save this line by simply using t8_element_array_iterator::value_type. See comment below.

[jmark]

Suggested change

	[&maxlevel, &ts] (const t8_linearidx_t element_id_, const element_ptr_t &elem_ptr) {
	[&maxlevel, &ts] (const t8_linearidx_t element_id_, const t8_element_array_iterator::value_type &elem_ptr) {