Bug fix: get_used_symbols

[koniksedy]

This PR fixes bugs in the method get_used_symbols.

[jurajsic]

This PR fixes bugs in the method get_used_symbols.

Which bugs?

[Adda0]

LGTM.

[Adda0]

This PR fixes bugs in the method get_used_symbols.

Which bugs?

The optimized variants of the method by @kilohsakul had some bugs with using uninitialized memory, which caused segfaults. @koniksedy fixed these, and apparently some variants are now generally faster than our currently used implementation. We can experiment with the variants in, for example, benchmarks from Noodler and see whether there is any noticeable impact on the performance.

[koniksedy]

The function utils::reserve_on_insert that @kilohsakul used inside Delta::get_used_symbols_bv and Delta::get_used_symbols_chv is tricky.

mata/include/mata/utils/utils.hh

Lines 230 to 238 in 65cb9c6

	template<class Vector>
	void inline reserve_on_insert(Vector & vec,size_t needed_capacity = 0,size_t extension = 32) {
	//return; //Try this to see the effect of calling this. It should not affect functionality.
	if (vec.capacity() < extension) //if the size is already large enough, leave it to the default doubling strategy. This if seems to make a barely noticeable difference :).
	{
	if (vec.capacity() < std::max(vec.size() + 1, needed_capacity))
	vec.reserve(vec.size() + extension);
	}
	}

It should only be used before insert operations, not direct access. The function extends the vector size only if it is less than extension = 32. Since this function was the only one used in Delta::get_used_symbols_chv to resize the output vector, it never actually resized the vector, leading to a segmentation fault.

Perhaps the function utils::reserve_on_insert should be renamed to something like first_reserve_before_insert.

[Adda0]

More like reserve_before_first_insert, or reserve_on_first_insert no?

[koniksedy]

reserve_on_first_insert sounds the best.