[Feat] Easier to upgrade the default maximum number of function/closure args

[LamKamhang]

The tinyexpr++ project is very good, thank you for maintaining a C++ version.

In my personal use, I found that the custom function can only support up to 7 parameters, and I actually need more. After carefully reading the code of tinyexpr.h, I found the specific reason: because the code uses std::variant to store the specific type of var, and only up to 7 input parameters are defined for the function.

tinyexpr-plusplus/tinyexpr.h

Lines 106 to 123 in 7559e42

	using te_fun0 = te_type (*)();
	using te_fun1 = te_type (*)(te_type);
	using te_fun2 = te_type (*)(te_type, te_type);
	using te_fun3 = te_type (*)(te_type, te_type, te_type);
	using te_fun4 = te_type (*)(te_type, te_type, te_type, te_type);
	using te_fun5 = te_type (*)(te_type, te_type, te_type, te_type, te_type);
	using te_fun6 = te_type (*)(te_type, te_type, te_type, te_type, te_type, te_type);
	using te_fun7 = te_type (*)(te_type, te_type, te_type, te_type, te_type, te_type, te_type);
	// context functions (where te_variable passes a client's te_expr as the first argument)
	using te_confun0 = te_type (*)(const te_expr*);
	using te_confun1 = te_type (*)(const te_expr*, te_type);
	using te_confun2 = te_type (*)(const te_expr*, te_type, te_type);
	using te_confun3 = te_type (*)(const te_expr*, te_type, te_type, te_type);
	using te_confun4 = te_type (*)(const te_expr*, te_type, te_type, te_type, te_type);
	using te_confun5 = te_type (*)(const te_expr*, te_type, te_type, te_type, te_type, te_type);
	using te_confun6 = te_type (*)(const te_expr*, te_type, te_type, te_type, te_type, te_type,
	te_type);
	using te_confun7 = te_type (*)(const te_expr*, te_type, te_type, te_type, te_type, te_type, te_type,

If I want to expand it to support more parameters, I need to manually add this part of the code and rewrite te_variant_type:

tinyexpr-plusplus/tinyexpr.h

Lines 133 to 141 in 7559e42

	// do not change the ordering of these, the indices are used to determine
	// the value type of a te_variable
	using te_variant_type =
	std::variant<te_type, const te_type*, // indices 0-1
	// indices 2-9
	te_fun0, te_fun1, te_fun2, te_fun3, te_fun4, te_fun5, te_fun6, te_fun7,
	// indices 10-17
	te_confun0, te_confun1, te_confun2, te_confun3, te_confun4, te_confun5, te_confun6,
	te_confun7>;

However, I found some bad code smells here. The underline index of the type is used to locate the concrete type of te_variant_type, which makes it not easy to expand the number of args：

tinyexpr-plusplus/tinyexpr.h

Lines 803 to 806 in 7559e42

	constexpr static bool is_function(const te_variant_type& var) noexcept
	{
	return (var.index() >= 2 && var.index() <= 9);
	}

tinyexpr-plusplus/tinyexpr.h

Lines 913 to 916 in 7559e42

	constexpr static bool is_closure(const te_variant_type& var) noexcept
	{
	return (var.index() >= 10 && var.index() <= 17);
	}

I spent some time rewriting this code using some simple template techniques. Now it should be easier to upgrade the maximum args of a function/closure.

[Blake-Madden]

Thanks, I'll check out the PR later this week.

I don't know if I would call it a code smell necessarily. Using variant indices is OK as long as you don't intend to change the variant data type. I was planning on capping the arguments to 7, so that was the case. Using indices like this provides a small optimization, although admittedly nothing too drastic.

Another issue is the built-in variadic functions like SUM. Those are hard coded to 7 arguments because I have to review each argument to see if they are NaN or not. If I expand support to 24 parameters, then those will all need to be updated too. I'll see if I can get that all working with your PR later this week or so...