Simplify / speed up _Index_tuple and _Build_index_tuple

[Daniel Frey <d dot frey at gmx dot de>]

This is about <functional>'s internal helpers _Index_tuple and _Build_index_tuple.

I noticed that _Build_index_tuple is very complex and does not reuse instantiations with lower numbers. To test, I instantiated it with _Num = { 400, 401, 402, ..., 419 }, which took 6.5s to compile on my machine. The improved version, which does reuse the instantiation of _Build_index_tuple/_Index_tuple for _Num=418 for _Num=419, etc. compiles in 0.3s - approx. 20x faster.

Of course the above test is an extreme case, but it's IMHO also easier to understand the new code. What do you think, worth it?

BTW: A shame that these little helpers are not part of the upcoming standard...

Regards,

Daniel

Attachment: Index_tuple.patch
Description: Binary data