quickmerge.cpp simplification; function merge

[anadon]

reported blurb as follows:

also in quickmerge.cpp, there's a lot of code repetition between sortDoubleSizeTPairHighToLow and sortDoubleSizeTPairLowToHigh, as might be expected from their function names.
this can actually be avoided with templating and some appropriately designed helper functions.
while also generalizing for more/any pair types.

here's an implementation of the inverse problem.

bool is_sorted(const std::vector<T>& v, bool (*compare)(const T, const T)) {
    size_t len = v.size();
    if(len < 2) return true;
    len--;
    for(size_t i = 0; i < len; ++i) {
        if(!compare(v[i],v[i+1])) return false;
    }
    return true;
}

template <typename T>
constexpr bool is_LE(const T x, const T y) {
    return (x<=y);
}

template <typename T>
constexpr bool is_GE(const T x, const T y) {
    return (x>=y);
}
// consider: is_sorted(vec, is_LE)

(g++ does not really like just letting you use operator<= there, sadly.)
(but constexpr and const T for small types should allow the compiler to essentially eliminate any resulting comparison overhead directly anyways.)

[anadon]

I don't see a way to implement this practically. It could be done with function pointers, but that involves an overhead I'd rather not incur. Maybe through some use of templating, but even then it gets complex. This code is for when even an ordering reversal is considered too costly to perform. Though there are opportunities for multithreading here which have not yet been leveraged so I'll keep this open. I'd also be open to being proved wrong about an efficient way to make this more concise.

[anadon]

This should proceed by templating the sorts, and create a 'blank' which takes a lambda function which decides the greater than or less than for ordering and define function stubs for high to low and low to high. The templating should have the added benefit of allowing more data types to be applied.

However, this isn't an urgent change as it is not broken and this would be harder than the other issues and fixes reported.