C++ algorithm scrapbook

algo.hpp

// argsort(first, last, a_first, [compare])
// ---
// Store the order of the input range [`first`, `last`) in the output range
// starting at `a_first`.  Both iterator types must allow random access.
//
template<class ValueIt, class IndexIt,
    class Op = std::less<typename std::iterator_traits<ValueIt>::value_type>>
void argsort(ValueIt first, ValueIt last, IndexIt a_first, Op compare = Op())
{
    using T = typename std::iterator_traits<IndexIt>::value_type;
    IndexIt a_last = std::next(a_first, std::distance(first, last));
    std::iota(a_first, a_last, 0);
    std::stable_sort(a_first, a_last, [&](const T& i, const T& j) {
        return compare(*std::next(first, i), *std::next(first, j));
    });
}