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@(@\newcommand{\W}[1]{ \; #1 \; } \newcommand{\R}[1]{ {\rm #1} } \newcommand{\B}[1]{ {\bf #1} } \newcommand{\D}[2]{ \frac{\partial #1}{\partial #2} } \newcommand{\DD}[3]{ \frac{\partial^2 #1}{\partial #2 \partial #3} } \newcommand{\Dpow}[2]{ \frac{\partial^{#1}}{\partial {#2}^{#1}} } \newcommand{\dpow}[2]{ \frac{ {\rm d}^{#1}}{{\rm d}\, {#2}^{#1}} }@)@ This is cppad-20221105 documentation. Here is a link to its current documentation .
Index Sort: Example and Test
# include <cppad/utility/index_sort.hpp>
# include <cppad/utility/vector.hpp>
# include <valarray>
# include <vector>


namespace{
    // class that uses < to compare a pair of size_t values
    class Key {
    public:
        size_t first_;
        size_t second_;
        //
        Key(void)
        { }
        //
        Key(size_t first, size_t second)
        : first_(first), second_(second)
        { }
        //
        bool operator<(const Key& other) const
        {   if( first_ == other.first_ )
                return second_ < other.second_;
            return first_ < other.first_;
        }
    };

    template <class KeyVector, class SizeVector>
    bool vector_case(void)
    {   bool ok = true;
        size_t i, j;
        size_t first[]  =  { 4, 4, 3, 3, 2, 2, 1, 1};
        size_t second[] = { 0, 1, 0, 1, 0, 1, 0, 1};
        size_t size     = sizeof(first) / sizeof(first[0]);

        KeyVector keys(size);
        for(i = 0; i < size; i++)
            keys[i] = Key(first[i], second[i]);

        SizeVector ind(size);
        CppAD::index_sort(keys, ind);

        // check that all the indices are different
        for(i = 0; i < size; i++)
        {   for(j = 0; j < size; j++)
                ok &= (i == j) | (ind[i] != ind[j]);
        }

        // check for increasing order
        for(i = 0; i < size-1; i++)
        {   if( first[ ind[i] ] == first[ ind[i+1] ] )
                ok &= second[ ind[i] ] <= second[ ind[i+1] ];
            else
                ok &= first[ ind[i] ] < first[ ind[i+1] ];
        }

        return ok;
    }
}

bool index_sort(void)
{   bool ok = true;

    // some example simple vector template classes
    ok &= vector_case<  std::vector<Key>,  std::valarray<size_t> >();
    ok &= vector_case< std::valarray<Key>, CppAD::vector<size_t> >();
    ok &= vector_case< CppAD::vector<Key>,   std::vector<size_t> >();

    return ok;
}
Input File: example/utility/index_sort.cpp