algorithm.h

#pragma once
 
#include <utility>
 
namespace xrpl {
 
// Requires: [first1, last1) and [first2, last2) are ordered ranges according to
// comp.
 
// Effects: For each pair of elements {i, j} in the intersection of the sorted
// sequences [first1, last1) and [first2, last2), perform action(i, j).
 
// Note: This algorithm is evolved from std::set_intersection.
template <class InputIter1, class InputIter2, class Action, class Comp>
void
generalized_set_intersection(
    InputIter1 first1,
    InputIter1 last1,
    InputIter2 first2,
    InputIter2 last2,
    Action action,
    Comp comp)
{
    while (first1 != last1 && first2 != last2)
    {
        if (comp(*first1, *first2))  // if *first1 < *first2
            ++first1;                //     then reduce first range
        else
        {
            if (!comp(*first2, *first1))   // if *first1 == *first2
            {                              //     then this is an intersection
                action(*first1, *first2);  //     do the action
                ++first1;                  //     reduce first range
            }
            ++first2;  // Reduce second range because *first2 <= *first1
        }
    }
}
 
// Requires: [first1, last1) and [first2, last2) are ordered ranges according to
// comp.
 
// Effects: Eliminates all the elements i in the range [first1, last1) which are
// equivalent to some value in [first2, last2) or for which pred(i) returns
// true.
 
// Returns: A FwdIter1 E such that [first1, E) is the range of elements not
// removed by this algorithm.
 
// Note: This algorithm is evolved from std::remove_if and
// std::set_intersection.
template <class FwdIter1, class InputIter2, class Pred, class Comp>
FwdIter1
remove_if_intersect_or_match(
    FwdIter1 first1,
    FwdIter1 last1,
    InputIter2 first2,
    InputIter2 last2,
    Pred pred,
    Comp comp)
{
    // [original-first1, current-first1) is the set of elements to be preserved.
    // [current-first1, i) is the set of elements that have been removed.
    // [i, last1) is the set of elements not tested yet.
 
    // Test each *i in [first1, last1) against [first2, last2) and pred
    for (auto i = first1; i != last1;)
    {
        // if (*i is not in [first2, last2)
        if (first2 == last2 || comp(*i, *first2))
        {
            if (!pred(*i))
            {
                // *i should not be removed, so append it to the preserved set
                if (i != first1)
                    *first1 = std::move(*i);
                ++first1;
            }
            // *i has been fully tested, prepare for next i by
            // appending i to the removed set, whether or not
            // it has been moved from above.
            ++i;
        }
        else  // *i might be in [first2, last2) because *i >= *first2
        {
            if (!comp(*first2, *i))  // if *i == *first2
                ++i;                 //    then append *i to the removed set
            // All elements in [i, last1) are known to be greater than *first2,
            // so reduce the second range.
            ++first2;
        }
        // Continue to test *i against [first2, last2) and pred
    }
    return first1;
}
 
}  // namespace xrpl