-/* A thread pool (C++ version). */
-
/* Copyright (c) 2004-2023 The SimGrid Team. All rights reserved. */
/* This program is free software; you can redistribute it and/or modify it
* under the terms of the license (GNU LGPL) which comes with this package. */
-#ifndef XBT_SUBSETS_HPP
-#define XBT_SUBSETS_HPP
+#ifndef XBT_UTILS_ITER_SUBSETS_HPP
+#define XBT_UTILS_ITER_SUBSETS_HPP
+#include <boost/iterator/iterator_facade.hpp>
#include <functional>
-#include <unordered_set>
+#include <numeric>
+#include <optional>
#include <vector>
namespace simgrid::xbt {
+/**
+ * @brief A higher-order forward-iterator which traverses all possible subsets
+ * of a given fixed size `k` of an iterable sequence
+ *
+ * @class Iterator: The iterator over which this higher-order iterator
+ * generates elements.
+ */
+template <class Iterator>
+struct subsets_iterator : public boost::iterator_facade<subsets_iterator<Iterator>, const std::vector<Iterator>,
+ boost::forward_traversal_tag> {
+ subsets_iterator();
+ explicit subsets_iterator(unsigned k);
+ explicit subsets_iterator(unsigned k, Iterator begin, Iterator end = Iterator());
+
+private:
+ unsigned k; // The size of the subsets generated
+ std::optional<Iterator> end = std::nullopt;
+ std::vector<Iterator> current_subset;
+ std::vector<unsigned> P; // Increment counts to determine which combinations need to be traversed
+
+ // boost::iterator_facade<...> interface to implement
+ void increment();
+ bool equal(const subsets_iterator<Iterator>& other) const;
+ const std::vector<Iterator>& dereference() const;
+
+ // Allows boost::iterator_facade<...> to function properly
+ friend class boost::iterator_core_access;
+};
+
+template <typename Iterator> subsets_iterator<Iterator>::subsets_iterator() : subsets_iterator<Iterator>(0) {}
+
+template <typename Iterator>
+subsets_iterator<Iterator>::subsets_iterator(unsigned k)
+ : k(k), current_subset(std::vector<Iterator>(k)), P(std::vector<unsigned>(k))
+{
+ std::iota(P.begin(), P.end(), k);
+}
+
+template <typename Iterator>
+subsets_iterator<Iterator>::subsets_iterator(unsigned k, Iterator begin, Iterator end)
+ : k(k), end(std::optional<Iterator>{end}), current_subset(std::vector<Iterator>(k)), P(std::vector<unsigned>(k))
+{
+ for (unsigned i = 0; i < k; i++) {
+ // Less than `k` elements to choose
+ if (begin == end) {
+ // We want to initialize the object then to be equivalent
+ // to the end iterator so that there are no items to iterate
+ // over
+ this->end = std::nullopt;
+ std::iota(P.begin(), P.end(), k);
+ return;
+ }
+ current_subset[i] = begin++;
+ }
+ std::iota(P.begin(), P.end(), 0);
+}
+
+template <typename Iterator> bool subsets_iterator<Iterator>::equal(const subsets_iterator<Iterator>& other) const
+{
+ if (this->end == std::nullopt && other.end == std::nullopt) {
+ return true;
+ }
+ if (this->k != other.k) {
+ return false;
+ }
+ if (this->k == 0) { // this->k == other.k == 0
+ return true;
+ }
+ return this->end != std::nullopt && other.end != std::nullopt && this->P[0] == other.P[0];
+}
+
+template <typename Iterator> const std::vector<Iterator>& subsets_iterator<Iterator>::dereference() const
+{
+ return this->current_subset;
+}
+
+template <typename Iterator> void subsets_iterator<Iterator>::increment()
+{
+ // If k == 0, there's nothing to do
+ // If end == std::nullopt, we've finished
+ // iterating over all subsets of size `k`
+ if (end == std::nullopt || k == 0) {
+ return;
+ }
+
+ // Move the last element over each time
+ ++current_subset[k - 1];
+ ++P[k - 1];
+
+ const bool shift_other_elements = current_subset[k - 1] == end.value();
+
+ if (shift_other_elements) {
+ if (k == 1) {
+ // We're done in the case that k = 1; here, we've iterated
+ // through the list once, which is all that is needed
+ end = std::nullopt;
+ return;
+ }
+
+ // At this point, k >= 2
+
+ // The number of elements is now equal to the "index"
+ // of the last element (it is at the end, which means we added
+ // for the last time)
+ const unsigned n = P[k - 1];
+
+ // We're looking to determine
+ //
+ // argmax_{0 <= j <= k - 2}(P[j] != (n - (k - j)))
+ //
+ // If P[j] == (n - (k - j)) for some `j`, that means
+ // the `j`th element of the current subset has moved
+ // "as far as it can move" to the right; in other words,
+ // this is our signal that some element before the `j`th
+ // has to move over
+ //
+ // std::max_element() would work here too, but it seems
+ // overkill to create a vector full of numbers when a simple
+ // range-based for-loop can do the trick
+ unsigned l = 0;
+ for (unsigned j = k - 2; j > 0; j--) {
+ if (P[j] != (n - (k - j))) {
+ l = j;
+ break;
+ }
+ }
+
+ ++P[l];
+ ++current_subset[l];
+
+ // Plugging in `j = 0` to the above formula yields
+ // `n - k`, which is the furthest point that the first (i.e. `0th`)
+ // element can be located. Thus, if `P[0] > (n - k)`, this means
+ // we've sucessfully iterated through all subsets so we're done
+ if (P[0] > (n - k)) {
+ end = std::nullopt;
+ return;
+ }
+
+ // Otherwise, all elements past element `l` are reset
+ // to follow one after another immediately after `l`
+ auto iter_at_l = current_subset[l];
+ for (auto i = l + 1; i <= (k - 1); i++) {
+ P[i] = P[l] + (i - l);
+ current_subset[i] = ++iter_at_l;
+ }
+ }
+}
+
} // namespace simgrid::xbt
#endif
