Add bidirectional iterator support

include/boost/container/experimental/segmented_partition.hpp

@@ -32,8 +32,12 @@ FwdIt segmented_partition(FwdIt first, Sent last, Pred pred);
 
 namespace detail_algo {
 
-template <class FwdIt, class OutIter, class Pred>
-OutIter partition_scan(FwdIt first, FwdIt last, OutIter result, Pred pred, non_segmented_iterator_tag)
+//////////////////////////////////////////////
+// Forward (Lomuto-style) partition
+//////////////////////////////////////////////
+
+template <class FwdIt, class Sent, class OutIter, class Pred>
+OutIter partition_scan(FwdIt first, Sent last, OutIter result, Pred pred, non_segmented_iterator_tag, const std::forward_iterator_tag &)
 {
    for(; first != last; ++first) {
       if(pred(*first)) {
@@ -45,7 +49,7 @@ OutIter partition_scan(FwdIt first, FwdIt last, OutIter result, Pred pred, non_s
 }
 
 template <class SegIt, class OutIter, class Pred>
-OutIter partition_scan(SegIt first, SegIt last, OutIter result, Pred pred, segmented_iterator_tag)
+OutIter partition_scan(SegIt first, SegIt last, OutIter result, Pred pred, segmented_iterator_tag, const std::forward_iterator_tag &)
 {
    typedef segmented_iterator_traits<SegIt>  traits;
    typedef typename traits::local_iterator   local_iterator;
@@ -69,40 +73,140 @@ OutIter partition_scan(SegIt first, SegIt last, OutIter result, Pred pred, segme
    }
 }
 
-template <class SegIter, class Pred>
-BOOST_CONTAINER_FORCEINLINE SegIter segmented_partition_dispatch
-   (SegIter first, SegIter last, Pred pred, segmented_iterator_tag)
+//////////////////////////////////////////////
+// Bidirectional (Hoare-style) partition
+//////////////////////////////////////////////
+
+template <class BidirIt, class Pred>
+BidirIt partition_scan(BidirIt first, BidirIt last, Pred pred, non_segmented_iterator_tag, const std::bidirectional_iterator_tag&)
 {
-   return partition_scan(first, last, first, pred, segmented_iterator_tag());
+   while(true) {
+      while(first != last && pred(*first))
+         ++first;
+      if(first == last)
+         return first;
+      --last;
+      while(first != last && !pred(*last))
+         --last;
+      if(first == last)
+         return first;
+      boost::adl_move_swap(*first, *last);
+      ++first;
+   }
+}
+
+template <class SegIt, class Pred>
+SegIt partition_scan(SegIt first, SegIt last, Pred pred, segmented_iterator_tag, const std::bidirectional_iterator_tag& cat)
+{
+   typedef segmented_iterator_traits<SegIt>  traits;
+   typedef typename traits::local_iterator   local_iterator;
+   typedef typename traits::segment_iterator segment_iterator;
+   typedef typename segmented_iterator_traits<local_iterator>::is_segmented_iterator is_local_seg_t;
+
+   if(first == last) return first;
+
+   segment_iterator sf = traits::segment(first);
+   segment_iterator sl = traits::segment(last);
+
+   if(sf == sl) {
+      local_iterator r = partition_scan(traits::local(first), traits::local(last), pred, is_local_seg_t(), cat);
+      return traits::compose(sf, r);
+   }
+
+   local_iterator f_loc = traits::local(first);
+   local_iterator f_end = traits::end(sf);
+   local_iterator l_loc = traits::local(last);
+   local_iterator l_beg = traits::begin(sl);
+
+   for(;;) {
+      // Phase 1: advance front to find element NOT satisfying pred
+      while(f_loc != f_end && pred(*f_loc))
+         ++f_loc;
+
+      if(f_loc == f_end) {
+         ++sf;
+         if(sf == sl) {
+            local_iterator r = partition_scan(l_beg, l_loc, pred, is_local_seg_t(), cat);
+            return traits::compose(sf, r);
+         }
+         f_loc = traits::begin(sf);
+         f_end = traits::end(sf);
+         continue;
+      }
+
+      // Phase 2: retreat back to find element satisfying pred
+      for(;;) {
+         if(l_loc == l_beg) {
+            --sl;
+            if(sf == sl) {
+               local_iterator r = partition_scan(f_loc, f_end, pred, is_local_seg_t(), cat);
+               return traits::compose(sf, r);
+            }
+            l_beg = traits::begin(sl);
+            l_loc = traits::end(sl);
+         }
+         --l_loc;
+         if(pred(*l_loc)) break;
+      }
+
+      boost::adl_move_swap(*f_loc, *l_loc);
+      ++f_loc;
+   }
+}
+
+//////////////////////////////////////////////
+// Top-level dispatch
+//////////////////////////////////////////////
+
+template <class FwdIt, class Sent, class Pred, class Tag>
+BOOST_CONTAINER_FORCEINLINE
+FwdIt segmented_partition_dispatch(FwdIt first, Sent last, Pred pred, Tag tag, const std::bidirectional_iterator_tag& cat)
+{
+   return (partition_scan)(first, last, pred, tag, cat);
 }
 
 template <class FwdIt, class Sent, class Pred, class Tag>
-typename algo_enable_if_c<
-   !Tag::value || is_sentinel<Sent, FwdIt>::value, FwdIt>::type
-segmented_partition_dispatch(FwdIt first, Sent last, Pred pred, Tag)
+BOOST_CONTAINER_FORCEINLINE
+FwdIt segmented_partition_dispatch(FwdIt first, Sent last, Pred pred, Tag tag, const std::forward_iterator_tag &cat)
 {
-   FwdIt result = first;
-   for(; first != last; ++first) {
-      if(pred(*first)) {
-         boost::adl_move_swap(*result, *first);
-         ++result;
-      }
-   }
-   return result;
+   return (partition_scan)(first, last, first, pred, tag, cat);
 }
 
+template<class It, class Sent, class Seg, class Tag>
+struct sent_filter
+{
+   typedef std::forward_iterator_tag   cat_t;
+   typedef non_segmented_iterator_tag  seg_t;
+};
+
+template<class It, class Seg, class Tag>
+struct sent_filter<It, It, Seg, Tag>
+{
+   typedef Tag cat_t;
+   typedef Seg seg_t;
+};
+
 } // namespace detail_algo
 
 //! Reorders elements in [first, last) so that elements satisfying
-//! \c pred come before those that do not (Lomuto-style partition).
+//! \c pred come before those that do not.
+//! For forward iterators, uses a Lomuto-style scan.
+//! For bidirectional (or stronger) iterators, uses a Hoare-style
+//! partition that swaps from both ends, reducing the number of swaps.
 //! Returns an iterator to the partition point.
 template <class FwdIt, class Sent, class Pred>
 BOOST_CONTAINER_FORCEINLINE
 FwdIt segmented_partition(FwdIt first, Sent last, Pred pred)
 {
    typedef segmented_iterator_traits<FwdIt> traits;
+   typedef typename boost::container::iterator_traits<FwdIt>::iterator_category cat_t;
+   typedef typename traits::is_segmented_iterator seg_t;
+
+   typedef detail_algo::sent_filter<FwdIt, Sent, seg_t, cat_t> sent_filter_t;
    return detail_algo::segmented_partition_dispatch
-      (first, last, pred, typename traits::is_segmented_iterator());
+      ( first, last, pred
+      , typename sent_filter_t::seg_t()
+      , typename sent_filter_t::cat_t());
 }
 
 } // namespace container