From 704a6dbe33b094c0639aacadc81be8fa43d20bc5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ion=20Gazta=C3=B1aga?= <igaztanaga@gmail.com>
Date: Sun, 29 Mar 2026 19:07:08 +0200
Subject: [PATCH] Unroll segmented_transform

---
 .../experimental/segmented_transform.hpp      | 54 +++++++++++++++----
 1 file changed, 45 insertions(+), 9 deletions(-)

diff --git a/include/boost/container/experimental/segmented_transform.hpp b/include/boost/container/experimental/segmented_transform.hpp
index ab441cc..ba44646 100644
--- a/include/boost/container/experimental/segmented_transform.hpp
+++ b/include/boost/container/experimental/segmented_transform.hpp
@@ -21,6 +21,7 @@
 #include <boost/container/detail/config_begin.hpp>
 #include <boost/container/detail/workaround.hpp>
 #include <boost/container/experimental/segmented_iterator_traits.hpp>
+#include <boost/container/detail/iterator.hpp>
 
 namespace boost {
 namespace container {
@@ -30,21 +31,56 @@ OutIter segmented_transform(InIter first, Sent last, OutIter result, UnaryOp op)
 
 namespace detail_algo {
 
+#if defined(BOOST_CONTAINER_SEGMENTED_LOOP_UNROLLING)
 
-template <class InIter, class Sent, class OutIter, class UnaryOp, class Tag>
+template <class InIter, class OutIter, class UnaryOp>
+OutIter segmented_transform_dispatch
+   (InIter first, InIter last, OutIter result, UnaryOp op, const non_segmented_iterator_tag &, const std::random_access_iterator_tag &)
+{
+   typedef typename iterator_traits<InIter>::difference_type difference_type;
+   difference_type n = last - first;
+
+   while(n >= difference_type(4)) {
+      *result = op(*first); ++first; ++result;
+      *result = op(*first); ++first; ++result;
+      *result = op(*first); ++first; ++result;
+      *result = op(*first); ++first; ++result;
+      n -= 4;
+   }
+
+   switch (n % 4) {
+      case 3:
+         *result = op(*first); ++first; ++result;
+      BOOST_FALLTHROUGH;
+      case 2:
+         *result = op(*first); ++first; ++result;
+      BOOST_FALLTHROUGH;
+      case 1:
+         *result = op(*first); ++first; ++result;
+      BOOST_FALLTHROUGH;
+      default:
+         break;
+   }
+   return result;
+}
+
+#endif   //BOOST_CONTAINER_SEGMENTED_LOOP_UNROLLING
+
+
+template <class InIter, class Sent, class OutIter, class UnaryOp, class Tag, class Cat>
 typename algo_enable_if_c<
    !Tag::value || is_sentinel<Sent, InIter>::value, OutIter>::type
 segmented_transform_dispatch
-   (InIter first, Sent last, OutIter result, UnaryOp op, Tag)
+   (InIter first, Sent last, OutIter result, UnaryOp op, Tag, Cat)
 {
    for(; first != last; ++first, ++result)
       *result = op(*first);
    return result;
 }
 
-template <class SegIter, class OutIter, class UnaryOp>
+template <class SegIter, class OutIter, class UnaryOp, class Cat>
 OutIter segmented_transform_dispatch
-   (SegIter first, SegIter last, OutIter result, UnaryOp op, segmented_iterator_tag)
+   (SegIter first, SegIter last, OutIter result, UnaryOp op, segmented_iterator_tag, Cat)
 {
    typedef segmented_iterator_traits<SegIter> traits;
    typedef typename traits::local_iterator    local_iterator;
@@ -56,15 +92,15 @@ OutIter segmented_transform_dispatch
    segment_iterator const slast  = traits::segment(last);
 
    if(sfirst == slast) {
-      return (segmented_transform_dispatch)(traits::local(first), traits::local(last), result, op, is_local_seg_t());
+      return (segmented_transform_dispatch)(traits::local(first), traits::local(last), result, op, is_local_seg_t(), Cat());
    }
    else {
-      result = (segmented_transform_dispatch)(traits::local(first), traits::end(sfirst), result, op, is_local_seg_t());
+      result = (segmented_transform_dispatch)(traits::local(first), traits::end(sfirst), result, op, is_local_seg_t(), Cat());
 
       for(++sfirst; sfirst != slast; ++sfirst)
-         result = (segmented_transform_dispatch)(traits::begin(sfirst), traits::end(sfirst), result, op, is_local_seg_t());
+         result = (segmented_transform_dispatch)(traits::begin(sfirst), traits::end(sfirst), result, op, is_local_seg_t(), Cat());
 
-      return (segmented_transform_dispatch)(traits::begin(sfirst), traits::local(last), result, op, is_local_seg_t());
+      return (segmented_transform_dispatch)(traits::begin(sfirst), traits::local(last), result, op, is_local_seg_t(), Cat());
    }
 }
 
@@ -79,7 +115,7 @@ OutIter segmented_transform (InIter first, Sent last, OutIter result, UnaryOp op
 {
    typedef segmented_iterator_traits<InIter> traits;
    return detail_algo::segmented_transform_dispatch
-      (first, last, result, op, typename traits::is_segmented_iterator());
+      (first, last, result, op, typename traits::is_segmented_iterator(), typename iterator_traits<InIter>::iterator_category());
 }
 
 } // namespace container