[libcxx-commits] [libcxx] Added segmented iterator for std::equal (PR #106254)

[via libcxx-commits]

https://github.com/NoumanAmir657 created https://github.com/llvm/llvm-project/pull/106254

For the issue #102817
@philnik777
Hi, not sure if my implementation for this is correct or not. Would appreciate some pointers regarding it.

>From 69b32e82068d33c2403739710f66751351383750 Mon Sep 17 00:00:00 2001
From: nouman-10x <noumanamir453 at gmail.com>
Date: Tue, 27 Aug 2024 22:27:37 +0500
Subject: [PATCH] Added segmented iterator for equal

---
 libcxx/include/__algorithm/equal.h | 134 +++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)

diff --git a/libcxx/include/__algorithm/equal.h b/libcxx/include/__algorithm/equal.h
index bfc8f72f6eb195..c7d8eb54026bba 100644
--- a/libcxx/include/__algorithm/equal.h
+++ b/libcxx/include/__algorithm/equal.h
@@ -24,6 +24,7 @@
 #include <__type_traits/is_equality_comparable.h>
 #include <__type_traits/is_volatile.h>
 #include <__utility/move.h>
+#include <__iterator/segmented_iterator.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -54,9 +55,68 @@ __equal_iter_impl(_Tp* __first1, _Tp* __last1, _Up* __first2, _BinaryPredicate&)
   return std::__constexpr_memcmp_equal(__first1, __first2, __element_count(__last1 - __first1));
 }
 
+template <class _SegmentedIterator1, class _SegmentedIterator2, class _BinaryPredicate>
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+__equal_segment_iter(_SegmentedIterator1 __first1, _SegmentedIterator1 __last1, _SegmentedIterator2 __first2, _BinaryPredicate __pred) {
+  using _Traits1 = __segmented_iterator_traits<_SegmentedIterator1>;
+  using _Traits2 = __segmented_iterator_traits<_SegmentedIterator2>;
+
+  auto __sfirst1 = _Traits1::__segment(__first1);
+  auto __slast1  = _Traits1::__segment(__last1);
+
+  auto __sfirst2 = _Traits2::__segment(__first2);
+
+  // Both have only 1 segment
+  if (__sfirst1 == __slast1)
+    return std::__equal_iter_impl(
+      std::__unwrap_iter(_Traits1::__local(__first1)),
+      std::__unwrap_iter(_Traits1::__local(__last1)),
+      std::__unwrap_iter(_Traits2::__local(__first2)),
+      __pred);
+
+  { // We have more than one segment. Iterate over the first segment, since we might not start at the beginning
+    if (!std::__equal_iter_impl(
+      std::__unwrap_iter(_Traits1::__local(__first1)),
+      std::__unwrap_iter(_Traits1::__end(__sfirst1)),
+      std::__unwrap_iter(_Traits2::__local(__first2)),
+      __pred)) {
+        return false;
+    }
+  }
+  ++__sfirst1;
+  ++__sfirst2;
+
+  // Iterate over the segments which are guaranteed to be completely in the range
+  while (__sfirst1 != __slast1) {
+    if (!std::__equal_iter_impl(
+      std::__unwrap_iter(_Traits1::__begin(__sfirst1)),
+      std::__unwrap_iter(_Traits1::__end(__sfirst1)),
+      std::__unwrap_iter(_Traits2::__begin(__sfirst2)),
+      __pred)) {
+        return false;
+      }
+    ++__sfirst1;
+    ++__sfirst2;
+  }
+
+  // Iterate over the last segment
+  if (!std::__equal_iter_impl(
+    std::__unwrap_iter(_Traits1::__begin(__sfirst1)),
+    std::__unwrap_iter(_Traits1::__local(__last1)),
+    std::__unwrap_iter(_Traits2::__begin(__sfirst2)),
+    __pred)) {
+      return false;
+  }
+
+  return true;
+}
+
 template <class _InputIterator1, class _InputIterator2, class _BinaryPredicate>
 _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
 equal(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __first2, _BinaryPredicate __pred) {
+  if constexpr (__is_segmented_iterator<_InputIterator1>::value && __is_segmented_iterator<_InputIterator2>::value) {
+    return std::__equal_segment_iter(__first1, __last1, __first2, __pred);
+  }
   return std::__equal_iter_impl(
       std::__unwrap_iter(__first1), std::__unwrap_iter(__last1), std::__unwrap_iter(__first2), __pred);
 }
@@ -95,6 +155,75 @@ __equal_impl(_Tp* __first1, _Tp* __last1, _Up* __first2, _Up*, _Pred&, _Proj1&,
   return std::__constexpr_memcmp_equal(__first1, __first2, __element_count(__last1 - __first1));
 }
 
+template <class _SegmentedIterator1, class _SegmentedIterator2, class _BinaryPredicate, class _Proj1, class _Proj2>
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+__equal_segment(_SegmentedIterator1 __first1, _SegmentedIterator1 __last1, _SegmentedIterator2 __first2, _SegmentedIterator2 __last2, _BinaryPredicate __pred, _Proj1& __proj1, _Proj2& __proj2) {
+  using _Traits1 = __segmented_iterator_traits<_SegmentedIterator1>;
+  using _Traits2 = __segmented_iterator_traits<_SegmentedIterator2>;
+
+  auto __sfirst1 = _Traits1::__segment(__first1);
+  auto __slast1  = _Traits1::__segment(__last1);
+
+  auto __sfirst2 = _Traits2::__segment(__first2);
+  auto __slast2  = _Traits2::__segment(__last2);
+
+  // Both have only 1 segment
+  if (__sfirst1 == __slast1 && __sfirst2 == __slast2)
+    return std::__equal_impl(
+      std::__unwrap_iter(_Traits1::__local(__first1)),
+      std::__unwrap_iter(_Traits1::__local(__last1)),
+      std::__unwrap_iter(_Traits2::__local(__first2)),
+      std::__unwrap_iter(_Traits2::__local(__last2)),
+      __pred,
+      __proj1,
+      __proj2);
+
+  { // We have more than one segment. Iterate over the first segment, since we might not start at the beginning
+    if (!std::__equal_impl(
+      std::__unwrap_iter(_Traits1::__local(__first1)),
+      std::__unwrap_iter(_Traits1::__end(__sfirst1)),
+      std::__unwrap_iter(_Traits2::__local(__first2)),
+      std::__unwrap_iter(_Traits2::__end(__sfirst2)),
+      __pred,
+      __proj1,
+      __proj2)) {
+        return false;
+      }
+  }
+  ++__sfirst1;
+  ++__sfirst2;
+
+  // Iterate over the segments which are guaranteed to be completely in the range
+  while (__sfirst1 != __slast1 && __sfirst2 != __slast2) {
+    if (!std::__equal_impl(
+      std::__unwrap_iter(_Traits1::__begin(__sfirst1)),
+      std::__unwrap_iter(_Traits1::__end(__sfirst1)),
+      std::__unwrap_iter(_Traits2::__begin(__sfirst2)),
+      std::__unwrap_iter(_Traits2::__end(__sfirst2)),
+      __pred,
+      __proj1,
+      __proj2)) {
+        return false;
+      }
+    ++__sfirst1;
+    ++__sfirst2;
+  }
+
+  // Iterate over the last segment
+  if (!std::__equal_impl(
+    std::__unwrap_iter(_Traits1::__begin(__sfirst1)),
+    std::__unwrap_iter(_Traits1::__local(__last1)),
+    std::__unwrap_iter(_Traits2::__begin(__sfirst2)),
+    std::__unwrap_iter(_Traits2::__local(__last2)),
+    __pred,
+    __proj1,
+    __proj2)) {
+      return false;
+  }
+
+  return __sfirst1 == __slast1 && __sfirst2 == __slast2;
+}
+
 template <class _InputIterator1, class _InputIterator2, class _BinaryPredicate>
 _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
 equal(_InputIterator1 __first1,
@@ -108,6 +237,11 @@ equal(_InputIterator1 __first1,
       return false;
   }
   __identity __proj;
+
+  if constexpr (__is_segmented_iterator<_InputIterator1>::value && __is_segmented_iterator<_InputIterator2>::value) {
+    return std::__equal_segment(__first1, __last1, __first2, __last2, __pred, __proj, __proj);
+  }
+
   return std::__equal_impl(
       std::__unwrap_iter(__first1),
       std::__unwrap_iter(__last1),