[libcxx-commits] [libcxx] [libc++][pstl] Implementation of parallel std::lexicographical_compare() based on std::mismatch() (PR #212366)

[Louis Dionne via libcxx-commits]

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@@ -221,6 +222,33 @@ struct __find_first_of<__default_backend_tag, _ExecutionPolicy> {
 // mismatch family
 //////////////////////////////////////////////////////////////
 
+template <class _ExecutionPolicy>
+struct __lexicographical_compare<__default_backend_tag, _ExecutionPolicy> {
+  template <class _Policy, class _ForwardIterator1, class _ForwardIterator2, class _Comp>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
+  operator()(_Policy&& __policy,
+             _ForwardIterator1 __first1,
+             _ForwardIterator1 __last1,
+             _ForwardIterator2 __first2,
+             _ForwardIterator2 __last2,
+             _Comp __comp) const noexcept {
+    using _Mismatch = __dispatch<__mismatch, __current_configuration, _ExecutionPolicy>;
+    using _Ref1     = __iterator_reference<_ForwardIterator1>;
+    using _Ref2     = __iterator_reference<_ForwardIterator2>;
+    // find the first pair of elements that are not equal, or the end of one or both of the ranges
+    auto __res = _Mismatch()(__policy, __first1, __last1, __first2, __last2, [&](_Ref1 __lhs, _Ref2 __rhs) {
+      return !__comp(__lhs, __rhs) && !__comp(__rhs, __lhs); // derive equality from the less-than predicate
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ldionne wrote:

Per https://eel.is/c++draft/alg.sorting#general-3, `comp` is a strict weak order, so there's no question whether this is valid: it definitely is.

https://github.com/llvm/llvm-project/pull/212366