[libc++] Add ranges::fold_left_first and ranges::fold_left_first_with_iter

libcxx/docs/ReleaseNotes/21.rst

@@ -42,6 +42,7 @@ Implemented Papers
 - P0767R1: Deprecate POD (`Github <https://github.com/llvm/llvm-project/issues/104013>`__)
 - P1361R2: Integration of chrono with text formatting (`Github <https://github.com/llvm/llvm-project/issues/100014>`__)
 - P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (`Github <https://github.com/llvm/llvm-project/issues/105180>`__)
+- P2322R6: ``ranges::fold`` (implemented ``ranges::fold_left_first`` and ``ranges::fold_left_first_with_iter``) (`Github <https://github.com/llvm/llvm-project/issues/105208>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx23Papers.csv

@@ -65,7 +65,7 @@
 "`P2286R8 <https://wg21.link/P2286R8>`__","Formatting Ranges","2022-07 (Virtual)","|Complete|","16",""
 "`P2291R3 <https://wg21.link/P2291R3>`__","Add Constexpr Modifiers to Functions ``to_chars`` and ``from_chars`` for Integral Types in ``<charconv>`` Header","2022-07 (Virtual)","|Complete|","16",""
 "`P2302R4 <https://wg21.link/P2302R4>`__","``std::ranges::contains``","2022-07 (Virtual)","|Complete|","19",""
-"`P2322R6 <https://wg21.link/P2322R6>`__","``ranges::fold``","2022-07 (Virtual)","","",""
+"`P2322R6 <https://wg21.link/P2322R6>`__","``ranges::fold``","2022-07 (Virtual)","|Partial|","","Only ``in_value_result``, ``fold_left_with_iter``, ``fold_left``, ``fold_left_first_with_iter``, and ``fold_left_first`` are implemented."
 "`P2374R4 <https://wg21.link/P2374R4>`__","``views::cartesian_product``","2022-07 (Virtual)","","",""
 "`P2404R3 <https://wg21.link/P2404R3>`__","Move-only types for ``equality_comparable_with``, ``totally_ordered_with``, and ``three_way_comparable_with``","2022-07 (Virtual)","","",""
 "`P2408R5 <https://wg21.link/P2408R5>`__","Ranges iterators as inputs to non-Ranges algorithms","2022-07 (Virtual)","","",""

libcxx/include/__algorithm/ranges_fold.h

@@ -28,6 +28,7 @@
 #include <__type_traits/invoke.h>
 #include <__utility/forward.h>
 #include <__utility/move.h>
+#include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -62,6 +63,9 @@ struct in_value_result {
 template <class _Ip, class _Tp>
 using fold_left_with_iter_result = in_value_result<_Ip, _Tp>;
 
+template <class _Ip, class _Tp>
+using fold_left_first_with_iter_result = in_value_result<_Ip, _Tp>;
+
 template <class _Fp, class _Tp, class _Ip, class _Rp, class _Up = decay_t<_Rp>>
 concept __indirectly_binary_left_foldable_impl =
     convertible_to<_Rp, _Up> &&                    //
@@ -118,6 +122,55 @@ struct __fold_left {
 };
 
 inline constexpr auto fold_left = __fold_left();
+
+struct __fold_left_first_with_iter {
+  template <input_iterator _Ip, sentinel_for<_Ip> _Sp, __indirectly_binary_left_foldable<iter_value_t<_Ip>, _Ip> _Fp>
+    requires constructible_from<iter_value_t<_Ip>, iter_reference_t<_Ip>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Ip __first, _Sp __last, _Fp __f) {
+    using _Up = decltype(fold_left(std::move(__first), __last, iter_value_t<_Ip>(*__first), __f));
+
+    // case of empty range
+    if (__first == __last) {
+      return fold_left_first_with_iter_result<_Ip, optional<_Up>>{std::move(__first), optional<_Up>()};
+    }
+
+    _Up __result(*__first);
+    for (++__first; __first != __last; ++__first) {
+      __result = std::invoke(__f, std::move(__result), *__first);
+    }
+
+    return fold_left_first_with_iter_result<_Ip, optional<_Up>>{std::move(__first), optional<_Up>(std::move(__result))};
+  }
+
+  template <input_range _Rp, __indirectly_binary_left_foldable<range_value_t<_Rp>, iterator_t<_Rp>> _Fp>
+    requires constructible_from<range_value_t<_Rp>, range_reference_t<_Rp>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Rp&& __r, _Fp __f) {
+    auto __result = operator()(ranges::begin(__r), ranges::end(__r), std::ref(__f));
+
+    using _Up = decltype(fold_left(ranges::begin(__r), ranges::end(__r), range_value_t<_Rp>(*ranges::begin(__r)), __f));
+    return fold_left_first_with_iter_result<borrowed_iterator_t<_Rp>, optional<_Up>>{
+        std::move(__result.in), std::move(__result.value)};
+  }
+};
+
+inline constexpr auto fold_left_first_with_iter = __fold_left_first_with_iter();
+
+struct __fold_left_first {
+  template <input_iterator _Ip, sentinel_for<_Ip> _Sp, __indirectly_binary_left_foldable<iter_value_t<_Ip>, _Ip> _Fp>
+    requires constructible_from<iter_value_t<_Ip>, iter_reference_t<_Ip>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Ip __first, _Sp __last, _Fp __f) {
+    return fold_left_first_with_iter(std::move(__first), std::move(__last), std::ref(__f)).value;
+  }
+
+  template <input_range _Rp, __indirectly_binary_left_foldable<range_value_t<_Rp>, iterator_t<_Rp>> _Fp>
+    requires constructible_from<range_value_t<_Rp>, range_reference_t<_Rp>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Rp&& __r, _Fp __f) {
+    return fold_left_first_with_iter(ranges::begin(__r), ranges::end(__r), std::ref(__f)).value;
+  }
+};
+
+inline constexpr auto fold_left_first = __fold_left_first();
+
 } // namespace ranges
 
 #endif // _LIBCPP_STD_VER >= 23

libcxx/include/algorithm

@@ -941,6 +941,15 @@ namespace ranges {
   template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
     constexpr auto fold_left(R&& r, T init, F f);                                                   // since C++23
 
+  template<input_iterator I, sentinel_for<I> S,
+           indirectly-binary-left-foldable<T, I> F>
+    requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
+    constexpr see below fold_left_first(I first, S last, F f);                                      // since C++23
+
+  template<input_range R, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+    requires constructible_from<range_value_t<R>, range_reference_t<R>>
+    constexpr see below fold_left_first(R&& r, F f);                                                // since C++23
+
   template<class I, class T>
     using fold_left_with_iter_result = in_value_result<I, T>;                                       // since C++23
 
@@ -951,6 +960,18 @@ namespace ranges {
   template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
     constexpr see below fold_left_with_iter(R&& r, T init, F f);                                    // since C++23
 
+  template<class I, class T>
+    using fold_left_first_with_iter_result = in_value_result<I, T>;                                 // since C++23
+
+  template<input_iterator I, sentinel_for<I> S,
+           indirectly-binary-left-foldable<T, I> F>
+    requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
+    constexpr see below fold_left_first_with_iter(I first, S last, F f);                            // since C++23
+
+  template<input_range R, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+    requires constructible_from<range_value_t<R>, range_reference_t<R>>
+    constexpr see below fold_left_first_with_iter(R&& r, F f);                                      // since C++23
+
   template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2,
            class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
     requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>

libcxx/modules/std/algorithm.inc

@@ -162,15 +162,14 @@ export namespace std {
 
     // [alg.fold], fold
     using std::ranges::fold_left;
+    using std::ranges::fold_left_first;
+    using std::ranges::fold_left_first_with_iter;
+    using std::ranges::fold_left_first_with_iter_result;
     using std::ranges::fold_left_with_iter;
     using std::ranges::fold_left_with_iter_result;
 #  if 0
-    using std::ranges::fold_left_first;
     using std::ranges::fold_right;
     using std::ranges::fold_right_last;
-    using std::ranges::fold_left_with_iter;
-    using std::ranges::fold_left_first_with_iter;
-    using std::ranges::fold_left_first_with_iter;
 #  endif
 #endif // _LIBCPP_STD_VER >= 23
   }    // namespace ranges

libcxx/test/libcxx/diagnostics/algorithm.nodiscard.verify.cpp

@@ -392,5 +392,9 @@ void test() {
   // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
   std::ranges::fold_left_with_iter(iter, iter, 0, std::plus());
   // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::ranges::fold_left_first_with_iter(range, std::plus());
+  // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::ranges::fold_left_first_with_iter(iter, iter, std::plus());
+  // expected-warning@-1{{ignoring return value of function declared with 'nodiscard' attribute}}
 #endif
 }

libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp

@@ -39,6 +39,7 @@
 #include <string_view>
 #include <string>
 #include <vector>
+#include <optional>
 
 #include "test_macros.h"
 #include "test_range.h"
@@ -50,6 +51,8 @@
 #endif
 
 using std::ranges::fold_left;
+using std::ranges::fold_left_first;
+using std::ranges::fold_left_first_with_iter;
 using std::ranges::fold_left_with_iter;
 
 template <class Result, class Range, class T>
@@ -105,6 +108,48 @@ constexpr void check_iterator(R& r, T const& init, F f, Expected const& expected
   }
 }
 
+template <std::ranges::input_range R, class F, std::equality_comparable Expected>
+  requires std::copyable<R>
+constexpr void check_iterator(R& r, F f, std::optional<Expected> const& expected) {
+  {
+    is_in_value_result<R, std::optional<Expected>> decltype(auto) result =
+        fold_left_first_with_iter(r.begin(), r.end(), f);
+    assert(result.in == r.end());
+    assert(result.value == expected);
+  }
+
+  {
+    auto telemetry = invocable_telemetry();
+    auto f2        = invocable_with_telemetry(f, telemetry);
+    is_in_value_result<R, std::optional<Expected>> decltype(auto) result =
+        fold_left_first_with_iter(r.begin(), r.end(), f2);
+    assert(result.in == r.end());
+    assert(result.value == expected);
+    if (result.value.has_value()) {
+      assert(telemetry.invocations == std::ranges::distance(r) - 1);
+      assert(telemetry.moves == 0);
+      assert(telemetry.copies == 1);
+    }
+  }
+
+  {
+    std::same_as<std::optional<Expected>> decltype(auto) result = fold_left_first(r.begin(), r.end(), f);
+    assert(result == expected);
+  }
+
+  {
+    auto telemetry                                              = invocable_telemetry();
+    auto f2                                                     = invocable_with_telemetry(f, telemetry);
+    std::same_as<std::optional<Expected>> decltype(auto) result = fold_left_first(r.begin(), r.end(), f2);
+    assert(result == expected);
+    if (result.has_value()) {
+      assert(telemetry.invocations == std::ranges::distance(r) - 1);
+      assert(telemetry.moves == 0);
+      assert(telemetry.copies == 1);
+    }
+  }
+}
+
 template <std::ranges::input_range R, class T, class F, std::equality_comparable Expected>
   requires std::copyable<R>
 constexpr void check_lvalue_range(R& r, T const& init, F f, Expected const& expected) {
@@ -186,26 +231,37 @@ constexpr void check(R r, T const& init, F f, Expected const& expected) {
   check_rvalue_range(r, init, f, expected);
 }
 
+template <std::ranges::input_range R, class F, std::equality_comparable Expected>
+  requires std::copyable<R>
+constexpr void check(R r, F f, std::optional<Expected> const& expected) {
+  check_iterator(r, f, expected);
+}
+
 constexpr void empty_range_test_case() {
   auto const data = std::vector<int>{};
   check(data, 100, std::plus(), 100);
   check(data, -100, std::multiplies(), -100);
+  check(data, std::plus(), std::optional<int>());
 
   check(data | std::views::take_while([](auto) { return false; }), 1.23, std::plus(), 1.23);
   check(data, Integer(52), &Integer::plus, Integer(52));
+  check(data | std::views::take_while([](auto) { return false; }), std::plus(), std::optional<int>());
 }
 
 constexpr void common_range_test_case() {
   auto const data = std::vector<int>{1, 2, 3, 4};
   check(data, 0, std::plus(), triangular_sum(data));
   check(data, 1, std::multiplies(), factorial(data.back()));
+  check(data, std::plus(), std::optional(triangular_sum(data)));
+  check(data, std::multiplies(), std::optional(factorial(data.back())));
 
   auto multiply_with_prev = [n = 1](auto const x, auto const y) mutable {
     auto const result = x * y * n;
     n                 = y;
     return static_cast<std::size_t>(result);
   };
   check(data, 1, multiply_with_prev, factorial(data.size()) * factorial(data.size() - 1));
+  check(data, multiply_with_prev, std::optional(factorial(data.size()) * factorial(data.size() - 1)));
 
   auto fib = [n = 1](auto x, auto) mutable {
     auto old_x = x;
@@ -237,6 +293,7 @@ constexpr void non_common_range_test_case() {
     auto data  = std::vector<std::string>{"five", "three", "two", "six", "one", "four"};
     auto range = data | std::views::transform(parse);
     check(range, 0, std::plus(), triangular_sum(range));
+    check(range, std::plus(), std::optional(triangular_sum(range)));
   }
 
   {
@@ -248,6 +305,7 @@ constexpr void non_common_range_test_case() {
     auto range =
         std::views::lazy_split(data, ' ') | std::views::transform(to_string_view) | std::views::transform(parse);
     check(range, 0, std::plus(), triangular_sum(range));
+    check(range, std::plus(), std::optional(triangular_sum(range)));
   }
 }