Use C++20 three-way comparison everywhere

benchmark/sort_benchmark.cpp

@@ -72,7 +72,11 @@ int main()
         auto bench = an::Bench().relative(true).minEpochIterations(10);
 
         test_sort("random doubles (std)", std::ranges::sort, vec, bench);
-        test_sort("random doubles (flux)", flux::sort, vec, bench);
+        // Use a custom comparator because we know we don't have NaNs
+        auto custom_sort = [](auto& arg) {
+            return flux::sort(arg, flux::cmp::compare_floating_point_unchecked);
+        };
+        test_sort("random doubles (flux)", custom_sort, vec, bench);
     }
 
     {

docs/reference/adaptors.rst

@@ -1406,8 +1406,8 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
 ^^^^^^^^^^^^^^^^^^
 
 ..  function::
-    template <sequence Seq1, sequence Seq2, typename Cmp = std::ranges::less> \
-        requires strict_weak_order_for<Cmp, Seq1> && strict_weak_order_for<Cmp, Seq2> \
+    template <sequence Seq1, sequence Seq2, typename Cmp = std::compare_three_way> \
+        requires weak_ordering_for<Cmp, Seq1> && weak_ordering_for<Cmp, Seq2> \
     auto set_difference(Seq1 seq1, Seq2 seq2, Cmp cmp = {}) -> sequence auto;
 
     Returns a sequence adaptor which yields the set difference of the two input sequences :var:`seq1` and :var:`seq2`, ordered by the given comparison function :var:`cmp`.
@@ -1418,7 +1418,7 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
 
     :param seq1: The first sorted sequence.
     :param seq2: The second sorted sequence.
-    :param cmp: A binary predicate that takes two elements as arguments and returns true if the first element is less than the second.
+    :param cmp: A binary comparator whose return type is convertible to :type:`std::weak_ordering`. Both sequences must be sorted with respect to this comparator.
 
     :returns: A sequence adaptor that yields those elements of `seq1` which do not also appear in `seq2`.
 
@@ -1466,8 +1466,8 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
 ^^^^^^^^^^^^^^^^^^^^
 
 ..  function::
-    template <sequence Seq1, sequence Seq2, typename Cmp = std::ranges::less> \
-        requires strict_weak_order_for<Cmp, Seq1> && strict_weak_order_for<Cmp, Seq2> \
+    template <sequence Seq1, sequence Seq2, typename Cmp = std::compare_three_way> \
+        requires weak_ordering_for<Cmp, Seq1> && weak_ordering_for<Cmp, Seq2> \
     auto set_intersection(Seq1 seq1, Seq2 seq2, Cmp cmp = {}) -> sequence auto;
 
     Returns a sequence adaptor which yields the set intersection of the two input sequences :var:`seq1` and :var:`seq2`, ordered by the given comparison function :var:`cmp`.
@@ -1478,7 +1478,7 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
 
     :param seq1: The first sorted sequence.
     :param seq2: The second sorted sequence.
-    :param cmp: A binary predicate that takes two elements as arguments and returns true if the first element is less than the second.
+    :param cmp: A binary comparator whose return type is convertible to :type:`std::weak_ordering`. Both sequences must be sorted with respect to this comparator.
 
     :returns: A sequence adaptor that represents the set intersection of the two input sequences.
 
@@ -1526,7 +1526,7 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ..  function::
-    template <sequence Seq1, sequence Seq2, typename Cmp = std::ranges::less> \
+    template <sequence Seq1, sequence Seq2, typename Cmp = std::compare_three_way> \
         requires see_below \
     auto set_symmetric_difference(Seq1 seq1, Seq2 seq2, Cmp cmp = {}) -> sequence auto;
 
@@ -1546,11 +1546,13 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
             std::common_reference_with<element_t<Seq1>, element_t<Seq2>> &&
             std::common_reference_with<rvalue_element_t<Seq1>, rvalue_element_t<Seq2>> &&
             requires { typename std::common_type_t<value_t<Seq1>, value_t<Seq2>>; } &&
-            strict_weak_order_for<Cmp, Seq1> &&
-            strict_weak_order_for<Cmp, Seq2>
+            weak_ordering_for<Cmp, Seq1> &&
+            weak_ordering_for<Cmp, Seq2>
 
     :param seq1: The first sequence to merge.
     :param seq2: The second sequence to merge.
+    :param cmp: A binary comparator whose return type is convertible to :type:`std::weak_ordering`. Both sequences must be sorted with respect to this comparator.
+
     :returns: A sequence adaptor that yields elements of `seq1` and `seq2` which do not appear in both sequences.
 
     :models:
@@ -1597,7 +1599,7 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
 ^^^^^^^^^^^^^
 
 ..  function::
-    template <sequence Seq1, sequence Seq2, typename Cmp = std::ranges::less> \
+    template <sequence Seq1, sequence Seq2, typename Cmp = std::compare_three_way> \
         requires see_below \
     auto set_union(Seq1 seq1, Seq2 seq2, Cmp cmp = {}) -> sequence auto;
 
@@ -1613,12 +1615,12 @@ You can pass a reference to a sequence into an adaptor using :func:`flux::ref` o
             std::common_reference_with<element_t<Seq1>, element_t<Seq2>> &&
             std::common_reference_with<rvalue_element_t<Seq1>, rvalue_element_t<Seq2>> &&
             requires { typename std::common_type_t<value_t<Seq1>, value_t<Seq2>>; } &&
-            strict_weak_order_for<Cmp, Seq1> &&
-            strict_weak_order_for<Cmp, Seq2>
+            weak_ordering_for<Cmp, Seq1> &&
+            weak_ordering_for<Cmp, Seq2>
 
     :param seq1: The first sorted sequence to merge.
     :param seq2: The second sorted sequence to merge.
-    :param cmp: A binary predicate that takes two elements as arguments and returns true if the first element is less than the second.
+    :param cmp: A binary comparator whose return type is convertible to :type:`std::weak_ordering`. Both sequences must be sorted with respect to this comparator.
 
     :returns: A sequence adaptor that represents the set union of the two input sequences.
 

docs/reference/algorithms.rst

@@ -349,7 +349,7 @@ Algorithms
 ------------
 
 ..  function::
-    template <multipass_sequence Seq, strict_weak_order_for<Seq> Cmp = std::ranges::less> \
+    template <multipass_sequence Seq, weak_ordering_for<Seq> Cmp = std::compare_three_way> \
     auto find_max(Seq&& seq, Cmp cmp = {}) -> cursor_t<Seq>;
 
     Returns a cursor to the maximum element of :var:`seq`, compared using :var:`cmp`.
@@ -359,7 +359,7 @@ Algorithms
     ..  note:: This behaviour differs from :func:`std::max_element()`, which returns an iterator to the *first* maximal element.
 
     :param seq: A multipass sequence
-    :param cmp: A comparator to use to find the maximum element, defaulting to :type:`std::ranges::less`
+    :param cmp: A comparator to use to find the maximum element, defaulting to :type:`std::compare_three_way`
 
     :returns: A cursor pointing to the maximum element of :var:`seq`.
 
@@ -380,15 +380,15 @@ Algorithms
 ------------
 
 ..  function::
-    template <multipass_sequence Seq, strict_weak_order_for<Seq> Cmp = std::ranges::less> \
+    template <multipass_sequence Seq, weak_ordering_for<Seq> Cmp = std::compare_three_way> \
     auto find_min(Seq&& seq, Cmp cmp = {}) -> cursor_t<Seq>;
 
     Returns a cursor to the minimum element of :var:`seq`, compared using :var:`cmp`.
 
     If several elements are equally minimal, :func:`find_min` returns a cursor to the **first** such element.
 
     :param seq: A multipass sequence
-    :param cmp: A comparator to use to find the minimum element, defaulting to :type:`std::ranges::less`
+    :param cmp: A comparator to use to find the minimum element, defaulting to :type:`std::compare_three_way`
 
     :returns: A cursor pointing to the minimum element of :var:`seq`.
 
@@ -409,7 +409,7 @@ Algorithms
 ---------------
 
 ..  function::
-    template <multipass_sequence Seq, strict_weak_order_for<Seq> Cmp = std::ranges::less> \
+    template <multipass_sequence Seq, weak_ordering_for<Seq> Cmp = std::compare_three_way> \
     auto find_minmax(Seq&& seq, Cmp cmp = {}) -> minmax_result<cursor_t<Seq>>;
 
     Returns a pair of cursors to the minimum and maximum elements of :var:`seq`, compared using :var:`cmp`.
@@ -424,7 +424,7 @@ Algorithms
     but only does a single pass over :var:`seq`.
 
     :param seq: A multipass sequence
-    :param cmp: A comparator to use to find the maximum element, defaulting to :type:`std::ranges::less`
+    :param cmp: A comparator to use to find the maximum element, defaulting to :type:`std::compare_three_way`
 
     :returns: A cursor pointing to the maximum element of :var:`seq`.
 
@@ -489,15 +489,14 @@ Algorithms
 -------
 
 ..  function::
-    template <sequence Seq, typename Cmp = std::ranges::less> \
-        requires std::predicate<Cmp&, value_t<Seq>, element_t<Seq>> \
+    template <sequence Seq, weak_ordering_for<Seq> Cmp = std::compare_three_way> \
     auto max(Seq&& seq, Cmp cmp = {}) -> optional<value_t<Seq>>;
 
 ``min``
 -------
 
 ..  function::
-    template <sequence Seq, typename Cmp = std::ranges::less> \
+    template <sequence Seq, weak_ordering_for<Seq> Cmp = std::compare_three_way> \
         requires std::predicate<Cmp&, value_t<Seq>, element_t<Seq>> \
     auto min(Seq&& seq, Cmp cmp = {}) -> optional<value_t<Seq>>;
 
@@ -507,7 +506,7 @@ Algorithms
 ..  struct:: template <typename T> minmax_result;
 
 ..  function::
-    template <sequence Seq, typename Cmp = std::ranges::less> \
+    template <sequence Seq, weak_ordering_for<Seq> Cmp = std::compare_three_way> \
         requires std::predicate<Cmp&, value_t<Seq>, element_t<Seq>> \
     auto minmax(Seq&& seq, Cmp cmp = {}) -> optional<minmax_result<Seq>>;
 
@@ -547,7 +546,7 @@ Algorithms
 --------
 
 ..  function::
-    template <random_access_sequence Seq, typename Cmp = std::ranges::less> \
+    template <random_access_sequence Seq, typename Cmp = std::compare_three_way> \
         requires see_below \
     auto sort(Seq&& seq, Cmp cmp = {}) -> void;
 

docs/reference/concepts.rst

@@ -348,3 +348,88 @@ Concepts
 
 ..  concept::
     template <typename Seq, typename T> writable_sequence_of
+
+    A sequence :var:`Seq` models :expr:`writable_sequence_t<Seq, T>` for a type :var:`T` if :expr:`element_t<Seq>` is assignable from an object of type :var:`T`.
+
+    The :concept:`writable_sequence_of` concept is defined as::
+
+        template <typename Seq, typename T>
+        concept writable_sequence_of =
+            sequence<Seq> &&
+            requires (element_t<Seq> e, T&& item) {
+                { e = std::forward<T>(item) } -> std::same_as<element_t<Seq>&>;
+            };
+
+``element_swappable_with``
+--------------------------
+
+..  concept::
+    template <typename Seq1, typename Seq2> element_swappable_with
+
+    A pair of sequences :var:`Seq1` and :var:`Seq2` model :concept:`element_swappable_with` if their respective elements can be swapped, that is, we can assign to an element of :var:`Seq1` from an rvalue element of :var:`Seq2` and vice-versa.
+
+    Formally, the :concept:`element_swappable_with` concept is defined as::
+
+        template <typename Seq1, typename Seq2>
+        concept element_swappable_with =
+            std::constructible_from<value_t<Seq1>, rvalue_element_t<Seq1>> &&
+            std::constructible_from<value_t<Seq2>, rvalue_element_t<Seq2>> &&
+            writable_sequence_of<Seq1, rvalue_element_t<Seq2>> &&
+            writable_sequence_of<Seq1, value_t<Seq2>&&> &&
+            writable_sequence_of<Seq2, rvalue_element_t<Seq1>> &&
+            writable_sequence_of<Seq2, value_t<Seq1>&&>;
+
+
+``ordering_invocable``
+----------------------
+
+..  concept::
+    template <typename Fn, typename T, typename U, typename Cat = std::partial_ordering> \
+    ordering_invocable
+
+    The concept :concept:`ordering_invocable` signifies that the binary invocable :var:`Fn` return a value of one of the standard comparison categories, convertible to :var:`Cat`, for all combinations of arguments of types :var:`T` and :var:`U`
+
+    Semantic requirements:
+
+    * Let :expr:`r1 = fn(a, b)` and :expr:`r2 = fn(b, c)`. If :expr:`r1 == r2` and :expr:`r1 != std::partial_ordering::unordered`, then :expr:`fn(a, c) == r1`.
+    * :expr:`fn(a, b) == std::partial_ordering::less` if and only if :expr:`fn(b, a) == std::partial_ordering::greater`
+
+    The :concept:`ordering_invocable` concept is defined as::
+
+        template <typename Fn, typename T, typename U, typename Cat>
+        concept ordering_invocable_ = // exposition-only
+            std::regular_invocable<Fn, T, U> &&
+            std::same_as<
+                std::common_comparison_category_t<std::decay_t<std::invoke_result_t<Fn, T, U>>>,
+                                                  Cat>,
+                Cat>;
+
+        template <typename Fn, typename T, typename U, typename Cat = std::partial_ordering>
+        concept ordering_invocable =
+            ordering_invocable_<Fn, T, U, Cat> &&
+            ordering_invocable_<Fn, U, T, Cat> &&
+            ordering_invocable_<Fn, T, T, Cat> &&
+            ordering_invocable_<Fn, U, U, Cat>;
+
+
+``weak_ordering_for``
+----------------------------
+
+..  concept::
+    template <typename Fn, typename Seq1, typename Seq2 = Seq1> \
+    weak_ordering_for
+
+    Signifies that a binary callable :var:`Fn` forms a strict weak order over the elements of sequences :var:`Seq1` and :var:`Seq2`.
+
+    It is defined as::
+
+        template <typename Fn, typename Seq1, typename Seq2 = Seq1>
+        concept weak_ordering_for =
+            sequence<Seq1> &&
+            sequence<Seq2> &&
+            ordering_invocable<Fn&, element_t<Seq1>, element_t<Seq2>, std::weak_ordering> &&
+            ordering_invocable<Fn&, value_t<Seq1>&, element_t<Seq2>, std::weak_ordering> &&
+            ordering_invocable<Fn&, element_t<Seq1>, value_t<Seq2>&, std::weak_ordering> &&
+            ordering_invocable<Fn&, value_t<Seq1>&, value_t<Seq2>&, std::weak_ordering> &&
+            ordering_invocable<Fn&, common_element_t<Seq1>, common_element_t<Seq2>, std::weak_ordering>;
+

example/docs/find_max.cpp

@@ -21,7 +21,7 @@ int main()
     };
 
     // Get a cursor to the maximum of the people vector, according to age
-    auto max_cur = flux::find_max(people, flux::proj(std::less{}, &Person::age));
+    auto max_cur = flux::find_max(people, flux::proj(flux::cmp::compare, &Person::age));
 
     // The oldest person is 63
     assert(flux::read_at(people, max_cur).age == 63);

example/docs/find_min.cpp

@@ -21,7 +21,7 @@ int main()
     };
 
     // Get a cursor to the maximum of the people vector, according to age
-    auto min_cur = flux::find_min(people, flux::proj(std::less{}, &Person::age));
+    auto min_cur = flux::find_min(people, flux::proj(flux::cmp::compare, &Person::age));
 
     // The youngest person is 29
     assert(flux::read_at(people, min_cur).age == 29);

example/docs/find_minmax.cpp

@@ -22,7 +22,7 @@ int main()
 
     // find_minmax() returns a pair of cursors which we can destructure
     // Here we'll get the min and max of the people vector, according to age
-    auto [min, max] = flux::find_minmax(people, flux::proj(std::less{}, &Person::age));
+    auto [min, max] = flux::find_minmax(people, flux::proj(flux::cmp::compare, &Person::age));
 
     // The "minimum" is Chris. Dani is the same age, but Chris appears earlier
     // in the sequence

example/merge_intervals.cpp

@@ -27,7 +27,7 @@ bool is_overlapped(interval_t a, interval_t b)
 auto merge = [](flux::sequence auto seq) -> interval_t
 {
     auto begin = flux::front(seq)->begin;
-    auto end = flux::max(seq, flux::proj(std::less<>{}, &interval_t::end))->end;
+    auto end = flux::max(seq, flux::proj(flux::cmp::compare, &interval_t::end))->end;
     return {begin, end};
 };
 
@@ -36,7 +36,7 @@ int main()
     std::vector<interval_t> intervals = {{2, 4}, {7, 9}, {11, 13}, {6, 7}, {0, 3}};
 
     // sort intervals according to begin
-    flux::sort(intervals, flux::proj(std::less{}, &interval_t::begin));
+    flux::sort(intervals, flux::proj(flux::cmp::compare, &interval_t::begin));
 
     flux::ref(intervals)
          .chunk_by(is_overlapped)

include/flux/core/default_impls.hpp

@@ -6,6 +6,7 @@
 #ifndef FLUX_CORE_DEFAULT_IMPLS_HPP_INCLUDED
 #define FLUX_CORE_DEFAULT_IMPLS_HPP_INCLUDED
 
+#include <flux/core/numeric.hpp>
 #include <flux/core/sequence_access.hpp>
 
 #include <functional>