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✨ Add for_each_butlast and initial_medial_final
Problem: - There is no algorithm that does `for_each` on a range except for the last N elements. - There is a use case for a tripartite operation on a range: one operation for the initial element, another operation for each medial element, and a third operation for the final element. - The return values of `for_each` and `for_each_n` do not follow the law of Useful Return. Solution: - Add `for_each_butlastn`, `for_each_butlast`, and `initial_medial_final`. - Return all useful information from `for_each` and `for_each_n`.
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‎docs/algorithm.adoc

+53-2
Original file line numberDiff line numberDiff line change
@@ -14,23 +14,74 @@ variadic in its inputs.
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----
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template <typename InputIt, typename Operation, typename... InputItN>
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constexpr auto for_each(InputIt first, InputIt last,
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Operation op, InputItN... first_n) -> Operation;
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Operation op, InputItN... first_n)
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-> for_each_result<Operation, InputItN...>;
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----
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The return value is equivalent to a `tuple<Operation, InputItN...>`.
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In C\\++20 and later this is a `stdx::tuple`, in C++17 a `std::tuple`.
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NOTE: `stdx::for_each` is `constexpr` in C++20 and later, because it uses
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https://en.cppreference.com/w/cpp/utility/functional/invoke[`std::invoke`].
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`stdx::for_each_n` is just like `stdx::for_each`, but instead of taking two
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iterators to delimit the input range, it takes an iterator and size.
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Correspondingly, its return value includes the advanced `InputIt`.
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[source,cpp]
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----
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template <typename InputIt, typename Size, typename Operation,
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typename... InputItN>
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constexpr auto for_each_n(InputIt first, Size n,
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Operation op, InputItN... first_n) -> Operation;
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Operation op, InputItN... first_n);
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-> for_each_result<Operation, InputIt, InputItN...>;
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----
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=== `for_each_butlast`, `for_each_butlastn`
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`stdx::for_each_butlast` is like `for_each` but omits the last element of
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each range.
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[source,cpp]
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----
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template <typename FwdIt, typename Operation,
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typename... FwdItN>
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constexpr auto for_each_butlast(FwdIt first, FwdIt last,
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Operation op, FwdItN... first_n)
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-> for_each_result<Operation, FwdIt, FwdItN...>;
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----
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`stdx::for_each_butlastn` omits the last `n` elements of each range.
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[source,cpp]
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----
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template <typename FwdIt, typename Size, typename Operation,
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typename... FwdItN>
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constexpr auto for_each_butlastn(FwdIt first, FwdIt last, N n,
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Operation op, FwdItN... first_n)
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-> for_each_result<Operation, FwdIt, FwdItN...>;
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----
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NOTE: `for_each_butlast` and `for_each_butlastn` are defined for forward
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iterators (or stronger) only.
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=== `initial_medial_final`
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`initial_medial_final` iterates over a range, calling one function
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for the initial element, another function for each of the medial elements, and a third
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function for the final element.
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[source,cpp]
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----
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template <typename FwdIt, typename IOp, typename MOp, typename FOp>
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CONSTEXPR_INVOKE auto initial_medial_final(FwdIt first, FwdIt last, IOp iop,
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MOp mop, FOp fop)
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-> for_each_result<IOp, MOp, FOp>;
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----
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If the range is only two elements, there are no medial elements, so `mop` is not
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called. If the range is only one element, `fop` is not called. And if the range
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is empty, no functions are called.
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=== `transform` and `transform_n`
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‎include/stdx/algorithm.hpp

+71-4
Original file line numberDiff line numberDiff line change
@@ -1,11 +1,13 @@
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#pragma once
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#include <functional>
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#include <iterator>
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#if __cplusplus >= 202002L
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#include <stdx/tuple.hpp>
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#else
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#include <tuple>
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#endif
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#include <type_traits>
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namespace stdx {
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inline namespace v1 {
@@ -48,25 +50,90 @@ CONSTEXPR_INVOKE auto transform_n(InputIt first, Size n, OutputIt d_first,
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return {d_first, first, first_n...};
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}
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template <typename Op, typename... Is>
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using for_each_result = detail::result_tuple_t<Op, Is...>;
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template <typename InputIt, typename Operation, typename... InputItN>
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CONSTEXPR_INVOKE auto for_each(InputIt first, InputIt last, Operation op,
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InputItN... first_n) -> Operation {
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InputItN... first_n)
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-> for_each_result<Operation, InputItN...> {
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while (first != last) {
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std::invoke(op, *first, *first_n...);
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static_cast<void>(++first), (static_cast<void>(++first_n), ...);
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}
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return op;
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return {op, first_n...};
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}
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template <typename InputIt, typename Size, typename Operation,
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typename... InputItN>
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CONSTEXPR_INVOKE auto for_each_n(InputIt first, Size n, Operation op,
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InputItN... first_n) -> Operation {
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InputItN... first_n)
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-> for_each_result<Operation, InputIt, InputItN...> {
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while (n-- > 0) {
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std::invoke(op, *first, *first_n...);
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static_cast<void>(++first), (static_cast<void>(++first_n), ...);
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}
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return op;
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return {op, first, first_n...};
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}
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namespace detail {
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template <typename FwdIt, typename N, typename Operation, typename... FwdItN>
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CONSTEXPR_INVOKE auto for_each_butlastn(std::forward_iterator_tag, FwdIt first,
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FwdIt last, N n, Operation op,
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FwdItN... first_n)
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-> for_each_result<Operation, FwdIt, FwdItN...> {
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auto adv_it = first;
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for (auto i = N{}; i < n; ++i) {
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if (adv_it == last) {
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break;
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}
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++adv_it;
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}
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while (adv_it != last) {
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std::invoke(op, *first, *first_n...);
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static_cast<void>(++first), (static_cast<void>(++first_n), ...);
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++adv_it;
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}
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return {op, first, first_n...};
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}
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template <typename RandIt, typename N, typename Operation, typename... RandItN>
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CONSTEXPR_INVOKE auto for_each_butlastn(std::random_access_iterator_tag,
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RandIt first, RandIt last, N n,
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Operation op, RandItN... first_n) {
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auto const sz = std::distance(first, last);
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return for_each_n(first, sz - static_cast<decltype(sz)>(n), op, first_n...);
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}
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} // namespace detail
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template <typename FwdIt, typename N, typename Operation, typename... FwdItN>
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CONSTEXPR_INVOKE auto for_each_butlastn(FwdIt first, FwdIt last, N n,
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Operation op, FwdItN... first_n) {
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return detail::for_each_butlastn(
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typename std::iterator_traits<FwdIt>::iterator_category{}, first, last,
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n, op, first_n...);
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}
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template <typename FwdIt, typename Operation, typename... FwdItN>
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CONSTEXPR_INVOKE auto for_each_butlast(FwdIt first, FwdIt last, Operation op,
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FwdItN... first_n) {
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return for_each_butlastn(first, last, 1, op, first_n...);
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}
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template <typename FwdIt, typename IOp, typename MOp, typename FOp>
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CONSTEXPR_INVOKE auto initial_medial_final(FwdIt first, FwdIt last, IOp iop,
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MOp mop, FOp fop)
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-> for_each_result<IOp, MOp, FOp> {
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if (first != last) {
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iop(*first);
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auto [op, it] = for_each_butlast(++first, last, mop);
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if (it != last) {
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fop(*it);
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}
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return {iop, op, fop};
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}
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return {iop, mop, fop};
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}
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#undef CONSTEXPR_INVOKE

‎include/stdx/tuple.hpp

+3-4
Original file line numberDiff line numberDiff line change
@@ -105,11 +105,10 @@ template <std::size_t Index, typename T, typename... Ts> struct element {
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T value;
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private:
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[[nodiscard]] friend constexpr auto operator==(element const &,
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element const &)
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[[nodiscard]] friend auto operator==(element const &x, element const &y)
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-> bool = default;
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[[nodiscard]] friend constexpr auto operator<=>(element const &,
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element const &) = default;
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[[nodiscard]] friend auto operator<=>(element const &,
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element const &) = default;
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};
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template <typename Op, typename Value> struct fold_helper {

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