intermediate changes

ref:cde9a383711a11544ce7e107a78147fb96cc4029

1 files changed, 136 insertions, 0 deletions

diff --git a/library/cpp/iterator/concatenate.h b/library/cpp/iterator/concatenate.h
new file mode 100644
index 0000000000..64d2cd451a
--- /dev/null
+++ b/library/cpp/iterator/concatenate.h
@@ -0,0 +1,136 @@
+#pragma once
+
+#include <util/generic/store_policy.h>
+
+#include <iterator>
+#include <tuple>
+
+
+namespace NPrivate {
+
+    template <typename TValue_, typename... TContainers>
+    struct TConcatenator {
+        template <std::size_t... I>
+        struct TConcatenatorWithIndex {
+        private:
+            using THolders = std::tuple<TAutoEmbedOrPtrPolicy<TContainers>...>;
+            using TValue = TValue_;
+            using TIteratorState = std::tuple<decltype(std::begin(std::declval<TContainers&>()))...>;
+            using TSentinelState = std::tuple<decltype(std::end(std::declval<TContainers&>()))...>;
+
+            struct TIterator;
+            struct TSentinelCandidate {
+                TSentinelState Iterators_;
+                std::size_t Position_;
+                THolders* HoldersPtr_;
+            };
+            using TSentinel = std::conditional_t<std::is_same_v<TIteratorState, TSentinelState>,
+                                                 TIterator, TSentinelCandidate>;
+
+            struct TIterator {
+            private:
+                friend struct TConcatenatorWithIndex<I...>;
+
+                // important, that it is a static function, compiler better optimizes such code
+                template <std::size_t index = 0, typename TMaybeConstIteratorState>
+                static TValue GetCurrentValue(std::size_t position, TMaybeConstIteratorState& iterators) {
+                    if constexpr (index >= sizeof...(TContainers)) {
+                        // never happened when use of iterator is correct
+                        return *std::get<0>(iterators);
+                    } else {
+                        if (position == index) {
+                            return *std::get<index>(iterators);
+                        } else {
+                            return GetCurrentValue<index + 1>(position, iterators);
+                        }
+                    }
+                }
+
+                template <bool needIncrement, std::size_t index = 0>
+                void MaybeIncrementIteratorAndSkipExhaustedContainers() {
+                    if constexpr (index >= sizeof...(TContainers)) {
+                        return;
+                    } else {
+                        if (Position_ == index) {
+                            if constexpr (needIncrement) {
+                                ++std::get<index>(Iterators_);
+                            }
+                            if (!(std::get<index>(Iterators_) != std::end(*std::get<index>(*HoldersPtr_).Ptr()))) {
+                                ++Position_;
+                                MaybeIncrementIteratorAndSkipExhaustedContainers<false, index + 1>();
+                            }
+                        } else {
+                            MaybeIncrementIteratorAndSkipExhaustedContainers<needIncrement, index + 1>();
+                        }
+                    }
+                }
+            public:
+                using difference_type = std::ptrdiff_t;
+                using value_type = TValue;
+                using pointer = std::remove_reference_t<TValue>*;
+                using reference = std::remove_reference_t<TValue>&;
+                using iterator_category = std::input_iterator_tag;
+
+                TValue operator*() {
+                    return GetCurrentValue(Position_, Iterators_);
+                }
+                TValue operator*() const {
+                    return GetCurrentValue(Position_, Iterators_);
+                }
+                TIterator& operator++() {
+                    MaybeIncrementIteratorAndSkipExhaustedContainers<true>();
+                    return *this;
+                }
+                bool operator!=(const TSentinel& other) const {
+                    // give compiler an opportunity to optimize sentinel case (-70% of time)
+                    if (other.Position_ == sizeof...(TContainers)) {
+                        return Position_ < sizeof...(TContainers);
+                    } else {
+                        return (Position_ != other.Position_ ||
+                                ((std::get<I>(Iterators_) != std::get<I>(other.Iterators_)) || ...));
+                    }
+                }
+                bool operator==(const TSentinel& other) const {
+                    return !(*this != other);
+                }
+
+                TIteratorState Iterators_;
+                std::size_t Position_;
+                THolders* HoldersPtr_;
+            };
+        public:
+            using iterator = TIterator;
+            using const_iterator = TIterator;
+            using value_type = typename TIterator::value_type;
+            using reference = typename TIterator::reference;
+            using const_reference = typename TIterator::reference;
+
+            TIterator begin() const {
+                TIterator iterator{TIteratorState{std::begin(*std::get<I>(Holders_).Ptr())...}, 0, &Holders_};
+                iterator.template MaybeIncrementIteratorAndSkipExhaustedContainers<false>();
+                return iterator;
+            }
+
+            TSentinel end() const {
+                return {TSentinelState{std::end(*std::get<I>(Holders_).Ptr())...}, sizeof...(TContainers), &Holders_};
+            }
+
+            mutable THolders Holders_;
+        };
+
+        template <std::size_t... I>
+        static auto Concatenate(TContainers&&... containers, std::index_sequence<I...>) {
+            return TConcatenatorWithIndex<I...>{{std::forward<TContainers>(containers)...}};
+        }
+    };
+
+}
+
+
+//! Usage: for (auto x : Concatenate(a, b)) {...}
+template <typename TFirstContainer, typename... TContainers>
+auto Concatenate(TFirstContainer&& container, TContainers&&... containers) {
+    return NPrivate::TConcatenator<decltype(*std::begin(container)), TFirstContainer, TContainers...>::Concatenate(
+        std::forward<TFirstContainer>(container), std::forward<TContainers>(containers)...,
+        std::make_index_sequence<sizeof...(TContainers) + 1>{});
+}