From a9672d41155c0d5a541491f4b229a720c3eaf9e6 Mon Sep 17 00:00:00 2001
From: WiktorNowak <wnowak2f@gmail.com>
Date: Tue, 14 Jan 2025 08:02:51 +0100
Subject: [PATCH] feat: add skip to iterator interface

---
 include/rusty_iterators/interface.hpp | 10 +++++
 include/rusty_iterators/skip.hpp      | 50 +++++++++++++++++++++++
 tests/skip.test.cpp                   | 57 +++++++++++++++++++++++++++
 3 files changed, 117 insertions(+)
 create mode 100644 include/rusty_iterators/skip.hpp
 create mode 100644 tests/skip.test.cpp

diff --git a/include/rusty_iterators/interface.hpp b/include/rusty_iterators/interface.hpp
index f75b1a0..634123e 100644
--- a/include/rusty_iterators/interface.hpp
+++ b/include/rusty_iterators/interface.hpp
@@ -9,6 +9,7 @@
 #include "inspect.hpp"
 #include "map.hpp"
 #include "moving_window.hpp"
+#include "skip.hpp"
 #include "take.hpp"
 #include "zip.hpp"
 
@@ -45,6 +46,7 @@ using iterator::FilterMap;
 using iterator::Inspect;
 using iterator::Map;
 using iterator::MovingWindow;
+using iterator::Skip;
 using iterator::Take;
 using iterator::Zip;
 
@@ -149,6 +151,8 @@ class IterInterface
         requires ReduceFunctor<T, Functor>
     [[nodiscard]] auto reduce(Functor&& f) -> std::optional<T>;
 
+    [[nodiscard]] auto skip(size_t n) -> Skip<T, Derived>;
+
     template <class R = T>
         requires Summable<R>
     [[nodiscard]] auto sum() -> R;
@@ -443,6 +447,12 @@ auto rusty_iterators::interface::IterInterface<T, Derived>::reduce(Functor&& f)
     return fold(std::move(first.value()), std::forward<Functor>(f));
 }
 
+template <class T, class Derived>
+auto rusty_iterators::interface::IterInterface<T, Derived>::skip(size_t n) -> Skip<T, Derived>
+{
+    return Skip<T, Derived>{std::forward<Derived>(self()), n};
+}
+
 template <class T, class Derived>
 template <class R>
     requires rusty_iterators::concepts::Summable<R>
diff --git a/include/rusty_iterators/skip.hpp b/include/rusty_iterators/skip.hpp
new file mode 100644
index 0000000..6805015
--- /dev/null
+++ b/include/rusty_iterators/skip.hpp
@@ -0,0 +1,50 @@
+#pragma once
+
+#include "interface.fwd.hpp"
+
+#include <algorithm>
+#include <optional>
+
+namespace rusty_iterators::iterator
+{
+using interface::IterInterface;
+
+template <class T, class Other>
+class Skip : public IterInterface<T, Skip<T, Other>>
+{
+  public:
+    explicit Skip(Other&& it, size_t n) : it(std::forward<Other>(it)), n(n) {}
+
+    auto next() -> std::optional<T>;
+    [[nodiscard]] auto sizeHint() const -> std::optional<size_t>;
+
+  private:
+    Other it;
+    size_t n;
+};
+} // namespace rusty_iterators::iterator
+
+template <class T, class Other>
+auto rusty_iterators::iterator::Skip<T, Other>::next() -> std::optional<T>
+{
+    [[unlikely]] if (n > 0)
+    {
+        it = it.advanceBy(n);
+        n  = 0;
+        return std::move(it.next());
+    }
+    return std::move(it.next());
+}
+
+template <class T, class Other>
+auto rusty_iterators::iterator::Skip<T, Other>::sizeHint() const -> std::optional<size_t>
+{
+    auto underlyingSize = it.sizeHint();
+
+    if (!underlyingSize.has_value())
+    {
+        return std::nullopt;
+    }
+    int32_t estimatedSize = underlyingSize.value() - n;
+    return std::max(0, estimatedSize);
+}
diff --git a/tests/skip.test.cpp b/tests/skip.test.cpp
new file mode 100644
index 0000000..de6109a
--- /dev/null
+++ b/tests/skip.test.cpp
@@ -0,0 +1,57 @@
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <rusty_iterators/iterator.hpp>
+
+using ::rusty_iterators::iterator::LazyIterator;
+using ::testing::ElementsAreArray;
+
+TEST(TestSkipIterator, NextSkipsElements)
+{
+    auto vec = std::vector{1, 2, 3, 4, 5};
+    auto it  = LazyIterator{vec}.skip(3);
+
+    ASSERT_EQ(it.next(), 4);
+    ASSERT_EQ(it.next(), 5);
+    ASSERT_EQ(it.next(), std::nullopt);
+}
+
+TEST(TestSkipIterator, NextOnTooSmallIterator)
+{
+    auto vec = std::vector{1, 2, 3};
+    auto it  = LazyIterator{vec}.skip(5);
+
+    ASSERT_EQ(it.next(), std::nullopt);
+}
+
+TEST(TestSkipIterator, CollectAllItemsExceptSkippedOnes)
+{
+    auto vec = std::vector{1, 2, 3, 4, 5};
+    auto it  = LazyIterator{vec}.skip(3);
+
+    EXPECT_THAT(it.collect(), ElementsAreArray({4, 5}));
+}
+
+TEST(TestSkipIterator, SizeHintGivesEstimatedSize)
+{
+    auto vec = std::vector{1, 2, 3, 4, 5};
+    auto it  = LazyIterator{vec}.skip(3);
+
+    ASSERT_EQ(it.sizeHint(), 2);
+}
+
+TEST(TestSkipIterator, SizeHintOnInfiniteIterator)
+{
+    auto vec = std::vector{1, 2, 3};
+    auto it  = LazyIterator{vec}.cycle().skip(3);
+
+    ASSERT_EQ(it.sizeHint(), std::nullopt);
+}
+
+TEST(TestSkipIterator, TestSizeHintOnTooSmallIterator)
+{
+    auto vec = std::vector{1, 2, 3};
+    auto it  = LazyIterator{vec}.skip(10);
+
+    ASSERT_EQ(it.sizeHint(), 0);
+}