ksys/util: Add range() and indexIter() for convenience

- range() is similar to Python's range()

- indexIter() can be used to simplify `for (int i = 0, n = c.size(); i < n; ++i)`
  index-based loops

src/KingSystem/Utils/CMakeLists.txt

@@ -78,6 +78,7 @@ target_sources(uking PRIVATE
   HeapUtil.cpp
   HeapUtil.h
   InitTimeInfo.h
+  IteratorUtil.h
   ParamIO.cpp
   ParamIO.h
   SafeDelete.h

src/KingSystem/Utils/IteratorUtil.h

@@ -0,0 +1,130 @@
+#pragma once
+
+#include <type_traits>
+
+namespace ksys::util {
+
+template <typename IntType>
+class IntRange {
+public:
+    static_assert(std::is_integral_v<IntType>, "T is not a integer type");
+
+    class Iterator {
+    public:
+        constexpr explicit Iterator(IntType value) : mValue(value) {}
+
+        constexpr bool operator==(Iterator rhs) const { return mValue == rhs.mValue; }
+        constexpr bool operator!=(Iterator rhs) const { return !operator==(rhs); }
+
+        constexpr IntType operator*() const { return mValue; }
+
+        constexpr Iterator& operator++() {
+            ++mValue;
+            return *this;
+        }
+
+        constexpr Iterator operator++(int) const { return Iterator(mValue + 1); }
+
+    private:
+        IntType mValue;
+    };
+
+    constexpr IntRange(IntType begin, IntType end) : mBegin(begin), mEnd(end) {}
+
+    constexpr Iterator begin() const { return mBegin; }
+    constexpr Iterator end() const { return mEnd; }
+    constexpr Iterator cbegin() const { return mBegin; }
+    constexpr Iterator cend() const { return mEnd; }
+
+private:
+    Iterator mBegin;
+    Iterator mEnd;
+};
+
+/// Range [begin, end) with a step of 1.
+template <typename IntType>
+constexpr IntRange<IntType> range(IntType begin, IntType end) {
+    return {begin, end};
+}
+
+/// Range [0, end) with a step of 1.
+template <typename IntType>
+constexpr auto range(IntType end) {
+    return IntRange<IntType>{0, end};
+}
+
+/// Use this as a replacement for `for (int i = 0, n = c.size(); i < n; ++i)` loops.
+///
+/// @warning The even better option is to use the container's own iterators,
+/// as index-based loops can result in useless bounds checks at every iteration.
+template <typename Container>
+class IndexRange {
+public:
+    class ElementProxy {
+    public:
+        constexpr ElementProxy(Container& container, int index)
+            : mIndex(index), mContainer(container) {}
+
+        int getIndex() const { return mIndex; }
+        constexpr auto& get() const { return mContainer[mIndex]; }
+        constexpr auto& operator*() const { return get(); }
+        constexpr auto* operator->() const { return &get(); }
+
+    private:
+        int mIndex;
+        Container& mContainer;
+    };
+
+    class Iterator {
+    public:
+        constexpr Iterator(Container& container, int index)
+            : mIndex(index), mContainer(container) {}
+
+        constexpr bool operator==(Iterator rhs) const { return mIndex == rhs.mIndex; }
+        constexpr bool operator!=(Iterator rhs) const { return !operator==(rhs); }
+
+        int getIndex() const { return mIndex; }
+        constexpr ElementProxy operator*() const { return ElementProxy(mContainer, mIndex); }
+
+        constexpr Iterator& operator++() {
+            ++mIndex;
+            return *this;
+        }
+
+        constexpr Iterator operator++(int) const {
+            auto copy = *this;
+            ++copy;
+            return copy;
+        }
+
+    private:
+        int mIndex;
+        Container& mContainer;
+    };
+
+    class EndIterator {
+    public:
+        constexpr explicit EndIterator(int index) : mIndex(index) {}
+        constexpr bool notEnd(Iterator it) const { return it.getIndex() < mIndex; }
+        friend constexpr bool operator!=(EndIterator end, Iterator it) { return end.notEnd(it); }
+        friend constexpr bool operator!=(Iterator it, EndIterator end) { return end.notEnd(it); }
+
+    private:
+        int mIndex;
+    };
+
+    constexpr explicit IndexRange(Container& container) : mContainer(container) {}
+
+    constexpr Iterator begin() const { return Iterator(mContainer, 0); }
+    constexpr EndIterator end() const { return EndIterator(mContainer.size()); }
+
+private:
+    Container& mContainer;
+};
+
+template <typename Container>
+constexpr auto indexIter(Container& container) {
+    return IndexRange<Container>{container};
+}
+
+}  // namespace ksys::util