Simplify some C++ abstractions (#1518)

* Remove namespaces
* Prefer `bool operator==`, not `friend auto operator==`
* Prefer not to use `using`
* Use a `constexpr` function instead of a template for `flipTable`

include/itertools.hpp

@@ -6,52 +6,46 @@
 #include <tuple>
 #include <utility>
 
-template<typename T>
-class EnumSeqIterator {
-	T _value;
-
-public:
-	explicit EnumSeqIterator(T value) : _value(value) {}
-
-	EnumSeqIterator &operator++() {
-		_value = (T)(_value + 1);
-		return *this;
-	}
-
-	auto operator*() const { return _value; }
-
-	friend auto operator==(EnumSeqIterator const &lhs, EnumSeqIterator const &rhs) {
-		return lhs._value == rhs._value;
-	}
-
-	friend auto operator!=(EnumSeqIterator const &lhs, EnumSeqIterator const &rhs) {
-		return lhs._value != rhs._value;
-	}
-};
-
 template<typename T>
 class EnumSeq {
 	T _start;
 	T _stop;
 
+	class Iterator {
+		T _value;
+
+	public:
+		explicit Iterator(T value) : _value(value) {}
+
+		Iterator &operator++() {
+			_value = (T)(_value + 1);
+			return *this;
+		}
+
+		auto operator*() const { return _value; }
+
+		bool operator==(Iterator const &rhs) const { return _value == rhs._value; }
+		bool operator!=(Iterator const &rhs) const  { return _value != rhs._value; }
+	};
+
 public:
 	explicit EnumSeq(T stop) : _start((T)0), _stop(stop) {}
 	explicit EnumSeq(T start, T stop) : _start(start), _stop(stop) {}
 
-	EnumSeqIterator<T> begin() { return EnumSeqIterator(_start); }
-	EnumSeqIterator<T> end() { return EnumSeqIterator(_stop); }
+	Iterator begin() { return Iterator(_start); }
+	Iterator end() { return Iterator(_stop); }
 };
 
 // This is not a fully generic implementation; its current use cases only require for-loop behavior.
 // We also assume that all iterators have the same length.
-template<typename... Iters>
-class Zip {
-	std::tuple<Iters...> _iters;
+template<typename... Ts>
+class ZipIterator {
+	std::tuple<Ts...> _iters;
 
 public:
-	explicit Zip(std::tuple<Iters...> &&iters) : _iters(iters) {}
+	explicit ZipIterator(std::tuple<Ts...> &&iters) : _iters(iters) {}
 
-	Zip &operator++() {
+	ZipIterator &operator++() {
 		std::apply([](auto &&...it) { (++it, ...); }, _iters);
 		return *this;
 	}
@@ -62,26 +56,24 @@ public:
 		);
 	}
 
-	friend auto operator==(Zip const &lhs, Zip const &rhs) {
-		return std::get<0>(lhs._iters) == std::get<0>(rhs._iters);
+	bool operator==(ZipIterator const &rhs) const {
+		return std::get<0>(_iters) == std::get<0>(rhs._iters);
 	}
 
-	friend auto operator!=(Zip const &lhs, Zip const &rhs) {
-		return std::get<0>(lhs._iters) != std::get<0>(rhs._iters);
+	bool operator!=(ZipIterator const &rhs) const {
+		return std::get<0>(_iters) != std::get<0>(rhs._iters);
 	}
 };
 
-namespace detail {
-template<typename... Containers>
+template<typename... Ts>
 class ZipContainer {
-	std::tuple<Containers...> _containers;
+	std::tuple<Ts...> _containers;
 
 public:
-	explicit ZipContainer(Containers &&...containers)
-	    : _containers(std::forward<Containers>(containers)...) {}
+	explicit ZipContainer(Ts &&...containers) : _containers(std::forward<Ts>(containers)...) {}
 
 	auto begin() {
-		return Zip(std::apply(
+		return ZipIterator(std::apply(
 		    [](auto &&...containers) {
 			    using std::begin;
 			    return std::make_tuple(begin(containers)...);
@@ -91,7 +83,7 @@ public:
 	}
 
 	auto end() {
-		return Zip(std::apply(
+		return ZipIterator(std::apply(
 		    [](auto &&...containers) {
 			    using std::end;
 			    return std::make_tuple(end(containers)...);
@@ -105,12 +97,11 @@ public:
 template<typename T>
 using Holder = std::
     conditional_t<std::is_lvalue_reference_v<T>, T, std::remove_cv_t<std::remove_reference_t<T>>>;
-} // namespace detail
 
 // Does the same number of iterations as the first container's iterator!
-template<typename... Containers>
-static constexpr auto zip(Containers &&...cs) {
-	return detail::ZipContainer<detail::Holder<Containers>...>(std::forward<Containers>(cs)...);
+template<typename... Ts>
+static constexpr auto zip(Ts &&...cs) {
+	return ZipContainer<Holder<Ts>...>(std::forward<Ts>(cs)...);
 }
 
 #endif // RGBDS_ITERTOOLS_HPP