Reduce and prevent unnecessary random-access to `List`

Random-access access to `List` when iterating is `O(n^2)` (`O(n)` when
accessing a single element)

* Removed subscript operator, in favor of a more explicit `get`
* Added conversion from `Iterator` to `ConstIterator`
* Remade existing operations into other solutions when applicable

1 files changed, 32 insertions, 24 deletions

diff --git a/core/templates/list.h b/core/templates/list.h
index b4d4beb930..6663f06c30 100644
--- a/core/templates/list.h
+++ b/core/templates/list.h
@@ -139,54 +139,58 @@ public:
 
 	typedef T ValueType;
 
-	struct Iterator {
-		_FORCE_INLINE_ T &operator*() const {
+	struct ConstIterator {
+		_FORCE_INLINE_ const T &operator*() const {
 			return E->get();
 		}
-		_FORCE_INLINE_ T *operator->() const { return &E->get(); }
-		_FORCE_INLINE_ Iterator &operator++() {
+		_FORCE_INLINE_ const T *operator->() const { return &E->get(); }
+		_FORCE_INLINE_ ConstIterator &operator++() {
 			E = E->next();
 			return *this;
 		}
-		_FORCE_INLINE_ Iterator &operator--() {
+		_FORCE_INLINE_ ConstIterator &operator--() {
 			E = E->prev();
 			return *this;
 		}
 
-		_FORCE_INLINE_ bool operator==(const Iterator &b) const { return E == b.E; }
-		_FORCE_INLINE_ bool operator!=(const Iterator &b) const { return E != b.E; }
+		_FORCE_INLINE_ bool operator==(const ConstIterator &b) const { return E == b.E; }
+		_FORCE_INLINE_ bool operator!=(const ConstIterator &b) const { return E != b.E; }
 
-		Iterator(Element *p_E) { E = p_E; }
-		Iterator() {}
-		Iterator(const Iterator &p_it) { E = p_it.E; }
+		_FORCE_INLINE_ ConstIterator(const Element *p_E) { E = p_E; }
+		_FORCE_INLINE_ ConstIterator() {}
+		_FORCE_INLINE_ ConstIterator(const ConstIterator &p_it) { E = p_it.E; }
 
 	private:
-		Element *E = nullptr;
+		const Element *E = nullptr;
 	};
 
-	struct ConstIterator {
-		_FORCE_INLINE_ const T &operator*() const {
+	struct Iterator {
+		_FORCE_INLINE_ T &operator*() const {
 			return E->get();
 		}
-		_FORCE_INLINE_ const T *operator->() const { return &E->get(); }
-		_FORCE_INLINE_ ConstIterator &operator++() {
+		_FORCE_INLINE_ T *operator->() const { return &E->get(); }
+		_FORCE_INLINE_ Iterator &operator++() {
 			E = E->next();
 			return *this;
 		}
-		_FORCE_INLINE_ ConstIterator &operator--() {
+		_FORCE_INLINE_ Iterator &operator--() {
 			E = E->prev();
 			return *this;
 		}
 
-		_FORCE_INLINE_ bool operator==(const ConstIterator &b) const { return E == b.E; }
-		_FORCE_INLINE_ bool operator!=(const ConstIterator &b) const { return E != b.E; }
+		_FORCE_INLINE_ bool operator==(const Iterator &b) const { return E == b.E; }
+		_FORCE_INLINE_ bool operator!=(const Iterator &b) const { return E != b.E; }
 
-		_FORCE_INLINE_ ConstIterator(const Element *p_E) { E = p_E; }
-		_FORCE_INLINE_ ConstIterator() {}
-		_FORCE_INLINE_ ConstIterator(const ConstIterator &p_it) { E = p_it.E; }
+		Iterator(Element *p_E) { E = p_E; }
+		Iterator() {}
+		Iterator(const Iterator &p_it) { E = p_it.E; }
+
+		operator ConstIterator() const {
+			return ConstIterator(E);
+		}
 
 	private:
-		const Element *E = nullptr;
+		Element *E = nullptr;
 	};
 
 	_FORCE_INLINE_ Iterator begin() {
@@ -519,7 +523,9 @@ public:
 		}
 	}
 
-	T &operator[](int p_index) {
+	// Random access to elements, use with care,
+	// do not use for iteration.
+	T &get(int p_index) {
 		CRASH_BAD_INDEX(p_index, size());
 
 		Element *I = front();
@@ -532,7 +538,9 @@ public:
 		return I->get();
 	}
 
-	const T &operator[](int p_index) const {
+	// Random access to elements, use with care,
+	// do not use for iteration.
+	const T &get(int p_index) const {
 		CRASH_BAD_INDEX(p_index, size());
 
 		const Element *I = front();