diff -r 918b6c0f8b5b -r ed9685f44ab3 src/ring.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/ring.h	Sat Dec 14 22:36:06 2019 +0200
@@ -0,0 +1,109 @@
+/*
+ *  LDForge: LDraw parts authoring CAD
+ *  Copyright (C) 2013 - 2018 Teemu Piippo
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+namespace iter
+{
+	namespace _imp
+	{
+		template<typename T>
+		class RingAdapter;
+	}
+
+	template<typename T>
+	_imp::RingAdapter<T> ring(T& collection);
+
+	template<typename T>
+	_imp::RingAdapter<T> ring(T& collection, int count);
+}
+
+/*
+ * Implements a ring adapter over T. This class corrects indices given to the element-operator so that they're within bounds.
+ * The maximum amount can be specified manually.
+ *
+ * Example:
+ *
+ *   int A[] = {10,20,30,40};
+ *   ring(A)[0]  ==     A[0 % 4]    == A[0]
+ *   ring(A)[5]  ==     A[5 % 4]    == A[1]
+ *   ring(A)[-1] == ring(A)[-1 + 4] == A[3]
+ */
+template<typename T>
+class iter::_imp::RingAdapter
+{
+private:
+	// The private section must come first because _collection is used in decltype() below.
+	T& collection;
+	const int count;
+
+public:
+	RingAdapter(T& collection, int count) :
+		collection {collection},
+		count {count} {}
+
+	template<typename IndexType>
+	decltype(collection[IndexType()]) operator[](IndexType index)
+	{
+		if (count == 0)
+		{
+			// Argh! ...let the collection deal with this case.
+			return this->collection[index];
+		}
+		else
+		{
+			index %= this->count;
+
+			// Fix negative modulus...
+			if (index < 0)
+				index += this->count;
+
+			return this->collection[index];
+		}
+	}
+
+	int size() const
+	{
+		return this->count;
+	}
+};
+
+/*
+ * Convenience function for RingAdapter so that the template parameter does not have to be provided. The ring amount is assumed
+ * to be the amount of elements in the collection.
+ */
+template<typename T>
+iter::_imp::RingAdapter<T> iter::ring(T& collection)
+{
+	return {collection, countof(collection)};
+}
+
+/*
+ * Version of ring() that allows manual specification of the count.
+ */
+template<typename T>
+iter::_imp::RingAdapter<T> iter::ring(T& collection, int count)
+{
+	return {collection, count};
+}
+
+template<typename T>
+int countof(const iter::_imp::RingAdapter<T>& ring)
+{
+	return ring.size();
+}