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diff --git a/chilbert/detail/BitVecIndex.hpp b/chilbert/detail/BitVecIndex.hpp
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+/*
+  Copyright (C) 2018 David Robillard <d@drobilla.net>
+  Copyright (C) 2006-2007 Chris Hamilton <chamilton@cs.dal.ca>
+
+  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 2 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 <https://www.gnu.org/licenses/>.
+*/
+
+#ifndef CHILBERT_BITVECINDEX_HPP
+#define CHILBERT_BITVECINDEX_HPP
+
+#include <cassert>
+#include <climits>
+#include <cstddef>
+
+namespace chilbert {
+namespace detail {
+
+/// Index into a multi-rack bit vector
+template <class BitVec>
+struct BitVecIndex
+{
+	using Rack = typename BitVec::Rack;
+
+	static constexpr size_t bits_per_rack = sizeof(Rack) * CHAR_BIT;
+
+	BitVecIndex(const size_t bits)
+	  : rack{bits / bits_per_rack}
+	  , bit{bits - rack * bits_per_rack}
+	{
+		assert(bit < bits_per_rack);
+	}
+
+	size_t rack;
+	size_t bit;
+};
+
+} // namespace detail
+} // namespace chilbert
+
+#endif