Generalize cycle detection.

libdevcore/Algorithms.h

@@ -0,0 +1,76 @@
+/*
+	This file is part of solidity.
+
+	solidity 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.
+
+	solidity 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 solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#pragma once
+
+
+#include <functional>
+#include <set>
+
+namespace dev
+{
+
+/**
+ * Detector for cycles in directed graphs. It returns the first
+ * vertex on the path towards a cycle or a nullptr if there is
+ * no reachable cycle starting from a given vertex.
+ */
+template <typename V>
+class CycleDetector
+{
+public:
+	/// Initializes the cycle detector
+	/// @param _visit function that is given the current vertex
+	///               and is supposed to call @a run on all
+	///               adjacent vertices.
+	explicit CycleDetector(std::function<void(V const&, CycleDetector&)> _visit):
+		m_visit(std::move(_visit))
+	{  }
+
+	/// Recursively perform cycle detection starting
+	/// (or continuing) with @param _vertex
+	/// @returns the first vertex on the path towards a cycle from @a _vertex
+	/// or nullptr if no cycle is reachable from @a _vertex.
+	V const* run(V const& _vertex)
+	{
+		if (m_firstCycleVertex)
+			return m_firstCycleVertex;
+		if (m_processed.count(&_vertex))
+			return nullptr;
+		else if (m_processing.count(&_vertex))
+			return m_firstCycleVertex = &_vertex;
+		m_processing.insert(&_vertex);
+
+		m_depth++;
+		m_visit(_vertex, *this);
+		m_depth--;
+		if (m_firstCycleVertex && m_depth == 1)
+			m_firstCycleVertex = &_vertex;
+
+		m_processing.erase(&_vertex);
+		m_processed.insert(&_vertex);
+		return m_firstCycleVertex;
+	}
+
+private:
+	std::function<void(V const&, CycleDetector&)> m_visit;
+	std::set<V const*> m_processing;
+	std::set<V const*> m_processed;
+	size_t m_depth = 0;
+	V const* m_firstCycleVertex = nullptr;
+};
+
+}