/** \file
 * \brief Implementation of an LP-based 1.39+epsilon Steiner tree
 * approximation algorithm by Goemans et al.
 *
 * \author Stephan Beyer
 *
 * \par License:
 * This file is part of the Open Graph Drawing Framework (OGDF).
 *
 * \par
 * Copyright (C)<br>
 * See README.md in the OGDF root directory for details.
 *
 * \par
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * Version 2 or 3 as published by the Free Software Foundation;
 * see the file LICENSE.txt included in the packaging of this file
 * for details.
 *
 * \par
 * 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.
 *
 * \par
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, see
 * http://www.gnu.org/copyleft/gpl.html
 */

#pragma once

#include <ogdf/graphalg/steiner_tree/Full2ComponentGenerator.h>
#include <ogdf/graphalg/steiner_tree/Full3ComponentGeneratorVoronoi.h>
#include <ogdf/graphalg/steiner_tree/FullComponentGeneratorCaller.h>
#include <ogdf/graphalg/steiner_tree/FullComponentGeneratorDreyfusWagner.h>
#include <ogdf/graphalg/steiner_tree/FullComponentGeneratorDreyfusWagnerWithoutMatrix.h>
#include <ogdf/graphalg/steiner_tree/LPRelaxationSER.h>
#include <ogdf/graphalg/steiner_tree/common_algorithms.h>
#include <ogdf/graphalg/steiner_tree/goemans/Approximation.h>

namespace ogdf {

/*!
 * \brief This class implements the (1.39+epsilon)-approximation algorithm
 * for the Steiner tree problem by Goemans et. al.
 *
 * @ingroup ga-steiner
 *
 * This implementation is based on:
 *
 * M.X. Goemans, N. Olver, T. Rothvoß, R. Zenklusen:
 * Matroids and Integrality Gaps for Hypergraphic Steiner Tree Relaxations.
 * STOC 2012, pages 1161-1176, 2012
 *
 * and
 *
 * S. Beyer, M. Chimani: Steiner Tree 1.39-Approximation in Practice.
 * MEMICS 2014, LNCS 8934, 60-72, Springer, 2014
 */
template<typename T>
class MinSteinerTreeGoemans139 : public MinSteinerTreeModule<T> {
private:
	class Main;

protected:
	int m_restricted;
	bool m_preprocess;
	bool m_use2approx;
	bool m_separateCycles;
	int m_seed;

public:
	MinSteinerTreeGoemans139()
		: m_restricted(3)
		, m_preprocess(true)
		, m_use2approx(false)
		, m_separateCycles(false)
		, m_seed(1337) { }

	virtual ~MinSteinerTreeGoemans139() { }

	/*!
	 * \brief Sets the maximal number of terminals in a full component
	 * @param k the maximal number of terminals in a full component
	 */
	void setMaxComponentSize(int k) { m_restricted = k; }

	/*!
	 * \brief Set seed for the random number generation.
	 * @param seed The seed
	 */
	void setSeed(int seed) { m_seed = seed; }

	/*!
	 * \brief Use Takahashi-Matsuyama 2-approximation as upper bounds
	 * \note not recommended to use in general
	 * @param use2approx True to apply the bound
	 */
	void use2Approximation(bool use2approx = true) { m_use2approx = use2approx; }

	/*!
	 * Disable preprocessing of LP solutions
	 * \note not recommended to use in general
	 * @param preprocess True to disable, false to enable
	 */
	void disablePreprocessing(bool preprocess = true) { m_preprocess = !preprocess; }

	/*!
	 * \brief Use stronger LP relaxation (not recommended in general)
	 * @param separateCycles True to turn the stronger LP relaxation on
	 */
	void separateCycles(bool separateCycles = true) { m_separateCycles = separateCycles; }

protected:
	/*!
	 * \brief Builds a minimum Steiner tree for a given weighted graph with terminals \see MinSteinerTreeModule::computeSteinerTree
	 * @param G The weighted input graph
	 * @param terminals The list of terminal nodes
	 * @param isTerminal A bool array of terminals
	 * @param finalSteinerTree The final Steiner tree
	 * @return The objective value (sum of edge costs) of the final Steiner tree
	 */
	virtual T computeSteinerTree(const EdgeWeightedGraph<T>& G, const List<node>& terminals,
			const NodeArray<bool>& isTerminal, EdgeWeightedGraphCopy<T>*& finalSteinerTree) override;
};

template<typename T>
T MinSteinerTreeGoemans139<T>::computeSteinerTree(const EdgeWeightedGraph<T>& G,
		const List<node>& terminals, const NodeArray<bool>& isTerminal,
		EdgeWeightedGraphCopy<T>*& finalSteinerTree) {
	std::minstd_rand rng(m_seed);
	List<node> sortedTerminals(terminals);
	MinSteinerTreeModule<T>::sortTerminals(sortedTerminals);
	Main main(G, sortedTerminals, isTerminal, m_restricted, m_use2approx, m_separateCycles);
	return main.getApproximation(finalSteinerTree, rng, m_preprocess);
}

//! \brief Class managing LP-based approximation
//! \todo should be refactored, done this way for historical reasons
template<typename T>
class MinSteinerTreeGoemans139<T>::Main {
	const EdgeWeightedGraph<T>& m_G;
	const NodeArray<bool>& m_isTerminal;
	const List<node>& m_terminals; //!< List of terminals
	steiner_tree::FullComponentWithExtraStore<T, double>
			m_fullCompStore; //!< all enumerated full components, with solution

	int m_restricted;
	enum class Approx2State {
		Off,
		On,
		JustUseIt,
	};
	Approx2State m_use2approx;

	const double m_eps; //!< epsilon for double operations

	EdgeWeightedGraphCopy<T>* m_approx2SteinerTree;
	T m_approx2Weight;

	//! \name Finding full components
	//! @{

	//! Find full components of size 2
	void findFull2Components(const NodeArray<NodeArray<T>>& distance,
			const NodeArray<NodeArray<edge>>& pred);
	//! Find full components of size 3
	void findFull3Components(const NodeArray<NodeArray<T>>& distance,
			const NodeArray<NodeArray<edge>>& pred);
	//! Find 3-restricted components
	void find3RestrictedComponents();
	//! Find full components using algorithm by Dreyfus-Wagner
	void findFullComponentsDW();
	//! Find full components using algorithm by Erickson et al
	void findFullComponentsEMV();
	//! Auxiliary function to retrieve components by Dreyfus-Wagner/Erickson et al implementation
	template<typename FCG>
	void retrieveComponents(const FCG& fcg);
	//! Find full components
	void findFullComponents();

	//! @}
	//! \name Preliminaries and preprocessing for the approximation algorithm
	//! @{

	//! Remove inactive components from m_fullCompStore (since we do not need them any longer)
	void removeInactiveComponents() {
		for (int k = m_fullCompStore.size() - 1; k >= 0; --k) {
			OGDF_ASSERT(k < m_fullCompStore.size());
			if (m_fullCompStore.extra(k) <= m_eps) {
				m_fullCompStore.remove(k);
			}
		}
	}

	//! Remove the full components with the given ids
	void removeComponents(ArrayBuffer<int>& ids) {
		ids.quicksort();
		for (int i = ids.size() - 1; i >= 0; --i) {
			m_fullCompStore.remove(ids[i]);
		}
	}

	//! Add a full component to the final solution (by changing nonterminals to terminals)
	void addComponent(NodeArray<bool>& isNewTerminal, int id) {
		m_fullCompStore.foreachNode(id, [&](node v) { isNewTerminal[v] = true; });
	}

	//! \brief Preprocess LP solution
	//! \pre every terminal is covered with >= 1
	void preprocess(NodeArray<bool>& isNewTerminal) {
		Graph H; // a graph where each component is a star
		NodeArray<int> id(H); // ids each center of the star to the component id
		NodeArray<node> copy(m_G, nullptr); // ids orig in m_G -> copy in H

		List<node> centers; // all centers
		for (int i = 0; i < m_fullCompStore.size(); ++i) {
			const node center = H.newNode();
			centers.pushBack(center);
			id[center] = i;

			for (node vG : m_fullCompStore.terminals(i)) {
				node vH = copy[vG];
				if (!vH) {
					vH = H.newNode();
					copy[vG] = vH;
				}
				H.newEdge(vH, center); // target is always center
			}
		}

		// find components to be inserted into the steinerTree and insert them
		ArrayBuffer<int> inactive; // ids of components we insert; we have to remove them from the set of active components afterwards
		bool changed;
		do {
			changed = false;
			ListIterator<node> it2;
			for (ListIterator<node> it = centers.begin(); it.valid(); it = it2) {
				it2 = it.succ();
				node c = *it;
				int innerNodes = 0; // count inner nodes
				for (adjEntry adj : c->adjEntries) {
					innerNodes += (adj->twinNode()->degree() != 1);
				}
				if (innerNodes <= 1) { // this center represents a component to add to steinerTree
					// insert component into steinerTree
					addComponent(isNewTerminal, id[c]);

					// remove center from H (adjacent leaves can remain being isolated nodes)
					inactive.push(id[c]);
					H.delNode(c);
					centers.del(it);

					changed = true;
				}
			}
		} while (changed);

		removeComponents(inactive);
	}

	//! @}

public:
	//! Initialize all attributes, sort the terminal list
	Main(const EdgeWeightedGraph<T>& G, const List<node>& terminals,
			const NodeArray<bool>& isTerminal, int restricted, bool use2approx, bool separateCycles,
			double eps = 1e-8)
		: m_G(G)
		, m_isTerminal(isTerminal)
		, m_terminals(terminals)
		, m_fullCompStore(G, m_terminals, isTerminal)
		, m_restricted(restricted)
		, m_use2approx(use2approx ? Approx2State::On : Approx2State::Off)
		, m_eps(eps)
		, m_approx2SteinerTree(nullptr)
		, m_approx2Weight(0) {
		if (m_use2approx == Approx2State::On) { // add upper bound by 2-approximation
			MinSteinerTreeTakahashi<T> mstT;
			m_approx2Weight = mstT.call(m_G, m_terminals, m_isTerminal, m_approx2SteinerTree);
		}

		if (m_restricted > m_terminals.size()) {
			m_restricted = m_terminals.size();
		}

		findFullComponents();

		steiner_tree::LPRelaxationSER<T> lp(m_G, m_terminals, m_isTerminal, m_fullCompStore,
				m_approx2Weight, separateCycles ? m_restricted + 1 : 0, m_eps);
		if (!lp.solve()) {
			OGDF_ASSERT(m_use2approx == Approx2State::On);
			m_use2approx = Approx2State::JustUseIt;
		}
	}

	~Main() { }

	//! Obtain an (1.39+epsilon)-approximation based on the LP solution
	T getApproximation(EdgeWeightedGraphCopy<T>*& finalSteinerTree, const std::minstd_rand& rng,
			const bool doPreprocessing = true);
};

template<typename T>
void MinSteinerTreeGoemans139<T>::Main::findFull2Components(const NodeArray<NodeArray<T>>& distance,
		const NodeArray<NodeArray<edge>>& pred) {
	steiner_tree::Full2ComponentGenerator<T> fcg;
	fcg.call(m_G, m_terminals, distance, pred, [&](node s, node t, T cost) {
		EdgeWeightedGraphCopy<T> minComp;
		minComp.createEmpty(m_G);
		minComp.newEdge(minComp.newNode(s), minComp.newNode(t), distance[s][t]);
		m_fullCompStore.insert(minComp);
	});
}

template<typename T>
void MinSteinerTreeGoemans139<T>::Main::findFull3Components(const NodeArray<NodeArray<T>>& distance,
		const NodeArray<NodeArray<edge>>& pred) {
	steiner_tree::Full3ComponentGeneratorVoronoi<T> fcg;
	fcg.call(m_G, m_terminals, m_isTerminal, distance, pred,
			[&](node t0, node t1, node t2, node minCenter, T minCost) {
				// create a full 3-component
				EdgeWeightedGraphCopy<T> minComp;
				minComp.createEmpty(m_G);
				node minCenterC = minComp.newNode(minCenter);
				minComp.newEdge(minComp.newNode(t0), minCenterC, distance[t0][minCenter]);
				minComp.newEdge(minComp.newNode(t1), minCenterC, distance[t1][minCenter]);
				minComp.newEdge(minComp.newNode(t2), minCenterC, distance[t2][minCenter]);
				m_fullCompStore.insert(minComp);
			});
}

template<typename T>
void MinSteinerTreeGoemans139<T>::Main::find3RestrictedComponents() {
	NodeArray<NodeArray<T>> distance;
	NodeArray<NodeArray<edge>> pred;

	steiner_tree::FullComponentGeneratorCaller<T>::computeDistanceMatrix(distance, pred, m_G,
			m_terminals, m_isTerminal, m_restricted);

	findFull2Components(distance, pred);
	if (m_restricted == 3) {
		findFull3Components(distance, pred);
	}
}

template<typename T>
template<typename FCG>
void MinSteinerTreeGoemans139<T>::Main::retrieveComponents(const FCG& fcg) {
	SubsetEnumerator<node> terminalSubset(m_terminals);
	for (terminalSubset.begin(2, m_restricted); terminalSubset.valid(); terminalSubset.next()) {
		EdgeWeightedGraphCopy<T> component;
		List<node> terminals;
		terminalSubset.list(terminals);
		fcg.getSteinerTreeFor(terminals, component);
		if (fcg.isValidComponent(component)) {
			m_fullCompStore.insert(component);
		}
	}
}

template<typename T>
void MinSteinerTreeGoemans139<T>::Main::findFullComponentsDW() {
	NodeArray<NodeArray<T>> distance;
	NodeArray<NodeArray<edge>> pred;
	steiner_tree::FullComponentGeneratorCaller<T>::computeDistanceMatrix(distance, pred, m_G,
			m_terminals, m_isTerminal, m_restricted);

	steiner_tree::FullComponentGeneratorDreyfusWagner<T> fcg(m_G, m_terminals, m_isTerminal,
			distance, pred);
	fcg.call(m_restricted);
	retrieveComponents(fcg);
}

template<typename T>
void MinSteinerTreeGoemans139<T>::Main::findFullComponentsEMV() {
	steiner_tree::FullComponentGeneratorDreyfusWagnerWithoutMatrix<T> fcg(m_G, m_terminals,
			m_isTerminal);
	fcg.call(m_restricted);
	retrieveComponents(fcg);
}

template<typename T>
void MinSteinerTreeGoemans139<T>::Main::findFullComponents() {
	if (m_restricted >= 4) { // use Dreyfus-Wagner based full component generation
		if (steiner_tree::FullComponentDecisions::shouldUseErickson(m_G.numberOfNodes(),
					m_G.numberOfEdges())) {
			findFullComponentsEMV();
		} else {
			findFullComponentsDW();
		}
	} else {
		find3RestrictedComponents();
	}
}

template<typename T>
T MinSteinerTreeGoemans139<T>::Main::getApproximation(EdgeWeightedGraphCopy<T>*& finalSteinerTree,
		const std::minstd_rand& rng, const bool doPreprocessing) {
	if (m_use2approx == Approx2State::JustUseIt) {
		// no remaining components
		finalSteinerTree = m_approx2SteinerTree;
		return m_approx2Weight;
	}

	removeInactiveComponents();

	NodeArray<bool> isNewTerminal(m_G, false);
	for (node v : m_terminals) {
		isNewTerminal[v] = true;
	}

	if (doPreprocessing) {
		preprocess(isNewTerminal);
	}

	if (!m_fullCompStore.isEmpty()) {
		steiner_tree::goemans::Approximation<T> approx(m_G, m_terminals, m_isTerminal,
				m_fullCompStore, rng, m_eps);
		approx.solve(isNewTerminal);
	}

	T cost = steiner_tree::obtainFinalSteinerTree(m_G, isNewTerminal, m_isTerminal, finalSteinerTree);

	if (m_use2approx != Approx2State::Off) {
		if (m_approx2Weight < cost) {
			delete finalSteinerTree;
			finalSteinerTree = m_approx2SteinerTree;
			cost = m_approx2Weight;
		} else {
			delete m_approx2SteinerTree;
		}
	}

	return cost;
}

}
