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package org.aksw.isomorphism;

import java.util.Collections;
import java.util.function.BinaryOperator;
import java.util.stream.Stream;

/**
 * Default implementation of the core algorithm for solving problems.
 * TODO Give it some fancy name
 *
 * @author Claus Stadler
 *
 * @param  The solution type
 */
public class ProblemSolver {
    protected ProblemContainer problemContainer;
    protected S baseSolution;
    protected BinaryOperator solutionCombiner;

    public ProblemSolver(ProblemContainer problemContainer, S baseSolution, BinaryOperator solutionCombiner) {
        super();
        this.problemContainer = problemContainer;
        this.baseSolution = baseSolution;
        this.solutionCombiner = solutionCombiner;
    }

    public Stream streamSolutions() {

        ProblemContainerPick pick = problemContainer.pick();

        Problem picked = pick.getPicked();
        ProblemContainer remaining = pick.getRemaining();

        Stream result = picked
            .generateSolutions()
            .flatMap(solutionContribution -> {
                S partialSolution = solutionCombiner.apply(baseSolution, solutionContribution);

                Stream r;
                // If the partial solution is null, then indicate the
                // absence of a solution by returning a stream that yields
                // null as a 'solution'
                if (partialSolution == null) {
                    r = Collections. singleton(null).stream();
                } else {
                    // This step is optional: it refines problems
                    // based on the current partial solution
                    // Depending on your setting, this can give a
                    // performance boost or penalty
                    //ProblemContainerImpl openProblems = remaining;
                    ProblemContainer openProblems = remaining.refine(partialSolution);

                    if (openProblems.isEmpty()) {
                        r = Collections. emptySet().stream();
                    } else {
                        ProblemSolver nextState = new ProblemSolver(openProblems, baseSolution, solutionCombiner);
                        r = nextState.streamSolutions();
                    }
                }
                return r;
            });

        return result;
    }

    public static  Stream solve(ProblemContainer problemContainer, S baseSolution, BinaryOperator solutionCombiner) {
        ProblemSolver problemSolver = new ProblemSolver(problemContainer, baseSolution, solutionCombiner);
        Stream result = problemSolver.streamSolutions();

        return result;
    }
}