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GraphHopper is a fast and memory efficient Java road routing engine working seamlessly with OpenStreetMap data.

There is a newer version: 0.7.0

package com.graphhopper.routing.util;

/**
 * Turns an unidirectional weight Approximation into a bidirectional consistent one.
 * 
 * Ikeda, T., Hsu, M.-Y., Imai, H., Nishimura, S., Shimoura, H., Hashimoto, T., Tenmoku, K., and
 * Mitoh, K. (1994). A fast algorithm for finding better routes by ai search techniques. In VNIS,
 * pages 291–296.
 * 
 * @author jansoe
 */
public class ConsistentWeightApproximator
{
    private final WeightApproximator uniDirApproximatorForward, uniDirApproximatorReverse;

    public ConsistentWeightApproximator( WeightApproximator weightApprox )
    {
        uniDirApproximatorForward = weightApprox;
        uniDirApproximatorReverse = weightApprox.duplicate();
    }

    public void setSourceNode( int sourceNode )
    {
        uniDirApproximatorReverse.setGoalNode(sourceNode);
    }

    public void setGoalNode( int goalNode )
    {
        uniDirApproximatorForward.setGoalNode(goalNode);
    }

    public double approximate( int fromNode, boolean reverse )
    {
        double weightApproximation = 0.5
                * (uniDirApproximatorForward.approximate(fromNode) - uniDirApproximatorReverse.approximate(fromNode));

        if (reverse)
            return -weightApproximation;

        return weightApproximation;
    }
}