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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.tsp.ers;

import java.util.ArrayList;
import java.util.List;

import org.bouncycastle.asn1.tsp.PartialHashtree;
import org.bouncycastle.operator.DigestCalculator;

/**
 * Calculator based on the use of a left weighted binary Merkle tree created
 * on top of the partial hash tree objects provided.
 */
public class BinaryTreeRootCalculator
    implements ERSRootNodeCalculator
{
    public byte[] computeRootHash(DigestCalculator digCalc, PartialHashtree[] nodes)
    {
        List hashes = new ArrayList();
        for (int i = 0; i <= nodes.length - 2; i += 2)
        {
            byte[] left = ERSUtil.computeNodeHash(digCalc, nodes[i]);
            byte[] right = ERSUtil.computeNodeHash(digCalc, nodes[i + 1]);

            hashes.add(ERSUtil.calculateBranchHash(digCalc, left, right));
        }

        if (nodes.length % 2 == 1)
        {
            hashes.add(ERSUtil.computeNodeHash(digCalc, nodes[nodes.length - 1]));
        }

        do
        {
            List newHashes = new ArrayList((hashes.size() + 1) / 2);

            for (int i = 0; i <= hashes.size() - 2; i += 2)
            {
                newHashes.add(ERSUtil.calculateBranchHash(digCalc, (byte[])hashes.get(i), (byte[])hashes.get(i + 1)));
            }

            if (hashes.size() % 2 == 1)
            {
                newHashes.add(hashes.get(hashes.size() - 1));
            }

            hashes = newHashes;
        }
        while (hashes.size() > 1);

        return (byte[])hashes.get(0);
    }
}