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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 1.8 and up. 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;
import org.bouncycastle.util.Arrays;

/**
 * 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
{
    private List> tree;

    public byte[] computeRootHash(DigestCalculator digCalc, PartialHashtree[] nodes)
    {
        SortedHashList hashes = new SortedHashList();
        for (int i = 0; i < nodes.length; i++)
        {
            byte[] left = ERSUtil.computeNodeHash(digCalc, nodes[i]);

            hashes.add(left);
        }

        List   hashValues = hashes.toList();

        tree = new ArrayList>();
        tree.add(hashValues);

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

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

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

                tree.add(newHashes);
                hashValues = newHashes;
            }
            while (hashValues.size() > 1);
        }

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

    public PartialHashtree[] computePathToRoot(DigestCalculator digCalc, PartialHashtree node, int index)
    {
        List path = new ArrayList();
        byte[] nodeHash = ERSUtil.computeNodeHash(digCalc, node);

        path.add(node);

        int row = 0;
        while (row < tree.size() - 1)
        {
            if (index == ((List)tree.get(row)).size() - 1)
            {
                while (true)
                {
                    // search back in case we are the odd one at the end
                    List hashes = (List)tree.get(row + 1);
                    if (!Arrays.areEqual(nodeHash, (byte[])hashes.get(hashes.size() - 1)))
                    {
                        break;
                    }

                    row++;
                    index = ((List)tree.get(row)).size() - 1;
                }
            }

            byte[] neighborHash;
            if ((index & 1) == 0)
            {
                neighborHash = (byte[])((List)tree.get(row)).get(index + 1);
            }
            else
            {
                neighborHash = (byte[])((List)tree.get(row)).get(index - 1);
            }

            path.add(new PartialHashtree(neighborHash));

            nodeHash = ERSUtil.calculateBranchHash(digCalc, nodeHash, neighborHash);
            index = index / 2;
            row++;
        }

        return (PartialHashtree[])path.toArray(new PartialHashtree[0]);
    }

    public byte[] recoverRootHash(DigestCalculator digCalc, PartialHashtree[] nodes)
    {
        byte[] baseHash = ERSUtil.computeNodeHash(digCalc, nodes[0]);

        for (int i = 1; i < nodes.length; i++)
        {
            baseHash = ERSUtil.calculateBranchHash(digCalc, baseHash, ERSUtil.computeNodeHash(digCalc, nodes[i]));
        }

        return baseHash;
    }
}