RHF32.java

/*
 * Copyright 2004 Sean Owen
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.planetj.math.rabinhash;

import java.security.DigestException;
import java.security.MessageDigestSpi;

/**
 * <p>Provides a {@link java.security.MessageDigest} based on {@link RabinHashFunction32}.</p>
 *
 * @author Sean Owen
 * @version 2.0
 * @since 2.0
 */
public final class RHF32 extends MessageDigestSpi implements Cloneable {

    private final RabinHashFunction32 rhf = RabinHashFunction32.DEFAULT_HASH_FUNCTION;
    private int hash;

    /**
     * @return 4
     */
    protected int engineGetDigestLength() {
        return 4;
    }

    protected void engineUpdate(final byte input) {
        hash = rhf.hash(new byte[]{input}, 0, 1, hash);
    }

    protected void engineUpdate(final byte[] input, final int offset, final int len) {
        hash = rhf.hash(input, offset, len, hash);
    }

    protected byte[] engineDigest() {
        final byte[] buf = new byte[4];
        hashToBuf(buf, 0);
        engineReset();
        return buf;
    }

    /**
     * @param buf buffer into which to write the digest
     * @param offset offset into buffer at which to start writing
     * @param len (not used)
     * @return 4
     * @throws DigestException if len is less than 4
     */
    protected int engineDigest(final byte[] buf, final int offset, final int len) throws DigestException {
        if (len < 4) {
            throw new DigestException("Output buffer is smaller than digest length of 4");
        }
        hashToBuf(buf, offset);
        engineReset();
        return 4;
    }

    protected void engineReset() {
        hash = 0;
    }

    private void hashToBuf(final byte[] buf, final int offset) {
        buf[offset] = (byte) (hash >> 24);
        buf[offset + 1] = (byte) (hash >> 16);
        buf[offset + 2] = (byte) (hash >> 8);
        buf[offset + 3] = (byte) hash;
    }

}