/*
 * Copyright (C) 2011 The Guava Authors
 *
 * 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.google.common.hash;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import java.io.Serializable;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

/**
 * {@link HashFunction} adapter for {@link MessageDigest} instances.
 *
 * @author Kevin Bourrillion
 * @author Dimitris Andreou
 */
final class MessageDigestHashFunction extends AbstractStreamingHashFunction implements Serializable {
	private final MessageDigest prototype;
	private final int bytes;
	private final boolean supportsClone;
	private final String toString;

	MessageDigestHashFunction(String algorithmName, String toString) {
		this.prototype = getMessageDigest(algorithmName);
		this.bytes = prototype.getDigestLength();
		this.toString = checkNotNull(toString);
		this.supportsClone = supportsClone();
	}

	MessageDigestHashFunction(String algorithmName, int bytes, String toString) {
		this.toString = checkNotNull(toString);
		this.prototype = getMessageDigest(algorithmName);
		int maxLength = prototype.getDigestLength();
		checkArgument(bytes >= 4 && bytes <= maxLength, "bytes (%s) must be >= 4 and < %s", bytes, maxLength);
		this.bytes = bytes;
		this.supportsClone = supportsClone();
	}

	private boolean supportsClone() {
		try {
			prototype.clone();
			return true;
		} catch (CloneNotSupportedException e) {
			return false;
		}
	}

	@Override
	public int bits() {
		return bytes * Byte.SIZE;
	}

	@Override
	public String toString() {
		return toString;
	}

	private static MessageDigest getMessageDigest(String algorithmName) {
		try {
			return MessageDigest.getInstance(algorithmName);
		} catch (NoSuchAlgorithmException e) {
			throw new AssertionError(e);
		}
	}

	@Override
	public Hasher newHasher() {
		if (supportsClone) {
			try {
				return new MessageDigestHasher((MessageDigest) prototype.clone(), bytes);
			} catch (CloneNotSupportedException e) {
				// falls through
			}
		}
		return new MessageDigestHasher(getMessageDigest(prototype.getAlgorithm()), bytes);
	}

	private static final class SerializedForm implements Serializable {
		private final String algorithmName;
		private final int bytes;
		private final String toString;

		private SerializedForm(String algorithmName, int bytes, String toString) {
			this.algorithmName = algorithmName;
			this.bytes = bytes;
			this.toString = toString;
		}

		private Object readResolve() {
			return new MessageDigestHashFunction(algorithmName, bytes, toString);
		}

		private static final long serialVersionUID = 0;
	}

	Object writeReplace() {
		return new SerializedForm(prototype.getAlgorithm(), bytes, toString);
	}

	/**
	 * Hasher that updates a message digest.
	 */
	private static final class MessageDigestHasher extends AbstractByteHasher {

		private final MessageDigest digest;
		private final int bytes;
		private boolean done;

		private MessageDigestHasher(MessageDigest digest, int bytes) {
			this.digest = digest;
			this.bytes = bytes;
		}

		@Override
		protected void update(byte b) {
			checkNotDone();
			digest.update(b);
		}

		@Override
		protected void update(byte[] b) {
			checkNotDone();
			digest.update(b);
		}

		@Override
		protected void update(byte[] b, int off, int len) {
			checkNotDone();
			digest.update(b, off, len);
		}

		private void checkNotDone() {
			checkState(!done, "Cannot re-use a Hasher after calling hash() on it");
		}

		@Override
		public HashCode hash() {
			checkNotDone();
			done = true;
			return (bytes == digest.getDigestLength()) ? HashCode.fromBytesNoCopy(digest.digest())
					: HashCode.fromBytesNoCopy(Arrays.copyOf(digest.digest(), bytes));
		}
	}
}
