001 /*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015 package com.google.common.hash;
016
017 import com.google.common.annotations.Beta;
018 import com.google.common.base.Preconditions;
019 import com.google.common.primitives.Ints;
020
021 import java.security.MessageDigest;
022
023 /**
024 * An immutable hash code of arbitrary bit length.
025 *
026 * @author Dimitris Andreou
027 * @since 11.0
028 */
029 @Beta
030 public abstract class HashCode {
031 HashCode() {}
032
033 /**
034 * Returns the first four bytes of {@linkplain #asBytes() this hashcode's bytes}, converted to
035 * an {@code int} value in little-endian order.
036 */
037 public abstract int asInt();
038
039 /**
040 * Returns the first eight bytes of {@linkplain #asBytes() this hashcode's bytes}, converted to
041 * a {@code long} value in little-endian order.
042 *
043 * @throws IllegalStateException if {@code bits() < 64}
044 */
045 public abstract long asLong();
046
047 /**
048 * Returns the value of this hash code as a byte array. The caller may modify the byte array;
049 * changes to it will <i>not</i> be reflected in this {@code HashCode} object or any other arrays
050 * returned by this method.
051 */
052 // TODO(user): consider ByteString here, when that is available
053 public abstract byte[] asBytes();
054
055 /**
056 * Copies bytes from this hash code into {@code dest}.
057 *
058 * @param dest the byte array into which the hash code will be written
059 * @param offset the start offset in the data
060 * @param maxLength the maximum number of bytes to write
061 * @return the number of bytes written to {@code dest}
062 * @throws IndexOutOfBoundsException if there is not enough room in {@code dest}
063 */
064 public int writeBytesTo(byte[] dest, int offset, int maxLength) {
065 byte[] hash = asBytes();
066 maxLength = Ints.min(maxLength, hash.length);
067 Preconditions.checkPositionIndexes(offset, offset + maxLength, dest.length);
068 System.arraycopy(hash, 0, dest, offset, maxLength);
069 return maxLength;
070 }
071
072 /**
073 * Returns the number of bits in this hash code; a positive multiple of 32.
074 */
075 public abstract int bits();
076
077 @Override public boolean equals(Object object) {
078 if (object instanceof HashCode) {
079 HashCode that = (HashCode) object;
080 // Undocumented: this is a non-short-circuiting equals(), in case this is a cryptographic
081 // hash code, in which case we don't want to leak timing information
082 return MessageDigest.isEqual(this.asBytes(), that.asBytes());
083 }
084 return false;
085 }
086
087 /**
088 * Returns a "Java hash code" for this {@code HashCode} instance; this is well-defined
089 * (so, for example, you can safely put {@code HashCode} instances into a {@code
090 * HashSet}) but is otherwise probably not what you want to use.
091 */
092 @Override public int hashCode() {
093 /*
094 * As long as the hash function that produced this isn't of horrible quality, this
095 * won't be of horrible quality either.
096 */
097 return asInt();
098 }
099
100 /**
101 * Returns a string containing each byte of {@link #asBytes}, in order, as a two-digit unsigned
102 * hexadecimal number in lower case.
103 *
104 * <p>Note that if the output is considered to be a single hexadecimal number, this hash code's
105 * bytes are the <i>big-endian</i> representation of that number. This may be surprising since
106 * everything else in the hashing API uniformly treats multibyte values as little-endian. But
107 * this format conveniently matches that of utilities such as the UNIX {@code md5sum} command.
108 */
109 @Override public String toString() {
110 byte[] bytes = asBytes();
111 // TODO(user): Use c.g.common.base.ByteArrays once it is open sourced.
112 StringBuilder sb = new StringBuilder(2 * bytes.length);
113 for (byte b : bytes) {
114 sb.append(hexDigits[(b >> 4) & 0xf]).append(hexDigits[b & 0xf]);
115 }
116 return sb.toString();
117 }
118
119 private static final char[] hexDigits = "0123456789abcdef".toCharArray();
120 }