1/*
2 * Copyright (C) 2011 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
5 * in compliance with the License. You may obtain a copy of the License at
6 *
7 * http://www.apache.org/licenses/LICENSE-2.0
8 *
9 * Unless required by applicable law or agreed to in writing, software distributed under the License
10 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
11 * or implied. See the License for the specific language governing permissions and limitations under
12 * the License.
13 */
14
15package com.google.common.hash;
16
17import com.google.common.annotations.Beta;
18import com.google.common.base.Preconditions;
19import com.google.common.primitives.Ints;
20
21import java.security.MessageDigest;
22
23/**
24 * An immutable hash code of arbitrary bit length.
25 *
26 * @author Dimitris Andreou
27 * @since 11.0
28 */
29@Beta
30public abstract class HashCode {
31  HashCode() {}
32
33  /**
34   * Returns the first four bytes of {@linkplain #asBytes() this hashcode's bytes}, converted to
35   * an {@code int} value in little-endian order.
36   */
37  public abstract int asInt();
38
39  /**
40   * Returns the first eight bytes of {@linkplain #asBytes() this hashcode's bytes}, converted to
41   * a {@code long} value in little-endian order.
42   *
43   * @throws IllegalStateException if {@code bits() < 64}
44   */
45  public abstract long asLong();
46
47  /**
48   * Returns the value of this hash code as a byte array. The caller may modify the byte array;
49   * changes to it will <i>not</i> be reflected in this {@code HashCode} object or any other arrays
50   * returned by this method.
51   */
52  // TODO(user): consider ByteString here, when that is available
53  public abstract byte[] asBytes();
54
55  /**
56   * Copies bytes from this hash code into {@code dest}.
57   *
58   * @param dest the byte array into which the hash code will be written
59   * @param offset the start offset in the data
60   * @param maxLength the maximum number of bytes to write
61   * @return the number of bytes written to {@code dest}
62   * @throws IndexOutOfBoundsException if there is not enough room in {@code dest}
63   */
64  public int writeBytesTo(byte[] dest, int offset, int maxLength) {
65    byte[] hash = asBytes();
66    maxLength = Ints.min(maxLength, hash.length);
67    Preconditions.checkPositionIndexes(offset, offset + maxLength, dest.length);
68    System.arraycopy(hash, 0, dest, offset, maxLength);
69    return maxLength;
70  }
71
72  /**
73   * Returns the number of bits in this hash code; a positive multiple of 32.
74   */
75  public abstract int bits();
76
77  @Override public boolean equals(Object object) {
78    if (object instanceof HashCode) {
79      HashCode that = (HashCode) object;
80      // Undocumented: this is a non-short-circuiting equals(), in case this is a cryptographic
81      // hash code, in which case we don't want to leak timing information
82      return MessageDigest.isEqual(this.asBytes(), that.asBytes());
83    }
84    return false;
85  }
86
87  /**
88   * Returns a "Java hash code" for this {@code HashCode} instance; this is well-defined
89   * (so, for example, you can safely put {@code HashCode} instances into a {@code
90   * HashSet}) but is otherwise probably not what you want to use.
91   */
92  @Override public int hashCode() {
93    /*
94     * As long as the hash function that produced this isn't of horrible quality, this
95     * won't be of horrible quality either.
96     */
97    return asInt();
98  }
99
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}
121