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
015package com.google.common.hash;
016
017import com.google.common.annotations.Beta;
018import com.google.common.base.Preconditions;
019import com.google.common.primitives.Ints;
020
021import 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
030public 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}