001/*
002 * Copyright (C) 2009 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.primitives;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021
022import com.google.common.annotations.VisibleForTesting;
023
024import sun.misc.Unsafe;
025
026import java.lang.reflect.Field;
027import java.nio.ByteOrder;
028import java.security.AccessController;
029import java.security.PrivilegedAction;
030import java.util.Comparator;
031
032/**
033 * Static utility methods pertaining to {@code byte} primitives that interpret
034 * values as <i>unsigned</i> (that is, any negative value {@code b} is treated
035 * as the positive value {@code 256 + b}). The corresponding methods that treat
036 * the values as signed are found in {@link SignedBytes}, and the methods for
037 * which signedness is not an issue are in {@link Bytes}.
038 *
039 * @author Kevin Bourrillion
040 * @author Martin Buchholz
041 * @author Hiroshi Yamauchi
042 * @since 1
043 */
044public final class UnsignedBytes {
045  private UnsignedBytes() {}
046
047  /**
048   * Returns the value of the given byte as an integer, when treated as
049   * unsigned. That is, returns {@code value + 256} if {@code value} is
050   * negative; {@code value} itself otherwise.
051   *
052   * @since 6
053   */
054  public static int toInt(byte value) {
055    return value & 0xFF;
056  }
057
058  /**
059   * Returns the {@code byte} value that, when treated as unsigned, is equal to
060   * {@code value}, if possible.
061   *
062   * @param value a value between 0 and 255 inclusive
063   * @return the {@code byte} value that, when treated as unsigned, equals
064   *     {@code value}
065   * @throws IllegalArgumentException if {@code value} is negative or greater
066   *     than 255
067   */
068  public static byte checkedCast(long value) {
069    checkArgument(value >> 8 == 0, "out of range: %s", value);
070    return (byte) value;
071  }
072
073  /**
074   * Returns the {@code byte} value that, when treated as unsigned, is nearest
075   * in value to {@code value}.
076   *
077   * @param value any {@code long} value
078   * @return {@code (byte) 255} if {@code value >= 255}, {@code (byte) 0} if
079   *     {@code value <= 0}, and {@code value} cast to {@code byte} otherwise
080   */
081  public static byte saturatedCast(long value) {
082    if (value > 255) {
083      return (byte) 255; // -1
084    }
085    if (value < 0) {
086      return (byte) 0;
087    }
088    return (byte) value;
089  }
090
091  /**
092   * Compares the two specified {@code byte} values, treating them as unsigned
093   * values between 0 and 255 inclusive. For example, {@code (byte) -127} is
094   * considered greater than {@code (byte) 127} because it is seen as having
095   * the value of positive {@code 129}.
096   *
097   * @param a the first {@code byte} to compare
098   * @param b the second {@code byte} to compare
099   * @return a negative value if {@code a} is less than {@code b}; a positive
100   *     value if {@code a} is greater than {@code b}; or zero if they are equal
101   */
102  public static int compare(byte a, byte b) {
103    return toInt(a) - toInt(b);
104  }
105
106  /**
107   * Returns the least value present in {@code array}.
108   *
109   * @param array a <i>nonempty</i> array of {@code byte} values
110   * @return the value present in {@code array} that is less than or equal to
111   *     every other value in the array
112   * @throws IllegalArgumentException if {@code array} is empty
113   */
114  public static byte min(byte... array) {
115    checkArgument(array.length > 0);
116    int min = toInt(array[0]);
117    for (int i = 1; i < array.length; i++) {
118      int next = toInt(array[i]);
119      if (next < min) {
120        min = next;
121      }
122    }
123    return (byte) min;
124  }
125
126  /**
127   * Returns the greatest value present in {@code array}.
128   *
129   * @param array a <i>nonempty</i> array of {@code byte} values
130   * @return the value present in {@code array} that is greater than or equal
131   *     to every other value in the array
132   * @throws IllegalArgumentException if {@code array} is empty
133   */
134  public static byte max(byte... array) {
135    checkArgument(array.length > 0);
136    int max = toInt(array[0]);
137    for (int i = 1; i < array.length; i++) {
138      int next = toInt(array[i]);
139      if (next > max) {
140        max = next;
141      }
142    }
143    return (byte) max;
144  }
145
146  /**
147   * Returns a string containing the supplied {@code byte} values separated by
148   * {@code separator}. For example, {@code join(":", (byte) 1, (byte) 2,
149   * (byte) 255)} returns the string {@code "1:2:255"}.
150   *
151   * @param separator the text that should appear between consecutive values in
152   *     the resulting string (but not at the start or end)
153   * @param array an array of {@code byte} values, possibly empty
154   */
155  public static String join(String separator, byte... array) {
156    checkNotNull(separator);
157    if (array.length == 0) {
158      return "";
159    }
160
161    // For pre-sizing a builder, just get the right order of magnitude
162    StringBuilder builder = new StringBuilder(array.length * 5);
163    builder.append(toInt(array[0]));
164    for (int i = 1; i < array.length; i++) {
165      builder.append(separator).append(toInt(array[i]));
166    }
167    return builder.toString();
168  }
169
170  /**
171   * Returns a comparator that compares two {@code byte} arrays
172   * lexicographically. That is, it compares, using {@link
173   * #compare(byte, byte)}), the first pair of values that follow any common
174   * prefix, or when one array is a prefix of the other, treats the shorter
175   * array as the lesser. For example, {@code [] < [0x01] < [0x01, 0x7F] <
176   * [0x01, 0x80] < [0x02]}. Values are treated as unsigned.
177   *
178   * <p>The returned comparator is inconsistent with {@link
179   * Object#equals(Object)} (since arrays support only identity equality), but
180   * it is consistent with {@link java.util.Arrays#equals(byte[], byte[])}.
181   *
182   * @see <a href="http://en.wikipedia.org/wiki/Lexicographical_order">
183   *     Lexicographical order article at Wikipedia</a>
184   * @since 2
185   */
186  public static Comparator<byte[]> lexicographicalComparator() {
187    return LexicographicalComparatorHolder.BEST_COMPARATOR;
188  }
189
190  @VisibleForTesting
191  static Comparator<byte[]> lexicographicalComparatorJavaImpl() {
192    return LexicographicalComparatorHolder.PureJavaComparator.INSTANCE;
193  }
194
195  /**
196   * Provides a lexicographical comparator implementation; either a Java
197   * implementation or a faster implementation based on {@link Unsafe}.
198   *
199   * <p>Uses reflection to gracefully fall back to the Java implementation if
200   * {@code Unsafe} isn't available.
201   */
202  @VisibleForTesting
203  static class LexicographicalComparatorHolder {
204    static final String UNSAFE_COMPARATOR_NAME =
205        LexicographicalComparatorHolder.class.getName() + "$UnsafeComparator";
206
207    static final Comparator<byte[]> BEST_COMPARATOR = getBestComparator();
208
209    @SuppressWarnings("unused") // only access this class via reflection!
210    enum UnsafeComparator implements Comparator<byte[]> {
211      INSTANCE;
212
213      static final boolean littleEndian =
214          ByteOrder.nativeOrder().equals(ByteOrder.LITTLE_ENDIAN);
215
216      /*
217       * The following static final fields exist for performance reasons.
218       *
219       * In UnsignedBytesBenchmark, accessing the following objects via static
220       * final fields is the fastest (more than twice as fast as the Java
221       * implementation, vs ~1.5x with non-final static fields, on x86_32)
222       * under the Hotspot server compiler. The reason is obviously that the
223       * non-final fields need to be reloaded inside the loop.
224       *
225       * And, no, defining (final or not) local variables out of the loop still
226       * isn't as good because the null check on the theUnsafe object remains
227       * inside the loop and BYTE_ARRAY_BASE_OFFSET doesn't get
228       * constant-folded.
229       *
230       * The compiler can treat static final fields as compile-time constants
231       * and can constant-fold them while (final or not) local variables are
232       * run time values.
233       */
234
235      static final Unsafe theUnsafe;
236
237      /** The offset to the first element in a byte array. */
238      static final int BYTE_ARRAY_BASE_OFFSET;
239
240      static {
241        theUnsafe = (Unsafe) AccessController.doPrivileged(
242            new PrivilegedAction<Object>() {
243              @Override
244              public Object run() {
245                try {
246                  Field f = Unsafe.class.getDeclaredField("theUnsafe");
247                  f.setAccessible(true);
248                  return f.get(null);
249                } catch (NoSuchFieldException e) {
250                  // It doesn't matter what we throw;
251                  // it's swallowed in getBestComparator().
252                  throw new Error();
253                } catch (IllegalAccessException e) {
254                  throw new Error();
255                }
256              }
257            });
258
259        BYTE_ARRAY_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);
260
261        // sanity check - this should never fail
262        if (theUnsafe.arrayIndexScale(byte[].class) != 1) {
263          throw new AssertionError();
264        }
265      }
266
267      /**
268       * Returns true if x1 is less than x2, when both values are treated as
269       * unsigned.
270       */
271      // TODO(kevinb): Should be a common method in primitives.UnsignedLongs.
272      static boolean lessThanUnsigned(long x1, long x2) {
273        return (x1 + Long.MIN_VALUE) < (x2 + Long.MIN_VALUE);
274      }
275
276      @Override public int compare(byte[] left, byte[] right) {
277        int minLength = Math.min(left.length, right.length);
278        int minWords = minLength / Longs.BYTES;
279
280        /*
281         * Compare 8 bytes at a time. Benchmarking shows comparing 8 bytes at a
282         * time is no slower than comparing 4 bytes at a time even on 32-bit.
283         * On the other hand, it is substantially faster on 64-bit.
284         */
285        for (int i = 0; i < minWords * Longs.BYTES; i += Longs.BYTES) {
286          long lw = theUnsafe.getLong(left, BYTE_ARRAY_BASE_OFFSET + (long) i);
287          long rw = theUnsafe.getLong(right, BYTE_ARRAY_BASE_OFFSET + (long) i);
288          long diff = lw ^ rw;
289
290          if (diff != 0) {
291            if (!littleEndian) {
292              return lessThanUnsigned(lw, rw) ? -1 : 1;
293            }
294
295            // Use binary search
296            int n = 0;
297            int y;
298            int x = (int) diff;
299            if (x == 0) {
300              x = (int) (diff >>> 32);
301              n = 32;
302            }
303
304            y = x << 16;
305            if (y == 0) {
306              n += 16;
307            } else {
308              x = y;
309            }
310
311            y = x << 8;
312            if (y == 0) {
313              n +=  8;
314            }
315            return (int) (((lw >>> n) & 0xFFL) - ((rw >>> n) & 0xFFL));
316          }
317        }
318
319        // The epilogue to cover the last (minLength % 8) elements.
320        for (int i = minWords * Longs.BYTES; i < minLength; i++) {
321          int result = UnsignedBytes.compare(left[i], right[i]);
322          if (result != 0) {
323            return result;
324          }
325        }
326        return left.length - right.length;
327      }
328    }
329
330    enum PureJavaComparator implements Comparator<byte[]> {
331      INSTANCE;
332
333      @Override public int compare(byte[] left, byte[] right) {
334        int minLength = Math.min(left.length, right.length);
335        for (int i = 0; i < minLength; i++) {
336          int result = UnsignedBytes.compare(left[i], right[i]);
337          if (result != 0) {
338            return result;
339          }
340        }
341        return left.length - right.length;
342      }
343    }
344
345    /**
346     * Returns the Unsafe-using Comparator, or falls back to the pure-Java
347     * implementation if unable to do so.
348     */
349    static Comparator<byte[]> getBestComparator() {
350      try {
351        Class<?> theClass = Class.forName(UNSAFE_COMPARATOR_NAME);
352
353        // yes, UnsafeComparator does implement Comparator<byte[]>
354        @SuppressWarnings("unchecked")
355        Comparator<byte[]> comparator =
356            (Comparator<byte[]>) theClass.getEnumConstants()[0];
357        return comparator;
358      } catch (Throwable t) { // ensure we really catch *everything*
359        return lexicographicalComparatorJavaImpl();
360      }
361    }
362  }
363}