/*
 * Copyright (C) 2009 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.primitives;

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

import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;

import sun.misc.Unsafe;

import java.nio.ByteOrder;
import java.util.Comparator;

/**
 * Static utility methods pertaining to {@code byte} primitives that interpret
 * values as <i>unsigned</i> (that is, any negative value {@code b} is treated
 * as the positive value {@code 256 + b}). The corresponding methods that treat
 * the values as signed are found in {@link SignedBytes}, and the methods for
 * which signedness is not an issue are in {@link Bytes}.
 *
 * <p>See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/PrimitivesExplained">
 * primitive utilities</a>.
 *
 * @author Kevin Bourrillion
 * @author Martin Buchholz
 * @author Hiroshi Yamauchi
 * @author Louis Wasserman
 * @since 1.0
 */
public final class UnsignedBytes {
  private UnsignedBytes() {}

  /**
   * The largest power of two that can be represented as an unsigned {@code
   * byte}.
   *
   * @since 10.0
   */
  public static final byte MAX_POWER_OF_TWO = (byte) 0x80;

  /**
   * The largest value that fits into an unsigned byte.
   *
   * @since 13.0
   */
  public static final byte MAX_VALUE = (byte) 0xFF;

  private static final int UNSIGNED_MASK = 0xFF;

  /**
   * Returns the value of the given byte as an integer, when treated as
   * unsigned. That is, returns {@code value + 256} if {@code value} is
   * negative; {@code value} itself otherwise.
   *
   * @since 6.0
   */
  public static int toInt(byte value) {
    return value & UNSIGNED_MASK;
  }

  /**
   * Returns the {@code byte} value that, when treated as unsigned, is equal to
   * {@code value}, if possible.
   *
   * @param value a value between 0 and 255 inclusive
   * @return the {@code byte} value that, when treated as unsigned, equals
   *     {@code value}
   * @throws IllegalArgumentException if {@code value} is negative or greater
   *     than 255
   */
  public static byte checkedCast(long value) {
    checkArgument(value >> Byte.SIZE == 0, "out of range: %s", value);
    return (byte) value;
  }

  /**
   * Returns the {@code byte} value that, when treated as unsigned, is nearest
   * in value to {@code value}.
   *
   * @param value any {@code long} value
   * @return {@code (byte) 255} if {@code value >= 255}, {@code (byte) 0} if
   *     {@code value <= 0}, and {@code value} cast to {@code byte} otherwise
   */
  public static byte saturatedCast(long value) {
    if (value > toInt(MAX_VALUE)) {
      return MAX_VALUE; // -1
    }
    if (value < 0) {
      return (byte) 0;
    }
    return (byte) value;
  }

  /**
   * Compares the two specified {@code byte} values, treating them as unsigned
   * values between 0 and 255 inclusive. For example, {@code (byte) -127} is
   * considered greater than {@code (byte) 127} because it is seen as having
   * the value of positive {@code 129}.
   *
   * @param a the first {@code byte} to compare
   * @param b the second {@code byte} to compare
   * @return a negative value if {@code a} is less than {@code b}; a positive
   *     value if {@code a} is greater than {@code b}; or zero if they are equal
   */
  public static int compare(byte a, byte b) {
    return toInt(a) - toInt(b);
  }

  /**
   * Returns the least value present in {@code array}.
   *
   * @param array a <i>nonempty</i> array of {@code byte} values
   * @return the value present in {@code array} that is less than or equal to
   *     every other value in the array
   * @throws IllegalArgumentException if {@code array} is empty
   */
  public static byte min(byte... array) {
    checkArgument(array.length > 0);
    int min = toInt(array[0]);
    for (int i = 1; i < array.length; i++) {
      int next = toInt(array[i]);
      if (next < min) {
        min = next;
      }
    }
    return (byte) min;
  }

  /**
   * Returns the greatest value present in {@code array}.
   *
   * @param array a <i>nonempty</i> array of {@code byte} values
   * @return the value present in {@code array} that is greater than or equal
   *     to every other value in the array
   * @throws IllegalArgumentException if {@code array} is empty
   */
  public static byte max(byte... array) {
    checkArgument(array.length > 0);
    int max = toInt(array[0]);
    for (int i = 1; i < array.length; i++) {
      int next = toInt(array[i]);
      if (next > max) {
        max = next;
      }
    }
    return (byte) max;
  }

  /**
   * Returns a string representation of x, where x is treated as unsigned.
   *
   * @since 13.0
   */
  @Beta
  public static String toString(byte x) {
    return toString(x, 10);
  }

  /**
   * Returns a string representation of {@code x} for the given radix, where {@code x} is treated
   * as unsigned.
   *
   * @param x the value to convert to a string.
   * @param radix the radix to use while working with {@code x}
   * @throws IllegalArgumentException if {@code radix} is not between {@link Character#MIN_RADIX}
   *         and {@link Character#MAX_RADIX}.
   * @since 13.0
   */
  @Beta
  public static String toString(byte x, int radix) {
    checkArgument(radix >= Character.MIN_RADIX && radix <= Character.MAX_RADIX,
        "radix (%s) must be between Character.MIN_RADIX and Character.MAX_RADIX", radix);
    // Benchmarks indicate this is probably not worth optimizing.
    return Integer.toString(toInt(x), radix);
  }

  /**
   * Returns the unsigned {@code byte} value represented by the given decimal string.
   *
   * @throws NumberFormatException if the string does not contain a valid unsigned {@code byte}
   *         value
   * @throws NullPointerException if {@code s} is null 
   *         (in contrast to {@link Byte#parseByte(String)})
   * @since 13.0
   */
  @Beta
  public static byte parseUnsignedByte(String string) {
    return parseUnsignedByte(string, 10);
  }

  /**
   * Returns the unsigned {@code byte} value represented by a string with the given radix.
   *
   * @param string the string containing the unsigned {@code byte} representation to be parsed.
   * @param radix the radix to use while parsing {@code string}
   * @throws NumberFormatException if the string does not contain a valid unsigned {@code byte}
   *         with the given radix, or if {@code radix} is not between {@link Character#MIN_RADIX}
   *         and {@link Character#MAX_RADIX}.
   * @throws NullPointerException if {@code s} is null 
   *         (in contrast to {@link Byte#parseByte(String)})
   * @since 13.0
   */
  @Beta
  public static byte parseUnsignedByte(String string, int radix) {
    int parse = Integer.parseInt(checkNotNull(string), radix);
    // We need to throw a NumberFormatException, so we have to duplicate checkedCast. =(
    if (parse >> Byte.SIZE == 0) {
      return (byte) parse;
    } else {
      throw new NumberFormatException("out of range: " + parse);
    }
  }

  /**
   * Returns a string containing the supplied {@code byte} values separated by
   * {@code separator}. For example, {@code join(":", (byte) 1, (byte) 2,
   * (byte) 255)} returns the string {@code "1:2:255"}.
   *
   * @param separator the text that should appear between consecutive values in
   *     the resulting string (but not at the start or end)
   * @param array an array of {@code byte} values, possibly empty
   */
  public static String join(String separator, byte... array) {
    checkNotNull(separator);
    if (array.length == 0) {
      return "";
    }

    // For pre-sizing a builder, just get the right order of magnitude
    StringBuilder builder = new StringBuilder(array.length * (3 + separator.length()));
    builder.append(toInt(array[0]));
    for (int i = 1; i < array.length; i++) {
      builder.append(separator).append(toString(array[i]));
    }
    return builder.toString();
  }

  /**
   * Returns a comparator that compares two {@code byte} arrays
   * lexicographically. That is, it compares, using {@link
   * #compare(byte, byte)}), the first pair of values that follow any common
   * prefix, or when one array is a prefix of the other, treats the shorter
   * array as the lesser. For example, {@code [] < [0x01] < [0x01, 0x7F] <
   * [0x01, 0x80] < [0x02]}. Values are treated as unsigned.
   *
   * <p>The returned comparator is inconsistent with {@link
   * Object#equals(Object)} (since arrays support only identity equality), but
   * it is consistent with {@link java.util.Arrays#equals(byte[], byte[])}.
   *
   * @see <a href="http://en.wikipedia.org/wiki/Lexicographical_order">
   *     Lexicographical order article at Wikipedia</a>
   * @since 2.0
   */
  public static Comparator<byte[]> lexicographicalComparator() {
    return LexicographicalComparatorHolder.BEST_COMPARATOR;
  }

  @VisibleForTesting
  static Comparator<byte[]> lexicographicalComparatorJavaImpl() {
    return LexicographicalComparatorHolder.PureJavaComparator.INSTANCE;
  }

  /**
   * Provides a lexicographical comparator implementation; either a Java
   * implementation or a faster implementation based on {@link Unsafe}.
   *
   * <p>Uses reflection to gracefully fall back to the Java implementation if
   * {@code Unsafe} isn't available.
   */
  @VisibleForTesting
  static class LexicographicalComparatorHolder {
    static final String UNSAFE_COMPARATOR_NAME =
        LexicographicalComparatorHolder.class.getName() + "$UnsafeComparator";

    static final Comparator<byte[]> BEST_COMPARATOR = getBestComparator();

    @VisibleForTesting
    enum UnsafeComparator implements Comparator<byte[]> {
      INSTANCE;

      static final boolean BIG_ENDIAN =
          ByteOrder.nativeOrder().equals(ByteOrder.BIG_ENDIAN);

      /*
       * The following static final fields exist for performance reasons.
       *
       * In UnsignedBytesBenchmark, accessing the following objects via static
       * final fields is the fastest (more than twice as fast as the Java
       * implementation, vs ~1.5x with non-final static fields, on x86_32)
       * under the Hotspot server compiler. The reason is obviously that the
       * non-final fields need to be reloaded inside the loop.
       *
       * And, no, defining (final or not) local variables out of the loop still
       * isn't as good because the null check on the theUnsafe object remains
       * inside the loop and BYTE_ARRAY_BASE_OFFSET doesn't get
       * constant-folded.
       *
       * The compiler can treat static final fields as compile-time constants
       * and can constant-fold them while (final or not) local variables are
       * run time values.
       */

      static final Unsafe theUnsafe;

      /** The offset to the first element in a byte array. */
      static final int BYTE_ARRAY_BASE_OFFSET;

      static {
        theUnsafe = getUnsafe();

        BYTE_ARRAY_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);

        // sanity check - this should never fail
        if (theUnsafe.arrayIndexScale(byte[].class) != 1) {
          throw new AssertionError();
        }
      }
      
      /**
       * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
       * Replace with a simple call to Unsafe.getUnsafe when integrating
       * into a jdk.
       *
       * @return a sun.misc.Unsafe
       */
      private static sun.misc.Unsafe getUnsafe() {
          try {
              return sun.misc.Unsafe.getUnsafe();
          } catch (SecurityException tryReflectionInstead) {}
          try {
              return java.security.AccessController.doPrivileged
              (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                  public sun.misc.Unsafe run() throws Exception {
                      Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                      for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                          f.setAccessible(true);
                          Object x = f.get(null);
                          if (k.isInstance(x))
                              return k.cast(x);
                      }
                      throw new NoSuchFieldError("the Unsafe");
                  }});
          } catch (java.security.PrivilegedActionException e) {
              throw new RuntimeException("Could not initialize intrinsics",
                                         e.getCause());
          }
      }

      @Override public int compare(byte[] left, byte[] right) {
        int minLength = Math.min(left.length, right.length);
        int minWords = minLength / Longs.BYTES;

        /*
         * Compare 8 bytes at a time. Benchmarking shows comparing 8 bytes at a
         * time is no slower than comparing 4 bytes at a time even on 32-bit.
         * On the other hand, it is substantially faster on 64-bit.
         */
        for (int i = 0; i < minWords * Longs.BYTES; i += Longs.BYTES) {
          long lw = theUnsafe.getLong(left, BYTE_ARRAY_BASE_OFFSET + (long) i);
          long rw = theUnsafe.getLong(right, BYTE_ARRAY_BASE_OFFSET + (long) i);
          if (lw != rw) {
            if (BIG_ENDIAN) {
              return UnsignedLongs.compare(lw, rw);
            }

            /*
             * We want to compare only the first index where left[index] != right[index].
             * This corresponds to the least significant nonzero byte in lw ^ rw, since lw
             * and rw are little-endian.  Long.numberOfTrailingZeros(diff) tells us the least 
             * significant nonzero bit, and zeroing out the first three bits of L.nTZ gives us the 
             * shift to get that least significant nonzero byte.
             */
            int n = Long.numberOfTrailingZeros(lw ^ rw) & ~0x7;
            return (int) (((lw >>> n) & UNSIGNED_MASK) - ((rw >>> n) & UNSIGNED_MASK));
          }
        }

        // The epilogue to cover the last (minLength % 8) elements.
        for (int i = minWords * Longs.BYTES; i < minLength; i++) {
          int result = UnsignedBytes.compare(left[i], right[i]);
          if (result != 0) {
            return result;
          }
        }
        return left.length - right.length;
      }
    }

    enum PureJavaComparator implements Comparator<byte[]> {
      INSTANCE;

      @Override public int compare(byte[] left, byte[] right) {
        int minLength = Math.min(left.length, right.length);
        for (int i = 0; i < minLength; i++) {
          int result = UnsignedBytes.compare(left[i], right[i]);
          if (result != 0) {
            return result;
          }
        }
        return left.length - right.length;
      }
    }

    /**
     * Returns the Unsafe-using Comparator, or falls back to the pure-Java
     * implementation if unable to do so.
     */
    static Comparator<byte[]> getBestComparator() {
      try {
        Class<?> theClass = Class.forName(UNSAFE_COMPARATOR_NAME);

        // yes, UnsafeComparator does implement Comparator<byte[]>
        @SuppressWarnings("unchecked")
        Comparator<byte[]> comparator =
            (Comparator<byte[]>) theClass.getEnumConstants()[0];
        return comparator;
      } catch (Throwable t) { // ensure we really catch *everything*
        return lexicographicalComparatorJavaImpl();
      }
    }
  }
}