code: /*
   * Copyright (c) 2017-2018 The Regents of the University of California
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are
   * met:
   *
   * 1. Redistributions of source code must retain the above copyright
   * notice, this list of conditions and the following disclaimer.
   *
   * 2. Redistributions in binary form must reproduce the above copyright
   * notice, this list of conditions and the following disclaimer in the
   * documentation and/or other materials provided with the distribution.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
   * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  
  package edu.berkeley.cs.jqf.fuzz.guidance;
  
  import java.io.EOFException;
  import java.io.IOException;
  import java.io.InputStream;
  import java.nio.ByteBuffer;
  import java.nio.ByteOrder;
  import java.util.Random;
  
  
  /**
   * This class extends {@link Random} to act as a generator of
   * "random" values, which themselves are read from a static file.
   *
   * The file-backed random number generator can be used for tuning the
   * "random" choices made by various <code>junit-quickcheck</code>
   * generators using a mutation-based genetic algorithm, in order to
   * maximize some objective function that can be measured from the
   * execution of each trial, such as code coverage.
   *
   *
   */
  public class StreamBackedRandom extends Random {
      private final InputStream inputStream;
      private final ByteBuffer byteBuffer = ByteBuffer.allocate(4);
      private int totalBytesRead = 0;
      private int leadingBytesToIgnore = 0;
  
      /**
       * Constructs a stream-backed random generator.
       *
       * Also sets the seed of the underlying pseudo-random number
       * generator deterministically to zero.
       *
       * @param source  a generator of "random" bytes
       */
      public StreamBackedRandom(InputStream source) {
          super(0x5DEECE66DL);
          // Open the backing file source as a buffered input stream
          this.inputStream = source;
          // Force encoding to little-endian so that we can read small ints
          // by reading partially into the start of the buffer
          byteBuffer.order(ByteOrder.LITTLE_ENDIAN);
      }
  
      /**
       * Convenience constructor for use with the junit-quickcheck framework.
       *
       * The junit-quickcheck SourceOfRandomness annoyingly reads 8 bytes of
       * random data as soon as it is instantiated, only to configure its own
       * seed. As the seed is meaningless here, the 8 bytes get wasted. For
       * this purpose, this constructor allows specifying how many bytes of
       * requested random data to skip before starting to read from file.
       *
       * @param source  a generator of "random" bytes
       * @param leadinBytesToIgnore the number of leading bytes to ignore
       */
      public StreamBackedRandom(InputStream source, int leadinBytesToIgnore) {
          this(source);
          this.leadingBytesToIgnore = leadinBytesToIgnore;
      }
  
      /**
       * Generates upto 32 bits of random data for internal use by the Random
       * class.
       *
       * <p>Attempts to read up to 4 bytes of data from the input file, and
       * returns the requested lower order bits as a pseudo-random value.</p>
       *
       * <p>If end-of-file is reached before reading 4 bytes,
       * an {@link IllegalStateException} is thrown.</p>
       *
       * @param bits   the number of random bits to retain (1 to 32 inclusive)
       * @return the integer value whose lower <code>bits</code> bits contain the
       *    next random data available in the backing source
       * @throws IllegalStateException  if EOF has already been reached
       */
      @Override
      public int next(int bits) {
          // Ensure that up to 32 bits are being requested
          if (bits < 0 || bits > 32) {
              throw new IllegalArgumentException("Must read 1-32 bits at a time");
          }
  
          // Zero out the byte buffer before reading from the source
          byteBuffer.putInt(0, 0);
  
          try {
              // Read up to 4 bytes from the backing source
              int maxBytesToRead = ((bits + 7) / 8);
              assert(maxBytesToRead*8 >= bits && maxBytesToRead <= 4);
  
              if (this.leadingBytesToIgnore > 0) {
                  int bytesToIgnore = Math.min(maxBytesToRead, this.leadingBytesToIgnore);
                  this.leadingBytesToIgnore -= bytesToIgnore;
                  maxBytesToRead -= bytesToIgnore;
              }
  
              // If fewer bytes are read (because EOF is reached), the buffer
              // just keeps containing zeros
              int actualBytesRead = inputStream.read(byteBuffer.array(), 0, maxBytesToRead);
              totalBytesRead += actualBytesRead;
  
              // If EOF was reached, throw an exception
              if (actualBytesRead != maxBytesToRead) {
                  String message = String.format("EOF reached; total bytes read = %d, " +
                                  "last read got %d of %d bytes",
                          totalBytesRead, actualBytesRead, maxBytesToRead);
                  throw new IllegalStateException(new EOFException(message));
  
              }
  
          } catch (IOException e) {
              throw new GuidanceException(e);
          }
  
          // Interpret the bytes read as an integer
          int value = byteBuffer.getInt(0);
  
          // Return only the lower order bits as requested
          int mask = bits < 32 ? (1 << bits) - 1 : -1;
          return value & mask;
  
      }
  
      @Override
      public int nextInt(int bound) {
          if (bound <= 0)
              throw new IllegalArgumentException("bound must be positive");
          return next(31) % bound;
      }
  
      public byte nextByte() {
          return (byte) next(Byte.SIZE);
      }
  
      public short nextShort() {
          return (short) next(Short.SIZE);
      }
  
      public int getTotalBytesRead() {
          return this.totalBytesRead;
      }
  
  }
  