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   * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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   */
  
  package java.io;
  
  /**
   * A <code>PushbackInputStream</code> adds
   * functionality to another input stream, namely
   * the  ability to "push back" or "unread"
   * one byte. This is useful in situations where
   * it is  convenient for a fragment of code
   * to read an indefinite number of data bytes
   * that  are delimited by a particular byte
   * value; after reading the terminating byte,
   * the  code fragment can "unread" it, so that
   * the next read operation on the input stream
   * will reread the byte that was pushed back.
   * For example, bytes representing the  characters
   * constituting an identifier might be terminated
   * by a byte representing an  operator character;
   * a method whose job is to read just an identifier
   * can read until it  sees the operator and
   * then push the operator back to be re-read.
   *
   * @author  David Connelly
   * @author  Jonathan Payne
   * @since   JDK1.0
   */
  public
  class PushbackInputStream extends FilterInputStream {
      /**
       * The pushback buffer.
       * @since   JDK1.1
       */
      protected byte[] buf;
  
      /**
       * The position within the pushback buffer from which the next byte will
       * be read.  When the buffer is empty, <code>pos</code> is equal to
       * <code>buf.length</code>; when the buffer is full, <code>pos</code> is
       * equal to zero.
       *
       * @since   JDK1.1
       */
      protected int pos;
  
      /**
       * Check to make sure that this stream has not been closed
       */
      private void ensureOpen() throws IOException {
          if (in == null)
              throw new IOException("Stream closed");
      }
  
      /**
       * Creates a <code>PushbackInputStream</code>
       * with a pushback buffer of the specified <code>size</code>,
       * and saves its  argument, the input stream
       * <code>in</code>, for later use. Initially,
       * there is no pushed-back byte  (the field
       * <code>pushBack</code> is initialized to
       * <code>-1</code>).
       *
       * @param  in    the input stream from which bytes will be read.
       * @param  size  the size of the pushback buffer.
       * @exception IllegalArgumentException if {@code size <= 0}
       * @since  JDK1.1
       */
      public PushbackInputStream(InputStream in, int size) {
          super(in);
          if (size <= 0) {
              throw new IllegalArgumentException("size <= 0");
          }
          this.buf = new byte[size];
          this.pos = size;
      }
  
      /**
       * Creates a <code>PushbackInputStream</code>
       * and saves its  argument, the input stream
       * <code>in</code>, for later use. Initially,
       * there is no pushed-back byte  (the field
       * <code>pushBack</code> is initialized to
       * <code>-1</code>).
       *
       * @param   in   the input stream from which bytes will be read.
       */
      public PushbackInputStream(InputStream in) {
          this(in, 1);
      }
  
      /**
       * Reads the next byte of data from this input stream. The value
       * byte is returned as an <code>int</code> in the range
       * <code>0</code> to <code>255</code>. If no byte is available
       * because the end of the stream has been reached, the value
       * <code>-1</code> is returned. This method blocks until input data
       * is available, the end of the stream is detected, or an exception
       * is thrown.
       *
       * <p> This method returns the most recently pushed-back byte, if there is
       * one, and otherwise calls the <code>read</code> method of its underlying
       * input stream and returns whatever value that method returns.
       *
       * @return     the next byte of data, or <code>-1</code> if the end of the
       *             stream has been reached.
       * @exception  IOException  if this input stream has been closed by
       *             invoking its {@link #close()} method,
       *             or an I/O error occurs.
       * @see        InputStream#read()
       */
      public int read() throws IOException {
          ensureOpen();
          if (pos < buf.length) {
              return buf[pos++] & 0xff;
          }
          return super.read();
      }
  
      /**
       * Reads up to <code>len</code> bytes of data from this input stream into
       * an array of bytes.  This method first reads any pushed-back bytes; after
       * that, if fewer than <code>len</code> bytes have been read then it
       * reads from the underlying input stream. If <code>len</code> is not zero, the method
       * blocks until at least 1 byte of input is available; otherwise, no
       * bytes are read and <code>0</code> is returned.
       *
       * @param      b     the buffer into which the data is read.
       * @param      off   the start offset in the destination array <code>b</code>
       * @param      len   the maximum number of bytes read.
       * @return     the total number of bytes read into the buffer, or
       *             <code>-1</code> if there is no more data because the end of
       *             the stream has been reached.
       * @exception  NullPointerException If <code>b</code> is <code>null</code>.
       * @exception  IndexOutOfBoundsException If <code>off</code> is negative,
       * <code>len</code> is negative, or <code>len</code> is greater than
       * <code>b.length - off</code>
       * @exception  IOException  if this input stream has been closed by
       *             invoking its {@link #close()} method,
       *             or an I/O error occurs.
       * @see        InputStream#read(byte[], int, int)
       */
      public int read(byte[] b, int off, int len) throws IOException {
          ensureOpen();
          if (b == null) {
              throw new NullPointerException();
          } else if (off < 0 || len < 0 || len > b.length - off) {
              throw new IndexOutOfBoundsException();
          } else if (len == 0) {
              return 0;
          }
  
          int avail = buf.length - pos;
          if (avail > 0) {
              if (len < avail) {
                  avail = len;
              }
              System.arraycopy(buf, pos, b, off, avail);
              pos += avail;
              off += avail;
              len -= avail;
          }
          if (len > 0) {
              len = super.read(b, off, len);
              if (len == -1) {
                  return avail == 0 ? -1 : avail;
              }
              return avail + len;
          }
          return avail;
      }
  
      /**
       * Pushes back a byte by copying it to the front of the pushback buffer.
       * After this method returns, the next byte to be read will have the value
       * <code>(byte)b</code>.
       *
       * @param      b   the <code>int</code> value whose low-order
       *                  byte is to be pushed back.
       * @exception IOException If there is not enough room in the pushback
       *            buffer for the byte, or this input stream has been closed by
       *            invoking its {@link #close()} method.
       */
      public void unread(int b) throws IOException {
          ensureOpen();
          if (pos == 0) {
              throw new IOException("Push back buffer is full");
          }
          buf[--pos] = (byte)b;
      }
  
      /**
       * Pushes back a portion of an array of bytes by copying it to the front
       * of the pushback buffer.  After this method returns, the next byte to be
       * read will have the value <code>b[off]</code>, the byte after that will
       * have the value <code>b[off+1]</code>, and so forth.
       *
       * @param b the byte array to push back.
       * @param off the start offset of the data.
       * @param len the number of bytes to push back.
       * @exception IOException If there is not enough room in the pushback
       *            buffer for the specified number of bytes,
       *            or this input stream has been closed by
       *            invoking its {@link #close()} method.
       * @since     JDK1.1
       */
      public void unread(byte[] b, int off, int len) throws IOException {
          ensureOpen();
          if (len > pos) {
              throw new IOException("Push back buffer is full");
          }
          pos -= len;
          System.arraycopy(b, off, buf, pos, len);
      }
  
      /**
       * Pushes back an array of bytes by copying it to the front of the
       * pushback buffer.  After this method returns, the next byte to be read
       * will have the value <code>b[0]</code>, the byte after that will have the
       * value <code>b[1]</code>, and so forth.
       *
       * @param b the byte array to push back
       * @exception IOException If there is not enough room in the pushback
       *            buffer for the specified number of bytes,
       *            or this input stream has been closed by
       *            invoking its {@link #close()} method.
       * @since     JDK1.1
       */
      public void unread(byte[] b) throws IOException {
          unread(b, 0, b.length);
      }
  
      /**
       * Returns an estimate of the number of bytes that can be read (or
       * skipped over) from this input stream without blocking by the next
       * invocation of a method for this input stream. The next invocation might be
       * the same thread or another thread.  A single read or skip of this
       * many bytes will not block, but may read or skip fewer bytes.
       *
       * <p> The method returns the sum of the number of bytes that have been
       * pushed back and the value returned by {@link
       * FilterInputStream#available available}.
       *
       * @return     the number of bytes that can be read (or skipped over) from
       *             the input stream without blocking.
       * @exception  IOException  if this input stream has been closed by
       *             invoking its {@link #close()} method,
       *             or an I/O error occurs.
       * @see        FilterInputStream#in
       * @see        InputStream#available()
       */
      public int available() throws IOException {
          ensureOpen();
          int n = buf.length - pos;
          int avail = super.available();
          return n > (Integer.MAX_VALUE - avail)
                      ? Integer.MAX_VALUE
                      : n + avail;
      }
  
      /**
       * Skips over and discards <code>n</code> bytes of data from this
       * input stream. The <code>skip</code> method may, for a variety of
       * reasons, end up skipping over some smaller number of bytes,
       * possibly zero.  If <code>n</code> is negative, no bytes are skipped.
       *
       * <p> The <code>skip</code> method of <code>PushbackInputStream</code>
       * first skips over the bytes in the pushback buffer, if any.  It then
       * calls the <code>skip</code> method of the underlying input stream if
       * more bytes need to be skipped.  The actual number of bytes skipped
       * is returned.
       *
       * @param      n  {@inheritDoc}
       * @return     {@inheritDoc}
       * @exception  IOException  if the stream does not support seek,
       *            or the stream has been closed by
       *            invoking its {@link #close()} method,
       *            or an I/O error occurs.
       * @see        FilterInputStream#in
       * @see        InputStream#skip(long n)
       * @since      1.2
       */
      public long skip(long n) throws IOException {
          ensureOpen();
          if (n <= 0) {
              return 0;
          }
  
          long pskip = buf.length - pos;
          if (pskip > 0) {
              if (n < pskip) {
                  pskip = n;
              }
              pos += pskip;
              n -= pskip;
          }
          if (n > 0) {
              pskip += super.skip(n);
          }
          return pskip;
      }
  
      /**
       * Tests if this input stream supports the <code>mark</code> and
       * <code>reset</code> methods, which it does not.
       *
       * @return   <code>false</code>, since this class does not support the
       *           <code>mark</code> and <code>reset</code> methods.
       * @see     InputStream#mark(int)
       * @see     InputStream#reset()
       */
      public boolean markSupported() {
          return false;
      }
  
      /**
       * Marks the current position in this input stream.
       *
       * <p> The <code>mark</code> method of <code>PushbackInputStream</code>
       * does nothing.
       *
       * @param   readlimit   the maximum limit of bytes that can be read before
       *                      the mark position becomes invalid.
       * @see     InputStream#reset()
       */
      public synchronized void mark(int readlimit) {
      }
  
      /**
       * Repositions this stream to the position at the time the
       * <code>mark</code> method was last called on this input stream.
       *
       * <p> The method <code>reset</code> for class
       * <code>PushbackInputStream</code> does nothing except throw an
       * <code>IOException</code>.
       *
       * @exception  IOException  if this method is invoked.
       * @see     InputStream#mark(int)
       * @see     IOException
       */
      public synchronized void reset() throws IOException {
          throw new IOException("mark/reset not supported");
      }
  
      /**
       * Closes this input stream and releases any system resources
       * associated with the stream.
       * Once the stream has been closed, further read(), unread(),
       * available(), reset(), or skip() invocations will throw an IOException.
       * Closing a previously closed stream has no effect.
       *
       * @exception  IOException  if an I/O error occurs.
       */
      public synchronized void close() throws IOException {
          if (in == null)
              return;
          in.close();
          in = null;
          buf = null;
      }
  }
  