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   * Copyright (c) 1996, 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
   * or visit www.oracle.com if you need additional information or have any
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   */
  
  package java.io;
  
  
  /**
   * A character-stream reader that allows characters to be pushed back into the
   * stream.
   *
   * @author      Mark Reinhold
   * @since       JDK1.1
   */
  
  public class PushbackReader extends FilterReader {
  
      /** Pushback buffer */
      private char[] buf;
  
      /** Current position in buffer */
      private int pos;
  
      /**
       * Creates a new pushback reader with a pushback buffer of the given size.
       *
       * @param   in   The reader from which characters will be read
       * @param   size The size of the pushback buffer
       * @exception IllegalArgumentException if {@code size <= 0}
       */
      public PushbackReader(Reader in, int size) {
          super(in);
          if (size <= 0) {
              throw new IllegalArgumentException("size <= 0");
          }
          this.buf = new char[size];
          this.pos = size;
      }
  
      /**
       * Creates a new pushback reader with a one-character pushback buffer.
       *
       * @param   in  The reader from which characters will be read
       */
      public PushbackReader(Reader in) {
          this(in, 1);
      }
  
      /** Checks to make sure that the stream has not been closed. */
      private void ensureOpen() throws IOException {
          if (buf == null)
              throw new IOException("Stream closed");
      }
  
      /**
       * Reads a single character.
       *
       * @return     The character read, or -1 if the end of the stream has been
       *             reached
       *
       * @exception  IOException  If an I/O error occurs
       */
      public int read() throws IOException {
          synchronized (lock) {
              ensureOpen();
              if (pos < buf.length)
                  return buf[pos++];
              else
                  return super.read();
          }
      }
  
      /**
       * Reads characters into a portion of an array.
       *
       * @param      cbuf  Destination buffer
       * @param      off   Offset at which to start writing characters
       * @param      len   Maximum number of characters to read
       *
       * @return     The number of characters read, or -1 if the end of the
       *             stream has been reached
       *
       * @exception  IOException  If an I/O error occurs
       */
      public int read(char cbuf[], int off, int len) throws IOException {
          synchronized (lock) {
              ensureOpen();
              try {
                  if (len <= 0) {
                      if (len < 0) {
                          throw new IndexOutOfBoundsException();
                      } else if ((off < 0) || (off > cbuf.length)) {
                          throw new IndexOutOfBoundsException();
                      }
                      return 0;
                  }
                  int avail = buf.length - pos;
                  if (avail > 0) {
                      if (len < avail)
                          avail = len;
                      System.arraycopy(buf, pos, cbuf, off, avail);
                      pos += avail;
                      off += avail;
                      len -= avail;
                  }
                  if (len > 0) {
                      len = super.read(cbuf, off, len);
                      if (len == -1) {
                          return (avail == 0) ? -1 : avail;
                      }
                      return avail + len;
                  }
                  return avail;
              } catch (ArrayIndexOutOfBoundsException e) {
                  throw new IndexOutOfBoundsException();
              }
          }
      }
  
      /**
       * Pushes back a single character by copying it to the front of the
       * pushback buffer. After this method returns, the next character to be read
       * will have the value <code>(char)c</code>.
       *
       * @param  c  The int value representing a character to be pushed back
       *
       * @exception  IOException  If the pushback buffer is full,
       *                          or if some other I/O error occurs
       */
      public void unread(int c) throws IOException {
          synchronized (lock) {
              ensureOpen();
              if (pos == 0)
                  throw new IOException("Pushback buffer overflow");
              buf[--pos] = (char) c;
          }
      }
  
      /**
       * Pushes back a portion of an array of characters by copying it to the
       * front of the pushback buffer.  After this method returns, the next
       * character to be read will have the value <code>cbuf[off]</code>, the
       * character after that will have the value <code>cbuf[off+1]</code>, and
       * so forth.
       *
       * @param  cbuf  Character array
       * @param  off   Offset of first character to push back
       * @param  len   Number of characters to push back
       *
       * @exception  IOException  If there is insufficient room in the pushback
       *                          buffer, or if some other I/O error occurs
       */
      public void unread(char cbuf[], int off, int len) throws IOException {
          synchronized (lock) {
              ensureOpen();
              if (len > pos)
                  throw new IOException("Pushback buffer overflow");
              pos -= len;
              System.arraycopy(cbuf, off, buf, pos, len);
          }
      }
  
      /**
       * Pushes back an array of characters by copying it to the front of the
       * pushback buffer.  After this method returns, the next character to be
       * read will have the value <code>cbuf[0]</code>, the character after that
       * will have the value <code>cbuf[1]</code>, and so forth.
       *
       * @param  cbuf  Character array to push back
       *
       * @exception  IOException  If there is insufficient room in the pushback
       *                          buffer, or if some other I/O error occurs
       */
      public void unread(char cbuf[]) throws IOException {
          unread(cbuf, 0, cbuf.length);
      }
  
      /**
       * Tells whether this stream is ready to be read.
       *
       * @exception  IOException  If an I/O error occurs
       */
      public boolean ready() throws IOException {
          synchronized (lock) {
              ensureOpen();
              return (pos < buf.length) || super.ready();
          }
      }
  
      /**
       * Marks the present position in the stream. The <code>mark</code>
       * for class <code>PushbackReader</code> always throws an exception.
       *
       * @exception  IOException  Always, since mark is not supported
       */
      public void mark(int readAheadLimit) throws IOException {
          throw new IOException("mark/reset not supported");
      }
  
      /**
       * Resets the stream. The <code>reset</code> method of
       * <code>PushbackReader</code> always throws an exception.
       *
       * @exception  IOException  Always, since reset is not supported
       */
      public void reset() throws IOException {
          throw new IOException("mark/reset not supported");
      }
  
      /**
       * Tells whether this stream supports the mark() operation, which it does
       * not.
       */
      public boolean markSupported() {
          return false;
      }
  
      /**
       * Closes the stream and releases any system resources associated with
       * it. Once the stream has been closed, further read(),
       * unread(), ready(), or skip() invocations will throw an IOException.
       * Closing a previously closed stream has no effect.
       *
       * @exception  IOException  If an I/O error occurs
       */
      public void close() throws IOException {
          super.close();
          buf = null;
      }
  
      /**
       * Skips characters.  This method will block until some characters are
       * available, an I/O error occurs, or the end of the stream is reached.
       *
       * @param  n  The number of characters to skip
       *
       * @return    The number of characters actually skipped
       *
       * @exception  IllegalArgumentException  If <code>n</code> is negative.
       * @exception  IOException  If an I/O error occurs
       */
      public long skip(long n) throws IOException {
          if (n < 0L)
              throw new IllegalArgumentException("skip value is negative");
          synchronized (lock) {
              ensureOpen();
              int avail = buf.length - pos;
              if (avail > 0) {
                  if (n <= avail) {
                      pos += n;
                      return n;
                  } else {
                      pos = buf.length;
                      n -= avail;
                  }
              }
              return avail + super.skip(n);
          }
      }
  }
  