LinkedReaderAndWriter.java from DrJava at Krugle

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/*BEGIN_COPYRIGHT_BLOCK*

PLT Utilities BSD License

Copyright (c) 2007 JavaPLT group at Rice University
All rights reserved.

Developed by:   Java Programming Languages Team
                Rice University
                http://www.cs.rice.edu/~javaplt/

Redistribution and use in source and binary forms, with or without modification, are permitted 
provided that the following conditions are met:

    - Redistributions of source code must retain the above copyright notice, this list of conditions 
      and the following disclaimer.
    - 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.
    - Neither the name of the JavaPLT group, Rice University, nor the names of the library's 
      contributors may be used to endorse or promote products derived from this software without 
      specific prior written permission.

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 HOLDERS AND 
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 
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package edu.rice.cs.plt.io;

import java.io.Reader;
import java.io.Writer;
import java.io.IOException;

/**
 * Coordinates reading and writing from the same source.  Ensures
 * that the given Writer will never write past the location of the
 * Reader, potentially corrupting its contents.  When the Writer needs
 * to write more characters than have been read, a buffer is used to
 * store the characters in between.  In order to take advantage of these
 * linking features, all interaction with the user-provided Reader and Writer
 * must subsequently take place via the results of {@link #reader()} and {@link #writer()}, 
 * respectively.
 */
public class LinkedReaderAndWriter {
  
  private final DirectReader _linkedReader;
  private final DirectWriter _linkedWriter;
  private long _readIndex; // Must always be >= _writeIndex
  private boolean _eof;
  private long _writeIndex;
  
  /**
   * Link {@code r} and {@code w}.  Assumes that {@code r} and {@code w} are newly created, or at 
   * least that their underlying cursors point to the same place.  Also assumes that no further 
   * direct interaction with {@code r} or {@code w} will occur.
   */
  public LinkedReaderAndWriter(final Reader r, final Writer w) {
    _readIndex = 0;
    _writeIndex = 0;
    _eof = false;
    final ExpandingCharBuffer buffer = new ExpandingCharBuffer();
    final Reader fromBuffer = buffer.reader();
    final DirectWriter toBuffer = buffer.writer();
    
    _linkedReader = new DirectReader() {
      public int read(char[] cbuf, int offset, int chars) throws IOException {
        int read = 0;
        
        if (!buffer.isEmpty()) {
          int bufferReadResult = fromBuffer.read(cbuf, offset, chars);
          if (bufferReadResult < 0) {
            throw new IllegalStateException("Unexpected negative result from ExpandingCharBuffer read");
          }
          else if (bufferReadResult > 0) { read += bufferReadResult; chars -= bufferReadResult; }
        }
        
        if (buffer.isEmpty() && chars >= 0) {
          int readResult = r.read(cbuf, offset + read, chars);
          if (readResult < 0) {
            _eof = true;
            return read > 0 ? read : readResult;
          }
          else {
            read += readResult;
            chars -= readResult;
            _readIndex += readResult;
          }
        }
        return read;
      }
      
      public void close() throws IOException { r.close(); }
      
      public boolean ready() throws IOException { return r.ready(); }
    };
    
    _linkedWriter = new DirectWriter() {
      public void write(char[] cbuf, int offset, int chars) throws IOException {
        long newIndex = _writeIndex + chars;
        while (newIndex > _readIndex) {
          // If we've reached the eof, we don't try to buffer anything
          if (_eof) { _readIndex = newIndex; }
          else {
            int bufferWriteResult = toBuffer.write(r, (int) (newIndex - _readIndex));
            if (bufferWriteResult < 0) { _eof = true; }
            else { _readIndex += bufferWriteResult; }
          }
        }
        w.write(cbuf, offset, chars);
        _writeIndex = newIndex;
      }
      
      public void close() throws IOException { w.close(); }
      
      public void flush() throws IOException { w.flush(); }
    };
  }
  
  public DirectReader reader() { return _linkedReader; }
  
  public DirectWriter writer() { return _linkedWriter; }
    
}
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