/*
 * @(#)Reader.java	1.21 00/02/02
 *
 * Copyright 1996-2000 Sun Microsystems, Inc. All Rights Reserved.
 * 
 * This software is the proprietary information of Sun Microsystems, Inc.  
 * Use is subject to license terms.
 * 
 */

package java.io;


/**
 * Abstract class for reading character streams.  The only methods that a
 * subclass must implement are read(char[], int, int) and close().  Most
 * subclasses, however, will override some of the methods defined here in order
 * to provide higher efficiency, additional functionality, or both.
 *
 * 
 * @see BufferedReader
 * @see   LineNumberReader
 * @see CharArrayReader
 * @see InputStreamReader
 * @see   FileReader
 * @see FilterReader
 * @see   PushbackReader
 * @see PipedReader
 * @see StringReader
 * @see Writer
 *
 * @version 	1.21, 00/02/02
 * @author	Mark Reinhold
 * @since	JDK1.1
 */

public abstract class Reader {

    /**
     * The object used to synchronize operations on this stream.  For
     * efficiency, a character-stream object may use an object other than
     * itself to protect critical sections.  A subclass should therefore use
     * the object in this field rather than <tt>this</tt> or a synchronized
     * method.
     */
    protected Object lock;

    /**
     * Create a new character-stream reader whose critical sections will
     * synchronize on the reader itself.
     */
    protected Reader() {
	this.lock = this;
    }

    /**
     * Create a new character-stream reader whose critical sections will
     * synchronize on the given object.
     *
     * @param lock  The Object to synchronize on.
     */
    protected Reader(Object lock) {
	if (lock == null) {
	    throw new NullPointerException();
	}
	this.lock = lock;
    }

    /**
     * Read a single character.  This method will block until a character is
     * available, an I/O error occurs, or the end of the stream is reached.
     *
     * <p> Subclasses that intend to support efficient single-character input
     * should override this method.
     *
     * @return     The character read, as an integer in the range 0 to 65535
     *             (<tt>0x00-0xffff</tt>), or -1 if the end of the stream has
     *             been reached
     *
     * @exception  IOException  If an I/O error occurs
     */
    public int read() throws IOException {
	char cb[] = new char[1];
	if (read(cb, 0, 1) == -1)
	    return -1;
	else
	    return cb[0];
    }

    /**
     * Read characters into an array.  This method will block until some input
     * is available, an I/O error occurs, or the end of the stream is reached.
     *
     * @param       cbuf  Destination buffer
     *
     * @return      The number of bytes 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[]) throws IOException {
	return read(cbuf, 0, cbuf.length);
    }

    /**
     * Read characters into a portion of an array.  This method will block
     * until some input is available, an I/O error occurs, or the end of the
     * stream is reached.
     *
     * @param      cbuf  Destination buffer
     * @param      off   Offset at which to start storing 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
     */
    abstract public int read(char cbuf[], int off, int len) throws IOException;

    /** Maximum skip-buffer size */
    private static final int maxSkipBufferSize = 8192;

    /** Skip buffer, null until allocated */
    private char skipBuffer[] = null;

    /**
     * Skip 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");
	int nn = (int) Math.min(n, maxSkipBufferSize);
	synchronized (lock) {
	    if ((skipBuffer == null) || (skipBuffer.length < nn))
		skipBuffer = new char[nn];
	    long r = n;
	    while (r > 0) {
		int nc = read(skipBuffer, 0, (int)Math.min(r, nn));
		if (nc == -1)
		    break;
		r -= nc;
	    }
	    return n - r;
	}
    }

    /**
     * Tell whether this stream is ready to be read.
     *
     * @return True if the next read() is guaranteed not to block for input,
     * false otherwise.  Note that returning false does not guarantee that the
     * next read will block.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public boolean ready() throws IOException {
	return false;
    }

    /**
     * Tell whether this stream supports the mark() operation. The default
     * implementation always returns false. Subclasses should override this
     * method.
     *
     * @return true if and only if this stream supports the mark operation.
     */
    public boolean markSupported() {
	return false;
    }

    /**
     * Mark the present position in the stream.  Subsequent calls to reset()
     * will attempt to reposition the stream to this point.  Not all
     * character-input streams support the mark() operation.
     *
     * @param  readAheadLimit  Limit on the number of characters that may be
     *                         read while still preserving the mark.  After
     *                         reading this many characters, attempting to
     *                         reset the stream may fail.
     *
     * @exception  IOException  If the stream does not support mark(),
     *                          or if some other I/O error occurs
     */
    public void mark(int readAheadLimit) throws IOException {
	throw new IOException("mark() not supported");
    }

    /**
     * Reset the stream.  If the stream has been marked, then attempt to
     * reposition it at the mark.  If the stream has not been marked, then
     * attempt to reset it in some way appropriate to the particular stream,
     * for example by repositioning it to its starting point.  Not all
     * character-input streams support the reset() operation, and some support
     * reset() without supporting mark().
     *
     * @exception  IOException  If the stream has not been marked,
     *                          or if the mark has been invalidated,
     *                          or if the stream does not support reset(),
     *                          or if some other I/O error occurs
     */
    public void reset() throws IOException {
	throw new IOException("reset() not supported");
    }

    /**
     * Close the stream.  Once a stream has been closed, further read(),
     * ready(), mark(), or reset() invocations will throw an IOException.
     * Closing a previously-closed stream, however, has no effect.
     *
     * @exception  IOException  If an I/O error occurs
     */
     abstract public void close() throws IOException;

}