/*

 * Copyright (c) 1994, 2019, 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

 * questions.

 */

package java.io;

/**

 * A {@code PushbackInputStream} adds

 * functionality to another input stream, namely

 * the  ability to "push back" or "unread" bytes,

 * by storing pushed-back bytes in an internal buffer.

 * 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   1.0

 */

public

class PushbackInputStream extends FilterInputStream {

    /**

     * The pushback buffer.

     * @since   1.1

     */

    protected byte[] buf;

    /**

     * The position within the pushback buffer from which the next byte will

     * be read.  When the buffer is empty, {@code pos} is equal to

     * {@code buf.length}; when the buffer is full, {@code pos} is

     * equal to zero.

     *

     * @since   1.1

     */

    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}

     * with a pushback buffer of the specified {@code size},

     * and saves its argument, the input stream

     * {@code in}, for later use. Initially,

     * the pushback buffer is empty.

     *

     * @param  in    the input stream from which bytes will be read.

     * @param  size  the size of the pushback buffer.

     * @throws IllegalArgumentException if {@code size <= 0}

     * @since  1.1

     */

    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}

     * with a 1-byte pushback buffer, and saves its argument, the input stream

     * {@code in}, for later use. Initially,

     * the pushback buffer is empty.

     *

     * @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} in the range

     * {@code 0} to {@code 255}. If no byte is available

     * because the end of the stream has been reached, the value

     * {@code -1} 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} method of its underlying

     * input stream and returns whatever value that method returns.

     *

     * @return     the next byte of data, or {@code -1} if the end of the

     *             stream has been reached.

     * @throws     IOException  if this input stream has been closed by

     *             invoking its {@link #close()} method,

     *             or an I/O error occurs.

     * @see        java.io.InputStream#read()

     */

    public int read() throws IOException {

        ensureOpen();

        if (pos < buf.length) {

            return buf[pos++] & 0xff;

        }

        return super.read();

    }

    /**

     * Reads up to {@code len} 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} bytes have been read then it

     * reads from the underlying input stream. If {@code len} is not zero, the method

     * blocks until at least 1 byte of input is available; otherwise, no

     * bytes are read and {@code 0} is returned.

     *

     * @param      b     the buffer into which the data is read.

     * @param      off   the start offset in the destination array {@code b}

     * @param      len   the maximum number of bytes read.

     * @return     the total number of bytes read into the buffer, or

     *             {@code -1} if there is no more data because the end of

     *             the stream has been reached.

     * @throws     NullPointerException If {@code b} is {@code null}.

     * @throws     IndexOutOfBoundsException If {@code off} is negative,

     *             {@code len} is negative, or {@code len} is greater than

     *             {@code b.length - off}

     * @throws     IOException  if this input stream has been closed by

     *             invoking its {@link #close()} method,

     *             or an I/O error occurs.

     * @see        java.io.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}.

     *

     * @param      b   the {@code int} value whose low-order

     *                  byte is to be pushed back.

     * @throws    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]}, the byte after that will

     * have the value {@code b[off+1]}, 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.

     * @throws    NullPointerException If {@code b} is {@code null}.

     * @throws    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     1.1

     */

    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]}, the byte after that will have the

     * value {@code b[1]}, and so forth.

     *

     * @param     b the byte array to push back

     * @throws    NullPointerException If {@code b} is {@code null}.

     * @throws    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     1.1

     */

    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

     * java.io.FilterInputStream#available available}.

     *

     * @return     the number of bytes that can be read (or skipped over) from

     *             the input stream without blocking.

     * @throws     IOException  if this input stream has been closed by

     *             invoking its {@link #close()} method,

     *             or an I/O error occurs.

     * @see        java.io.FilterInputStream#in

     * @see        java.io.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} bytes of data from this

     * input stream. The {@code skip} method may, for a variety of

     * reasons, end up skipping over some smaller number of bytes,

     * possibly zero.  If {@code n} is negative, no bytes are skipped.

     *

     * <p> The {@code skip} method of {@code PushbackInputStream}

     * first skips over the bytes in the pushback buffer, if any.  It then

     * calls the {@code skip} 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}

     * @throws     IOException  if the stream has been closed by

     *             invoking its {@link #close()} method,

     *             {@code in.skip(n)} throws an IOException,

     *             or an I/O error occurs.

     * @see        java.io.FilterInputStream#in

     * @see        java.io.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} and

     * {@code reset} methods, which it does not.

     *

     * @return   {@code false}, since this class does not support the

     *           {@code mark} and {@code reset} methods.

     * @see      java.io.InputStream#mark(int)

     * @see      java.io.InputStream#reset()

     */

    public boolean markSupported() {

        return false;

    }

    /**

     * Marks the current position in this input stream.

     *

     * <p> The {@code mark} method of {@code PushbackInputStream}

     * does nothing.

     *

     * @param   readlimit   the maximum limit of bytes that can be read before

     *                      the mark position becomes invalid.

     * @see     java.io.InputStream#reset()

     */

    public synchronized void mark(int readlimit) {

    }

    /**

     * Repositions this stream to the position at the time the

     * {@code mark} method was last called on this input stream.

     *

     * <p> The method {@code reset} for class

     * {@code PushbackInputStream} does nothing except throw an

     * {@code IOException}.

     *

     * @throws  IOException  if this method is invoked.

     * @see     java.io.InputStream#mark(int)

     * @see     java.io.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.

     *

     * @throws     IOException  if an I/O error occurs.

     */

    public synchronized void close() throws IOException {

        if (in == null)

            return;

        in.close();

        in = null;

        buf = null;

    }

}