/*
 * Copyright (c) 1994, 2011, 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;

import java.io.InputStream;
import java.util.Enumeration;
import java.util.Vector;

/**
 * A <code>SequenceInputStream</code> represents
 * the logical concatenation of other input
 * streams. It starts out with an ordered
 * collection of input streams and reads from
 * the first one until end of file is reached,
 * whereupon it reads from the second one,
 * and so on, until end of file is reached
 * on the last of the contained input streams.
 *
 * @author Author van Hoff
 * @since 1.0
 */
public
class SequenceInputStream extends InputStream {
 Enumeration<? extends InputStream> e;
 InputStream in;

 /**
 * Initializes a newly created <code>SequenceInputStream</code>
 * by remembering the argument, which must
 * be an <code>Enumeration</code> that produces
 * objects whose run-time type is <code>InputStream</code>.
 * The input streams that are produced by
 * the enumeration will be read, in order,
 * to provide the bytes to be read from this
 * <code>SequenceInputStream</code>. After
 * each input stream from the enumeration
 * is exhausted, it is closed by calling its
 * <code>close</code> method.
 *
 * @param e an enumeration of input streams.
 * @see java.util.Enumeration
 */
 public SequenceInputStream(Enumeration<? extends InputStream> e) {
 this.e = e;
 peekNextStream();
 }

 /**
 * Initializes a newly
 * created <code>SequenceInputStream</code>
 * by remembering the two arguments, which
 * will be read in order, first <code>s1</code>
 * and then <code>s2</code>, to provide the
 * bytes to be read from this <code>SequenceInputStream</code>.
 *
 * @param s1 the first input stream to read.
 * @param s2 the second input stream to read.
 */
 public SequenceInputStream(InputStream s1, InputStream s2) {
 Vector<InputStream> v = new Vector<>(2);
 v.addElement(s1);
 v.addElement(s2);
 e = v.elements();
 peekNextStream();
 }

 /**
 * Continues reading in the next stream if an EOF is reached.
 */
 final void nextStream() throws IOException {
 if (in != null) {
 in.close();
 }
 peekNextStream();
 }

 private void peekNextStream() {
 if (e.hasMoreElements()) {
 in = (InputStream) e.nextElement();
 if (in == null)
 throw new NullPointerException();
 } else {
 in = null;
 }
 }

 /**
 * Returns an estimate of the number of bytes that can be read (or
 * skipped over) from the current underlying input stream without
 * blocking by the next invocation of a method for the current
 * underlying 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>
 * This method simply calls {@code available} of the current underlying
 * input stream and returns the result.
 *
 * @return an estimate of the number of bytes that can be read (or
 * skipped over) from the current underlying input stream
 * without blocking or {@code 0} if this input stream
 * has been closed by invoking its {@link #close()} method
 * @exception IOException if an I/O error occurs.
 *
 * @since 1.1
 */
 public int available() throws IOException {
 if (in == null) {
 return 0; // no way to signal EOF from available()
 }
 return in.available();
 }

 /**
 * Reads the next byte of data from this input stream. The 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
 * tries to read one character from the current substream. If it
 * reaches the end of the stream, it calls the <code>close</code>
 * method of the current substream and begins reading from the next
 * substream.
 *
 * @return the next byte of data, or <code>-1</code> if the end of the
 * stream is reached.
 * @exception IOException if an I/O error occurs.
 */
 public int read() throws IOException {
 while (in != null) {
 int c = in.read();
 if (c != -1) {
 return c;
 }
 nextStream();
 }
 return -1;
 }

 /**
 * Reads up to <code>len</code> bytes of data from this input stream
 * into an array of bytes. 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.
 * <p>
 * The <code>read</code> method of <code>SequenceInputStream</code>
 * tries to read the data from the current substream. If it fails to
 * read any characters because the substream has reached the end of
 * the stream, it calls the <code>close</code> method of the current
 * substream and begins reading from the next substream.
 *
 * @param b the buffer into which the data is read.
 * @param off the start offset in array <code>b</code>
 * at which the data is written.
 * @param len the maximum number of bytes read.
 * @return int the number of bytes read.
 * @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 an I/O error occurs.
 */
 public int read(byte b[], int off, int len) throws IOException {
 if (in == null) {
 return -1;
 } else 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;
 }
 do {
 int n = in.read(b, off, len);
 if (n > 0) {
 return n;
 }
 nextStream();
 } while (in != null);
 return -1;
 }

 /**
 * Closes this input stream and releases any system resources
 * associated with the stream.
 * A closed <code>SequenceInputStream</code>
 * cannot perform input operations and cannot
 * be reopened.
 * <p>
 * If this stream was created
 * from an enumeration, all remaining elements
 * are requested from the enumeration and closed
 * before the <code>close</code> method returns.
 *
 * @exception IOException if an I/O error occurs.
 */
 public void close() throws IOException {
 do {
 nextStream();
 } while (in != null);
 }
}