/*
    * Copyright (C) 2009, 2013 Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.util.io;
  
  import java.io.IOException;
  import java.io.InputStream;
  import java.util.Iterator;
  import java.util.LinkedList;
  
  /**
   * An InputStream which reads from one or more InputStreams.
   * <p>
   * This stream may enter into an EOF state, returning -1 from any of the read
   * methods, and then later successfully read additional bytes if a new
   * InputStream is added after reaching EOF.
   * <p>
   * Currently this stream does not support the mark/reset APIs. If mark and later
   * reset functionality is needed the caller should wrap this stream with a
   * {@link java.io.BufferedInputStream}.
   */
  public class UnionInputStream extends InputStream {
  	private static final InputStream EOF = new InputStream() {
  		@Override
  		public int read() throws IOException {
  			return -1;
  		}
  	};
  
  	private final LinkedList<InputStream> streams = new LinkedList<>();
  
  	/**
  	 * Create an empty InputStream that is currently at EOF state.
  	 */
  	public UnionInputStream() {
  		// Do nothing.
  	}
  
  	/**
  	 * Create an InputStream that is a union of the individual streams.
  	 * <p>
  	 * As each stream reaches EOF, it will be automatically closed before bytes
  	 * from the next stream are read.
  	 *
  	 * @param inputStreams
  	 *            streams to be pushed onto this stream.
  	 */
  	public UnionInputStream(InputStream... inputStreams) {
  		for (InputStream i : inputStreams)
  			add(i);
  	}
  
  	private InputStream head() {
  		return streams.isEmpty() ? EOF : streams.getFirst();
  	}
  
  	private void pop() throws IOException {
  		if (!streams.isEmpty())
  			streams.removeFirst().close();
  	}
  
  	/**
  	 * Add the given InputStream onto the end of the stream queue.
  	 * <p>
  	 * When the stream reaches EOF it will be automatically closed.
  	 *
  	 * @param in
  	 *            the stream to add; must not be null.
  	 */
  	public void add(InputStream in) {
  		streams.add(in);
  	}
  
  	/**
  	 * Returns true if there are no more InputStreams in the stream queue.
  	 * <p>
  	 * If this method returns {@code true} then all read methods will signal EOF
  	 * by returning -1, until another InputStream has been pushed into the queue
  	 * with {@link #add(InputStream)}.
  	 *
  	 * @return true if there are no more streams to read from.
  	 */
  	public boolean isEmpty() {
  		return streams.isEmpty();
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public int read() throws IOException {
  		for (;;) {
  			final InputStream in = head();
  			final int r = in.read();
 			if (0 <= r)
 				return r;
 			else if (in == EOF)
 				return -1;
 			else
 				pop();
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public int read(byte[] b, int off, int len) throws IOException {
 		if (len == 0)
 			return 0;
 		for (;;) {
 			final InputStream in = head();
 			final int n = in.read(b, off, len);
 			if (0 < n)
 				return n;
 			else if (in == EOF)
 				return -1;
 			else
 				pop();
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public int available() throws IOException {
 		return head().available();
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public long skip(long count) throws IOException {
 		long skipped = 0;
 		long cnt = count;
 		while (0 < cnt) {
 			final InputStream in = head();
 			final long n = in.skip(cnt);
 			if (0 < n) {
 				skipped += n;
 				cnt -= n;
 
 			} else if (in == EOF) {
 				return skipped;
 
 			} else {
 				// Is this stream at EOF? We can't tell from skip alone.
 				// Read one byte to test for EOF, discard it if we aren't
 				// yet at EOF.
 				//
 				final int r = in.read();
 				if (r < 0) {
 					pop();
 					if (0 < skipped)
 						break;
 				} else {
 					skipped += 1;
 					cnt -= 1;
 				}
 			}
 		}
 		return skipped;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void close() throws IOException {
 		IOException err = null;
 
 		for (Iterator<InputStream> i = streams.iterator(); i.hasNext();) {
 			try {
 				i.next().close();
 			} catch (IOException closeError) {
 				err = closeError;
 			}
 			i.remove();
 		}
 
 		if (err != null)
 			throw err;
 	}
 }