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/*
 * Original Guava code is copyright (C) 2015 The Guava Authors.
 * Modifications from Guava are copyright (C) 2015 DiffPlug.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.google.common.io;

import static com.google.common.base.Preconditions.checkNotNull;

import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;

import javax.annotation.Nullable;

/**
 * An {@link InputStream} that concatenates multiple substreams. At most
 * one stream will be open at a time.
 *
 * @author Chris Nokleberg
 * @since 1.0
 */
final class MultiInputStream extends InputStream {

	private Iterator it;
	private InputStream in;

	/**
	 * Creates a new instance.
	 *
	 * @param it an iterator of I/O suppliers that will provide each substream
	 */
	public MultiInputStream(
			Iterator it) throws IOException {
		this.it = checkNotNull(it);
		advance();
	}

	@Override
	public void close() throws IOException {
		if (in != null) {
			try {
				in.close();
			} finally {
				in = null;
			}
		}
	}

	/**
	 * Closes the current input stream and opens the next one, if any.
	 */
	private void advance() throws IOException {
		close();
		if (it.hasNext()) {
			in = it.next().openStream();
		}
	}

	@Override
	public int available() throws IOException {
		if (in == null) {
			return 0;
		}
		return in.available();
	}

	@Override
	public boolean markSupported() {
		return false;
	}

	@Override
	public int read() throws IOException {
		if (in == null) {
			return -1;
		}
		int result = in.read();
		if (result == -1) {
			advance();
			return read();
		}
		return result;
	}

	@Override
	public int read(@Nullable byte[] b, int off, int len) throws IOException {
		if (in == null) {
			return -1;
		}
		int result = in.read(b, off, len);
		if (result == -1) {
			advance();
			return read(b, off, len);
		}
		return result;
	}

	@Override
	public long skip(long n) throws IOException {
		if (in == null || n <= 0) {
			return 0;
		}
		long result = in.skip(n);
		if (result != 0) {
			return result;
		}
		if (read() == -1) {
			return 0;
		}
		return 1 + in.skip(n - 1);
	}
}