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The Apache Commons IO library contains utility classes, stream implementations, file filters, file comparators, endian transformation classes, and much more.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.io.input.buffer;

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


/** Implementation of a buffered input stream, which allows to peek into
 * the buffers first bytes. This comes in handy when manually implementing
 * scanners, lexers, parsers, or the like.
 */
public class PeekableInputStream extends CircularBufferInputStream {
	/** Creates a new instance, which filters the given input stream, and
	 * uses the given buffer size.
	 * @param pIn The input stream, which is being buffered.
	 * @param pBufferSize The size of the {@link CircularByteBuffer}, which is
	 * used internally.
	 */
	public PeekableInputStream(InputStream pIn, int pBufferSize) {
		super(pIn, pBufferSize);
	}

	/** Creates a new instance, which filters the given input stream, and
	 * uses a reasonable default buffer size (8192).
	 * @param pIn The input stream, which is being buffered.
	 */
	public PeekableInputStream(InputStream pIn) {
		super(pIn);
	}

	/**
	 * Returns, whether the next bytes in the buffer are as given by
	 * {@code pBuffer}. This is equivalent to {@link #peek(byte[],int,int)}
	 * with {@code pOffset} == 0, and {@code pLength} == {@code pBuffer.length}
	 * @param pBuffer the buffer to compare against
	 * @return true if the next bytes are as given
	 * @throws IOException Refilling the buffer failed.
	 */
	public boolean peek(byte[] pBuffer) throws IOException {
		Objects.requireNonNull(pBuffer, "Buffer");
		if (pBuffer.length > bufferSize) {
			throw new IllegalArgumentException("Peek request size of " + pBuffer.length
					                           + " bytes exceeds buffer size of " + bufferSize + " bytes");
		}
		if (buffer.getCurrentNumberOfBytes() < pBuffer.length) {
			fillBuffer();
		}
		return buffer.peek(pBuffer, 0, pBuffer.length);
	}

	/**
	 * Returns, whether the next bytes in the buffer are as given by
	 * {@code pBuffer}, {code pOffset}, and {@code pLength}.
	 * @param pBuffer the buffer to compare against
	 * @param pOffset the start offset
	 * @param pLength the length to compare
	 * @return true if the next bytes in the buffer are as given
	 * @throws IOException if there is a problem calling fillBuffer()
	 */
	public boolean peek(byte[] pBuffer, int pOffset, int pLength) throws IOException {
		Objects.requireNonNull(pBuffer, "Buffer");
		if (pBuffer.length > bufferSize) {
			throw new IllegalArgumentException("Peek request size of " + pBuffer.length
					                           + " bytes exceeds buffer size of " + bufferSize + " bytes");
		}
		if (buffer.getCurrentNumberOfBytes() < pBuffer.length) {
			fillBuffer();
		}
		return buffer.peek(pBuffer, pOffset, pLength);
	}
}