IO-288 Supply a ReversedLinesFileReader

N.B. Renamed test .txt files to .bin and made them application/octet-stream
as it's important not to change the EOL setting

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/io/trunk@1202500 13f79535-47bb-0310-9956-ffa450edef68

Author: sebbASF

src/main/java/org/apache/commons/io/ReversedLinesFileReader.java

@@ -0,0 +1,286 @@
+/*
+ * 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;
+
+import java.io.Closeable;
+import java.io.File;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.io.UnsupportedEncodingException;
+import java.nio.charset.Charset;
+import java.nio.charset.CharsetEncoder;
+
+/**
+ * Reads lines in a file reversely (similar to a BufferedReader, but starting at
+ * the last line). Useful for e.g. searching in log files.
+ *
+ */
+public class ReversedLinesFileReader implements Closeable {
+
+	private final int blockSize;
+	private final String encoding;
+
+	private final RandomAccessFile randomAccessFile;
+
+	private final long totalByteLength;
+	private final long totalBlockCount;
+
+	private final byte[][] newLineSequences;
+	private final int avoidNewlineSplitBufferSize;
+	private final int byteDecrement;
+	
+    private FilePart currentFilePart;
+
+	private boolean trailingNewlineOfFileSkipped = false;
+
+	/**
+	 * Creates a ReversedLinesFileReader with default block size of 4KB and the
+	 * platform's default encoding.
+	 *
+	 * @param file
+	 *            the file to be read
+	 * @throws IOException
+	 */
+	public ReversedLinesFileReader(final File file) throws IOException {
+		this(file, 4096, Charset.defaultCharset().toString());
+	}
+
+	/**
+	 * Creates a ReversedLinesFileReader with the given block size and encoding.
+	 *
+	 * @param file
+	 *            the file to be read
+	 * @param blockSize
+	 *            size of the internal buffer (for ideal performance this should
+	 *            match with the block size of the underlying file system).
+	 * @param encoding
+	 *            the encoding of the file
+	 * @throws IOException
+	 */
+	public ReversedLinesFileReader(final File file, final int blockSize, final String encoding) throws IOException {
+		this.blockSize = blockSize;
+		this.encoding = encoding;
+		
+		randomAccessFile = new RandomAccessFile(file, "r");
+		totalByteLength = randomAccessFile.length();
+		int lastBlockLength = (int) (totalByteLength % blockSize);
+		if (lastBlockLength > 0) {
+	        totalBlockCount = totalByteLength / blockSize + 1;
+		} else {
+            totalBlockCount = totalByteLength / blockSize;
+			if (totalByteLength > 0) {
+				lastBlockLength = blockSize;
+			}
+		}
+		currentFilePart = new FilePart(totalBlockCount, lastBlockLength, null);
+
+		// --- check & prepare encoding ---
+		Charset charset = Charset.forName(encoding);
+		CharsetEncoder charsetEncoder = charset.newEncoder();
+		float maxBytesPerChar = charsetEncoder.maxBytesPerChar();
+		if(maxBytesPerChar==1f) {
+			// all one byte encodings are no problem
+			byteDecrement = 1;
+		} else if(charset == Charset.forName("UTF-8")) {
+			// UTF-8 works fine out of the box, for multibyte sequences a second UTF-8 byte can never be a newline byte
+			// http://en.wikipedia.org/wiki/UTF-8
+			byteDecrement = 1;
+		} else if(charset == Charset.forName("Shift_JIS")) {
+			// Same as for UTF-8
+			// http://www.herongyang.com/Unicode/JIS-Shift-JIS-Encoding.html
+			byteDecrement = 1;
+		} else if(charset == Charset.forName("UTF-16BE") || charset == Charset.forName("UTF-16LE")) {
+			// UTF-16 new line sequences are not allowed as second tuple of four byte sequences, however byte order has to be specified
+			byteDecrement = 2;
+		} else if(charset == Charset.forName("UTF-16")) {
+			throw new UnsupportedEncodingException("For UTF-16, you need to specify the byte order (use UTF-16BE or UTF-16LE)");
+		} else {
+			throw new UnsupportedEncodingException("Encoding "+encoding+" is not supported yet (feel free to submit a patch)");
+		}
+		// NOTE: The new line sequences are matched in the order given, so it is important that \r\n is BEFORE \n
+		newLineSequences = new byte[][] { "\r\n".getBytes(encoding), "\n".getBytes(encoding), "\r".getBytes(encoding) };
+		
+		avoidNewlineSplitBufferSize = newLineSequences[0].length;
+
+	}
+
+	/**
+	 * Returns the lines of the file from bottom to top.
+	 *
+	 * @return the next line or null if the start of the file is reached
+	 * @throws IOException
+	 */
+	public String readLine() throws IOException {
+
+		String line = currentFilePart.readLine();
+		while (line == null) {
+			currentFilePart = currentFilePart.rollOver();
+			if (currentFilePart != null) {
+				line = currentFilePart.readLine();
+			} else {
+				// no more fileparts: we're done, leave line set to null
+				break;
+			}
+		}
+		
+		// aligned behaviour wiht BufferedReader that doesn't return a last, emtpy line
+		if("".equals(line) && !trailingNewlineOfFileSkipped) {
+			trailingNewlineOfFileSkipped = true;
+			line = readLine();
+		}
+		
+		return line;
+	}
+
+	/**
+	 * Closes underlying resources.
+	 */
+	public void close() throws IOException {
+		randomAccessFile.close();
+	}
+
+	private class FilePart {
+		private final long no;
+		private final byte[] data;
+
+		private byte[] leftOver;
+
+		private int currentLastBytePos;
+
+		private FilePart(final long no, final int length, final byte[] leftOverOfLastFilePart) throws IOException {
+			this.no = no;
+			int dataLength = length + (leftOverOfLastFilePart != null ? leftOverOfLastFilePart.length : 0);
+			this.data = new byte[dataLength];
+			final long off = (no - 1) * blockSize;
+
+			// read data
+			if (no > 0 /* file not empty */) {
+				randomAccessFile.seek(off);
+				final int countRead = randomAccessFile.read(data, 0, length);
+				if (countRead != length) {
+					throw new IllegalStateException("Count of requested bytes and actually read bytes don't match");
+				}
+			}
+			// copy left over part into data arr
+			if (leftOverOfLastFilePart != null) {
+				System.arraycopy(leftOverOfLastFilePart, 0, data, length, leftOverOfLastFilePart.length);
+			}
+			this.currentLastBytePos = data.length - 1;
+			this.leftOver = null;
+		}
+
+		private FilePart rollOver() throws IOException {
+
+			if (currentLastBytePos > -1) {
+				throw new IllegalStateException("Current currentLastCharPos unexpectedly positive... "
+						+ "last readLine() should have returned something! currentLastCharPos=" + currentLastBytePos);
+			}
+
+			if (no > 1) {
+				return new FilePart(no - 1, blockSize, leftOver);
+			} else {
+				// NO 1 was the last FilePart, we're finished
+				if (leftOver != null) {
+					throw new IllegalStateException("Unexpected leftover of the last block: leftOverOfThisFilePart="
+							+ new String(leftOver, encoding));
+				}
+				return null;
+			}
+		}
+
+		private String readLine() throws IOException {
+
+			String line = null;
+			int newLineMatchByteCount;
+			
+			boolean isLastFilePart = (no == 1);			
+
+			int i = currentLastBytePos;
+			while (i > -1) {
+				
+				if (!isLastFilePart && i < avoidNewlineSplitBufferSize) {
+					// avoidNewlineSplitBuffer: for all except the last file part we
+					// take a few bytes to the next file part to avoid splitting of newlines
+					createLeftOver();
+					break; // skip last few bytes and leave it to the next file part
+				}
+				
+				// --- check for newline ---
+				if ((newLineMatchByteCount = getNewLineMatchByteCount(data, i)) > 0 /* found newline */) {
+					final int lineStart = i + 1;
+					int lineLengthBytes = currentLastBytePos - lineStart + 1;
+
+					if (lineLengthBytes < 0) {
+						throw new IllegalStateException("Unexpected negative line length="+lineLengthBytes);
+					}
+					byte[] lineData = new byte[lineLengthBytes];
+					System.arraycopy(data, lineStart, lineData, 0, lineLengthBytes);
+
+					line = new String(lineData, encoding);
+					
+					currentLastBytePos = i - newLineMatchByteCount;
+					break; // found line
+				}
+
+				// --- move cursor ---
+				i -= byteDecrement;
+
+				// --- end of file part handling ---
+				if (i < 0) {
+					createLeftOver();
+					break; // end of file part
+				}
+			}
+			
+			// --- last file part handling ---
+			if (isLastFilePart && leftOver != null) {
+				// there will be no line break anymore, this is the first line of the file
+				line = new String(leftOver, encoding);
+				leftOver = null;
+			}			
+
+			return line;
+		}
+
+		private void createLeftOver() {
+			int lineLengthBytes = currentLastBytePos + 1;
+			if (lineLengthBytes > 0) {
+				// create left over for next block
+				leftOver = new byte[lineLengthBytes];
+				System.arraycopy(data, 0, leftOver, 0, lineLengthBytes);
+			} else {
+				leftOver = null;
+			}
+			currentLastBytePos = -1;
+		}
+
+		private int getNewLineMatchByteCount(byte[] data, int i) {
+			for (byte[] newLineSequence : newLineSequences) {
+				boolean match = true;
+				for (int j = newLineSequence.length - 1; j >= 0; j--) {
+					int k = i + j - (newLineSequence.length - 1);
+					match &= k >= 0 && data[k] == newLineSequence[j];
+				}
+				if (match) {
+					return newLineSequence.length;
+				}
+			}
+			return 0;
+		}
+	}
+
+}