tailfilter.java

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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.tools.ant.filters;

import java.io.IOException;
import java.io.Reader;
import java.util.LinkedList;
import org.apache.tools.ant.types.Parameter;
import org.apache.tools.ant.util.LineTokenizer;

/**
 * Reads the last <code>n</code> lines of a stream. (Default is last10 lines.)
 *
 * Example:
 *
 * <pre>&lt;tailfilter lines=&quot;3&quot;/&gt;</pre>
 *
 * Or:
 *
 * <pre>&lt;filterreader classname=&quot;org.apache.tools.ant.filters.TailFilter&quot;&gt;
 *   &lt;param name=&quot;lines&quot; value=&quot;3&quot;/&gt;
 * &lt;/filterreader&gt;</pre>
 *
 */
public final class TailFilter extends BaseParamFilterReader
    implements ChainableReader {
    /** Parameter name for the number of lines to be returned. */
    private static final String LINES_KEY = "lines";

    /** Parameter name for the number of lines to be skipped. */
    private static final String SKIP_KEY = "skip";

    /** Default number of lines to show */
    private static final int DEFAULT_NUM_LINES = 10;

    /** Number of lines to be returned in the filtered stream. */
    private long lines = DEFAULT_NUM_LINES;

    /** Number of lines to be skipped. */
    private long skip = 0;

    /** Whether or not read-ahead been completed. */
    private boolean completedReadAhead = false;

    /** A line tokenizer */
    private LineTokenizer lineTokenizer = null;

    /** the current line from the input stream */
    private String    line      = null;
    /** the position in the current line */
    private int       linePos   = 0;

    private LinkedList lineList = new LinkedList();

    /**
     * Constructor for "dummy" instances.
     *
     * @see BaseFilterReader#BaseFilterReader()
     */
    public TailFilter() {
        super();
    }

    /**
     * Creates a new filtered reader.
     *
     * @param in A Reader object providing the underlying stream.
     *           Must not be <code>null</code>.
     */
    public TailFilter(final Reader in) {
        super(in);
        lineTokenizer = new LineTokenizer();
        lineTokenizer.setIncludeDelims(true);
    }

    /**
     * Returns the next character in the filtered stream. If the read-ahead
     * has been completed, the next character in the buffer is returned.
     * Otherwise, the stream is read to the end and buffered (with the buffer
     * growing as necessary), then the appropriate position in the buffer is
     * set to read from.
     *
     * @return the next character in the resulting stream, or -1
     * if the end of the resulting stream has been reached
     *
     * @exception IOException if the underlying stream throws an IOException
     * during reading
     */
    public int read() throws IOException {
        if (!getInitialized()) {
            initialize();
            setInitialized(true);
        }

        while (line == null || line.length() == 0) {
            line = lineTokenizer.getToken(in);
            line = tailFilter(line);
            if (line == null) {
                return -1;
            }
            linePos = 0;
        }

        int ch = line.charAt(linePos);
        linePos++;
        if (linePos == line.length()) {
            line = null;
        }
        return ch;
    }

    /**
     * Sets the number of lines to be returned in the filtered stream.
     *
     * @param lines the number of lines to be returned in the filtered stream
     */
    public void setLines(final long lines) {
        this.lines = lines;
    }

    /**
     * Returns the number of lines to be returned in the filtered stream.
     *
     * @return the number of lines to be returned in the filtered stream
     */
    private long getLines() {
        return lines;
    }

    /**
     * Sets the number of lines to be skipped in the filtered stream.
     *
     * @param skip the number of lines to be skipped in the filtered stream
     */
    public void setSkip(final long skip) {
        this.skip = skip;
    }

    /**
     * Returns the number of lines to be skipped in the filtered stream.
     *
     * @return the number of lines to be skipped in the filtered stream
     */
    private long getSkip() {
        return skip;
    }

    /**
     * Creates a new TailFilter using the passed in
     * Reader for instantiation.
     *
     * @param rdr A Reader object providing the underlying stream.
     *            Must not be <code>null</code>.
     *
     * @return a new filter based on this configuration, but filtering
     *         the specified reader
     */
    public Reader chain(final Reader rdr) {
        TailFilter newFilter = new TailFilter(rdr);
        newFilter.setLines(getLines());
        newFilter.setSkip(getSkip());
        newFilter.setInitialized(true);
        return newFilter;
    }

    /**
     * Scans the parameters list for the "lines" parameter and uses
     * it to set the number of lines to be returned in the filtered stream.
     * also scan for "skip" parameter.
     */
    private void initialize() {
        Parameter[] params = getParameters();
        if (params != null) {
            for (int i = 0; i < params.length; i++) {
                if (LINES_KEY.equals(params[i].getName())) {
                    setLines(Long.parseLong(params[i].getValue()));
                    continue;
                }
                if (SKIP_KEY.equals(params[i].getName())) {
                    skip = Long.parseLong(params[i].getValue());
                    continue;
                }
            }
        }
    }

    /**
     * implement a tail filter on a stream of lines.
     * line = null is the end of the stream.
     * @return "" while reading in the lines,
     *         line while outputting the lines
     *         null at the end of outputting the lines
     */
    private String tailFilter(String line) {
        if (!completedReadAhead) {
            if (line != null) {
                lineList.add(line);
                if (lines == -1) {
                    if (lineList.size() > skip) {
                        return (String) lineList.removeFirst();
                    }
                } else {
                    long linesToKeep = lines + (skip > 0 ? skip : 0);
                    if (linesToKeep < lineList.size()) {
                        lineList.removeFirst();
                    }
                }
                return "";
            }
            completedReadAhead = true;
            if (skip > 0) {
                for (int i = 0; i < skip; ++i) {
                    lineList.removeLast();
                }
            }
            if (lines > -1) {
                while (lineList.size() > lines) {
                    lineList.removeFirst();
                }
            }
        }
        if (lineList.size() > 0) {
            return (String) lineList.removeFirst();
        }
        return null;
    }
}