/*******************************************************************************
 *
 * Copyright (c) 2010-2011 Sonatype, Inc.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *
 *
 *
 *
 *******************************************************************************/ 

package org.hudsonci.utils.io;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.FileChannel;

/**
 * Removes the beginning of a file.
 *
 * @author Jamie Whitehouse
 * @since 2.1.0
 */
public class FileBeheader {

    /**
     * Beware, the original file is modified when chopping; you relinquish all
     * control of the original file.
     *
     * Requires write access to the same directory as the original.
     *
     * @param original the original file
     * @param tailToKeep the number of bytes to keep at the end of the file
     *
     * @return the original file chopped; for convenience
     *
     * @throws IllegalArgumentException if original is a directory
     * @throws IllegalStateException if some error occurs; it's unreasonable to
     * try and recover from this
     */
    public static File chop(final File original, final long tailToKeep) {
        if (!original.isFile()) {
            throw new IllegalArgumentException("Cannot behead directories: " + original);
        }

        File truncatedFile = new File(original.getParentFile(), original.getName() + ".truncated");

        FileChannel whole = null;
        FileChannel chopped = null;
        try {
            whole = new FileInputStream(original).getChannel();
            chopped = new FileOutputStream(truncatedFile).getChannel();

            long originalLength = original.length();
            long lengthToKeep = originalLength < tailToKeep ? 0 : originalLength - tailToKeep;
            whole.transferTo(lengthToKeep, originalLength, chopped);
        } catch (IOException e) {
            throw new IllegalStateException("Cannot behead file due to inconsistent internal state.", e);
        } finally {
            Closer.close(whole, chopped);
        }

        // Replace original with chopped file.
        boolean isRenamed = truncatedFile.renameTo(original);
        if (!(isRenamed)) {
            IllegalStateException renameException =
                    new IllegalStateException("Cannot replace original with chopped file to: " + original);

            String originalPath = original.getAbsolutePath();
            boolean isDeleted = original.delete();
            if (isDeleted) {
                boolean isRenamedAgain = truncatedFile.renameTo(new File(originalPath));
                if (!isRenamedAgain) {
                    throw renameException;
                }
            } else {
                throw renameException;
            }
        }

        return original;
    }
}