/*
 * 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.vfs.provider.tar;

/**
 * This class provides static utility methods to work with byte streams.
 *
 * @author <a href="mailto:time@ice.com">Timothy Gerard Endres</a>
 * @author <a href="mailto:stefano@apache.org">Stefano Mazzocchi</a>
 * @version $Revision: 764356 $ $Date: 2009-04-13 12:06:01 +0800 (一, 2009-04-13) $
 */
class TarUtils
{
    /**
     * Parse the checksum octal integer from a header buffer.
     *
     * @param offset The offset into the buffer from which to parse.
     * @param length The number of header bytes to parse.
     * @param value  Description of Parameter
     * @param buf    Description of Parameter
     * @return The integer value of the entry's checksum.
     */
    public static int getCheckSumOctalBytes(final long value,
                                            final byte[] buf,
                                            final int offset,
                                            final int length)
    {
        getOctalBytes(value, buf, offset, length);

        buf[offset + length - 1] = (byte) ' ';
        buf[offset + length - 2] = 0;

        return offset + length;
    }

    /**
     * Parse an octal long integer from a header buffer.
     *
     * @param offset The offset into the buffer from which to parse.
     * @param length The number of header bytes to parse.
     * @param value  Description of Parameter
     * @param buf    Description of Parameter
     * @return The long value of the octal bytes.
     */
    public static int getLongOctalBytes(final long value,
                                        final byte[] buf,
                                        final int offset,
                                        final int length)
    {
        byte[] temp = new byte[length + 1];

        getOctalBytes(value, temp, 0, length + 1);
        System.arraycopy(temp, 0, buf, offset, length);

        return offset + length;
    }

    /**
     * Determine the number of bytes in an entry name.
     *
     * @param offset The offset into the buffer from which to parse.
     * @param length The number of header bytes to parse.
     * @param name   Description of Parameter
     * @param buffer Description of Parameter
     * @return The number of bytes in a header's entry name.
     */
    public static int getNameBytes(final StringBuffer name,
                                   final byte[] buffer,
                                   final int offset,
                                   final int length)
    {
        int i;

        for (i = 0; i < length && i < name.length(); ++i)
        {
            buffer[offset + i] = (byte) name.charAt(i);
        }

        for (; i < length; ++i)
        {
            buffer[offset + i] = 0;
        }

        return offset + length;
    }

    /**
     * Parse an octal integer from a header buffer.
     *
     * @param offset The offset into the buffer from which to parse.
     * @param length The number of header bytes to parse.
     * @return The integer value of the octal bytes.
     */
    public static int getOctalBytes(final long value,
                                    final byte[] buffer,
                                    final int offset,
                                    final int length)
    {
        int idx = length - 1;

        buffer[offset + idx] = 0;
        --idx;
        buffer[offset + idx] = (byte) ' ';
        --idx;

        if (value == 0)
        {
            buffer[offset + idx] = (byte) '0';
            --idx;
        }
        else
        {
            long val = value;
            while (idx >= 0 && val > 0)
            {
                buffer[offset + idx] = (byte) ((byte) '0' + (byte) (val & 7));
                val = val >> 3;
                idx--;
            }
        }

        while (idx >= 0)
        {
            buffer[offset + idx] = (byte) ' ';
            idx--;
        }

        return offset + length;
    }

    /**
     * Compute the checksum of a tar entry header.
     *
     * @param buffer The tar entry's header buffer.
     * @return The computed checksum.
     */
    public static long computeCheckSum(final byte[] buffer)
    {
        long sum = 0;

        for (int i = 0; i < buffer.length; ++i)
        {
            sum += 255 & buffer[i];
        }

        return sum;
    }

    /**
     * Parse an entry name from a header buffer.
     *
     * @param header The header buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @param length The number of header bytes to parse.
     * @return The header's entry name.
     */
    public static StringBuffer parseName(final byte[] header,
                                         final int offset,
                                         final int length)
    {
        StringBuffer result = new StringBuffer(length);
        int end = offset + length;

        for (int i = offset; i < end; ++i)
        {
            if (header[i] == 0)
            {
                break;
            }

            result.append((char) header[i]);
        }

        return result;
    }

    /**
     * Parse an octal string from a header buffer. This is used for the file
     * permission mode value.
     *
     * @param header The header buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @param length The number of header bytes to parse.
     * @return The long value of the octal string.
     */
    public static long parseOctal(final byte[] header,
                                  final int offset,
                                  final int length)
    {
        long result = 0;
        boolean stillPadding = true;
        int end = offset + length;

        for (int i = offset; i < end; ++i)
        {
            if (header[i] == 0)
            {
                break;
            }

            if (header[i] == (byte) ' ' || header[i] == '0')
            {
                if (stillPadding)
                {
                    continue;
                }

                if (header[i] == (byte) ' ')
                {
                    break;
                }
            }

            stillPadding = false;
            result = (result << 3) + (header[i] - '0');
        }

        return result;
    }
}
