bitshelper.java

snmp zip 包开发snmp协议

// NAME
//      $RCSfile: BitsHelper.java,v $
// DESCRIPTION
//      [given below in javadoc format]
// DELTA
//      $Revision: 3.1 $
// CREATED
//      $Date: 2006/03/23 15:09:48 $
// COPYRIGHT
//      Westhawk Ltd
// TO DO
//

/*
 * Copyright (C) 2006 by Westhawk Ltd
 * <a href="www.westhawk.co.uk">www.westhawk.co.uk</a>
 *
 * Permission to use, copy, modify, and distribute this software
 * for any purpose and without fee is hereby granted, provided
 * that the above copyright notices appear in all copies and that
 * both the copyright notice and this permission notice appear in
 * supporting documentation.
 * This software is provided "as is" without express or implied
 * warranty.
 * author <a href="mailto:[e-mail]">Tim Panton</a>
 */

package uk.co.westhawk.snmp.stack;

import java.io.*;

/**
 * This class help with the BITS construct. The AsnOctets class is
 * growing too big, because of all the different representations.
 *
 * Note that BITS is different from the ANS.1 BIT STRING. Apparently BIT
 * STRING was in v1, but was depreceated in favour of BITS.
 *
 * <pre>
 * An instance with bits 0, 3 and 10 set will have the value:
 *
 *      10010000 00100000
 *      ^  ^       ^
 * bit  0  3      10
 *
 * so will consist of the two bytes (octets) 0x90 & 0x20.
 * </pre>
 *
 * See also
 * section 7.1.4. of
 * <a href="http://www.ietf.org/rfc/rfc2578.txt">SNMPv2-SMI</a>
 * and section 8. (3) of
 * <a href="http://www.ietf.org/rfc/rfc3417.txt">SNMPv2-TM</a>.
 *
 * @author <a href="mailto:[e-mail]">Birgit Arkesteijn</a>
 * @version $Revision: 3.1 $ $Date: 2006/03/23 15:09:48 $
 * @since 5_0
 */
public class BitsHelper 
{
    private static final String     version_id =
        "@(#)$Id: BitsHelper.java,v 3.1 2006/03/23 15:09:48 birgit Exp $ Copyright Westhawk Ltd";


/**
 * Sets or unsets the flag (bit) on the specified index. The bit
 * will be set to zero if toset is false, set to one if toset is
 * true.
 *
 * Note, as a side effect the size of the asn octet might grow.
 *
 * @param oct The AsnOctets that represents the BITS
 * @param index The index (0 - X)
 * @param toset Whether to set (true) or unset (false) the bit
 */
public static void setFlagged(AsnOctets oct, int index, boolean toset)
throws IllegalArgumentException
{
    if (index < 0)
    {
        throw new IllegalArgumentException("Illegal value index (" + index
            + "). Shoud be greater than 0.");
    }
    else
    {
        byte mask;
        int byteNo = index / 8;
        int bitNo  = index % 8;

        // See if we've got enough bytes in our array.
        // Reallocate if necessary.
        int len = oct.value.length;
        if (byteNo >= len)
        {
            int newLen = byteNo+1;
            byte[] newValue = new byte[newLen];
            System.arraycopy(oct.value, 0, newValue, 0, len);
            oct.value = newValue;
        }

        if (toset == true)
        {
            mask = (byte) (0x80 >>> bitNo);
            oct.value[byteNo] = (byte) (oct.value[byteNo] | mask);
        }
        else
        {
            mask = (byte) (0x7F >>> bitNo);
            oct.value[byteNo] = (byte) (oct.value[byteNo] & mask);
        }
    }
}


/**
 * Returns if the flag (bit) on the specified index is set. The bit
 * will be set to zero if toset is false, set to one if toset is
 * true.
 *
 * @param oct The AsnOctets that represents the BITS
 * @param index The index (0 - X)
 * @return Whether the bit is set (true) or unset (false)
 */
public static boolean isFlagged(AsnOctets oct, int index)
throws IllegalArgumentException
{
    boolean isFlagged = false;
    if (index < 0)
    {
        throw new IllegalArgumentException("Illegal value index (" + index
            + "). Shoud be greater than 0.");
    }
    else
    {
        byte mask;
        int byteNo = index / 8;
        int bitNo  = index % 8;

        int len = oct.value.length;
        if (byteNo < len)
        {
            // Shift the bit we're interested in to the far left.
            // If the bit is one, the byte will be negative.
            byte res = (byte) (oct.value[byteNo] << bitNo);
            isFlagged = (res < 0);
        }
    }
    return isFlagged;
}


}