com.bigdata.io.ShortPacker Maven / Gradle / Ivy

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/**

Copyright (C) SYSTAP, LLC DBA Blazegraph 2006-2016.  All rights reserved.

Contact:
     SYSTAP, LLC DBA Blazegraph
     2501 Calvert ST NW #106
     Washington, DC 20008
     [email protected]

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/
/*
 * Created on Oct 24, 2005
 */
package com.bigdata.io;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.EOFException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Packing utility for non-negative short values.
 *  
 * @author Bryan Thompson
 * @version $Id$
 */

public class ShortPacker
{
    
    public ShortPacker() {
        super();
    }
    
    /**
     * Packs a non-negative short value into one or two bytes and writes them on
     * os . A short in [0:127] is packed into one byte. Larger values are
     * packed into two bytes. The high bit of the first byte is set if the value
     * was packed into two bytes. If the bit is set, clear the high bit, read
     * the next byte, and interpret the two bytes as a short value. Otherwise
     * interpret the byte as a short value.
     * 
     * @return The #of bytes into which the value was packed.
     */ 
    static public int packShort(final DataOutput os, final short v)
            throws IOException {

        /*
         * You can only pack non-negative values with this method.
         */
        if( v < 0 ) {
            throw new IllegalArgumentException( "negative value: v="+v );
        }
        if( v > 127 ) {
            // the value requires two bytes.
            os.write( (byte)((0xff & (v >> 8))|0x80) ); // note: set the high bit.
            os.write( (byte)(0xff & v) );
            return 2;
        } else {
            // the value fits in one byte.
            os.write( (byte)(0xff & v) );
            return 1;
        }
    }
    
    /**
     * Unpack a non-negative short value from the input stream.
     * 
     * @param is The input stream.
     * 
     * @return The short value.
     * 
     * @throws IOException
     */
    static public short unpackShort( final DataInput is ) throws IOException
    {
        short b = (short) is.readByte();
        short v;
        if( ( b & 0x80 ) != 0 ) {
            // high bit is set.
            v = (short) (( b & 0x7f ) << 8); // clear the high bit and shift over one byte.
            b = is.readByte(); // read the next byte.
            v |= ( b & 0xff ); // and combine it together with the high byte.
        } else {
            // high bit is clear.
            v = b; // interpret the byte as a short value.
        }
        return (short) v;
    }

    /**
     * Unpack a non-negative short value from the input stream.
     * 
     * @param is The input stream.
     * 
     * @return The short value.
     * 
     * @throws IOException
     */
    static public short unpackShort( final InputStream is ) throws IOException
    {
        short b = (short) readByte(is);
        short v;
        if( ( b & 0x80 ) != 0 ) {
            // high bit is set.
            v = (short) (( b & 0x7f ) << 8); // clear the high bit and shift over one byte.
            b = readByte(is); // read the next byte.
            v |= ( b & 0xff ); // and combine it together with the high byte.
        } else {
            // high bit is clear.
            v = b; // interpret the byte as a short value.
        }
        return (short) v;
    }

    /**
     * Read a byte from an {@link InputStream} ala {@link DataInput#readByte()}
     * 
     * @param is
     *            The input stream.
     * @return The byte.
     * @throws IOException
     * @throws EOFException
     *             if the end of the file has been reached.
     */
    private static byte readByte(final InputStream is) throws IOException {
        
        final int v = is.read();
        
        if (v == -1)
            throw new EOFException();
        
        return (byte) (v & 0xff);
        
    }

    /**
     * Returns the #of bytes into which a short value was packed based on the
     * first byte.
     * 
     * @param firstByte The first byte.
     * 
     * @return The #of bytes (either one (1) or two (2)).
     */
    static public int getNBytes( byte firstByte ) {
        if( ( firstByte & 0x80 ) != 0 ) {
            return 2;
        } else {
            return 1;
        }
    }
    
}