src/main/java/org/apache/pdfbox/jbig2/decoder/arithmetic/ArithmeticIntegerDecoder.java

/**
 * 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.pdfbox.jbig2.decoder.arithmetic;

import java.io.IOException;

/**
 * This class represents the arithmetic integer decoder, described in ISO/IEC 14492:2001 (Annex A).
 */
public class ArithmeticIntegerDecoder
{

    private final ArithmeticDecoder decoder;

    private int prev;

    public ArithmeticIntegerDecoder(ArithmeticDecoder decoder)
    {
        this.decoder = decoder;
    }

    /**
     * Arithmetic Integer Decoding Procedure, Annex A.2.
     * 
     * @param cxIAx to be decoded value
     * @return Decoded value.
     * @throws IOException if an underlying IO operation fails
     */
    public long decode(CX cxIAx) throws IOException
    {
        int v = 0;
        int d, s;

        int bitsToRead;
        int offset;

        if (cxIAx == null)
        {
            cxIAx = new CX(512, 1);
        }

        prev = 1;

        cxIAx.setIndex(prev);
        s = decoder.decode(cxIAx);
        setPrev(s);

        cxIAx.setIndex(prev);
        d = decoder.decode(cxIAx);
        setPrev(d);

        if (d == 1)
        {
            cxIAx.setIndex(prev);
            d = decoder.decode(cxIAx);
            setPrev(d);

            if (d == 1)
            {
                cxIAx.setIndex(prev);
                d = decoder.decode(cxIAx);
                setPrev(d);

                if (d == 1)
                {
                    cxIAx.setIndex(prev);
                    d = decoder.decode(cxIAx);
                    setPrev(d);

                    if (d == 1)
                    {
                        cxIAx.setIndex(prev);
                        d = decoder.decode(cxIAx);
                        setPrev(d);

                        if (d == 1)
                        {
                            bitsToRead = 32;
                            offset = 4436;
                        }
                        else
                        {
                            bitsToRead = 12;
                            offset = 340;
                        }
                    }
                    else
                    {
                        bitsToRead = 8;
                        offset = 84;
                    }
                }
                else
                {
                    bitsToRead = 6;
                    offset = 20;
                }
            }
            else
            {
                bitsToRead = 4;
                offset = 4;
            }
        }
        else
        {
            bitsToRead = 2;
            offset = 0;
        }

        for (int i = 0; i < bitsToRead; i++)
        {
            cxIAx.setIndex(prev);
            d = decoder.decode(cxIAx);
            setPrev(d);
            v = (v << 1) | d;
        }

        v += offset;

        if (s == 0)
        {
            return v;
        }
        else if (s == 1 && v > 0)
        {
            return -v;
        }

        return Long.MAX_VALUE;
    }

    private void setPrev(int bit)
    {
        if (prev < 256)
        {
            prev = ((prev << 1) | bit) & 0x1ff;
        }
        else
        {
            prev = ((((prev << 1) | bit) & 511) | 256) & 0x1ff;
        }
    }

    /**
     * The IAID decoding procedure, Annex A.3.
     * 
     * @param cxIAID - The contexts and statistics for decoding procedure.
     * @param symCodeLen - Symbol code length.
     * 
     * @return The decoded value.
     * 
     * @throws IOException if an underlying IO operation fails
     */
    public int decodeIAID(CX cxIAID, long symCodeLen) throws IOException
    {
        // A.3 1)
        prev = 1;

        // A.3 2)
        for (int i = 0; i < symCodeLen; i++)
        {
            cxIAID.setIndex(prev);
            prev = (prev << 1) | decoder.decode(cxIAID);
        }

        // A.3 3) & 4)
        return (prev - (1 << symCodeLen));
    }
}