/*
 * 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.imaging.formats.png.scanlinefilters;

import java.io.IOException;

import org.apache.commons.imaging.ImageReadException;

public class ScanlineFilterAverage implements ScanlineFilter {
    private final int bytesPerPixel;

    public ScanlineFilterAverage(final int bytesPerPixel) {
        this.bytesPerPixel = bytesPerPixel;
    }

    @Override
    public void unfilter(final byte[] src, final byte[] dst, final byte[] up)
            throws ImageReadException, IOException {
        for (int i = 0; i < src.length; i++) {
            int raw = 0;
            final int prevIndex = i - bytesPerPixel;
            if (prevIndex >= 0) {
                raw = dst[prevIndex];
            }

            int prior = 0;
            if (up != null) {
                prior = up[i];
            }

            final int average = ((0xff & raw) + (0xff & prior)) / 2;

            dst[i] = (byte) ((src[i] + average) % 256);
            // dst[i] = src[i];
            // dst[i] = (byte) 255;
        }
    }
}
