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Apache Commons Imaging (previously Sanselan) is a pure-Java image library.
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/*
 * 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.bmp;

import java.io.IOException;
import java.nio.ByteOrder;

import org.apache.commons.imaging.ImageReadException;

import static org.apache.commons.imaging.common.BinaryFunctions.*;

class PixelParserRgb extends PixelParserSimple {
    private int bytecount;
    private int cachedBitCount;
    private int cachedByte;

    public PixelParserRgb(final BmpHeaderInfo bhi, final byte[] colorTable, final byte[] imageData) {
        super(bhi, colorTable, imageData);

    }

    @Override
    public int getNextRGB() throws ImageReadException, IOException {

        if ((bhi.bitsPerPixel == 1) 
                || (bhi.bitsPerPixel == 4)) { // always grayscale?
            if (cachedBitCount < bhi.bitsPerPixel) {
                if (cachedBitCount != 0) {
                    throw new ImageReadException("Unexpected leftover bits: "
                            + cachedBitCount + "/" + bhi.bitsPerPixel);
                }

                cachedBitCount += 8;
                cachedByte = (0xff & imageData[bytecount]);
                bytecount++;
            }
            final int cacheMask = (1 << bhi.bitsPerPixel) - 1;
            final int sample = cacheMask & (cachedByte >> (8 - bhi.bitsPerPixel));
            cachedByte = 0xff & (cachedByte << bhi.bitsPerPixel);
            cachedBitCount -= bhi.bitsPerPixel;

            return getColorTableRGB(sample);
        } else if (bhi.bitsPerPixel == 8) { // always grayscale?
            final int sample = 0xff & imageData[bytecount + 0];

            final int rgb = getColorTableRGB(sample);

            bytecount += 1;

            return rgb;
        } else if (bhi.bitsPerPixel == 16) {
            final int data = read2Bytes("Pixel", is, "BMP Image Data", ByteOrder.LITTLE_ENDIAN);

            final int blue = (0x1f & (data >> 0)) << 3;
            final int green = (0x1f & (data >> 5)) << 3;
            final int red = (0x1f & (data >> 10)) << 3;
            final int alpha = 0xff;

            final int rgb = (alpha << 24) | (red << 16) | (green << 8) | (blue << 0);

            bytecount += 2;

            return rgb;
        } else if (bhi.bitsPerPixel == 24) {
            final int blue = 0xff & imageData[bytecount + 0];
            final int green = 0xff & imageData[bytecount + 1];
            final int red = 0xff & imageData[bytecount + 2];
            final int alpha = 0xff;

            final int rgb = (alpha << 24) | (red << 16) | (green << 8) | (blue << 0);

            bytecount += 3;

            return rgb;
        } else if (bhi.bitsPerPixel == 32) {
            final int blue = 0xff & imageData[bytecount + 0];
            final int green = 0xff & imageData[bytecount + 1];
            final int red = 0xff & imageData[bytecount + 2];
            final int alpha = 0xff;

            final int rgb = (alpha << 24) | (red << 16) | (green << 8) | (blue << 0);

            bytecount += 4;

            return rgb;
        }

        throw new ImageReadException("Unknown BitsPerPixel: "
                + bhi.bitsPerPixel);
    }

    @Override
    public void newline() throws ImageReadException, IOException {
        cachedBitCount = 0;

        while (((bytecount) % 4) != 0) {
            readByte("Pixel", is, "BMP Image Data");
            bytecount++;
        }
    }
}