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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.icns;

final class Rle24Compression {
    private Rle24Compression() {
    }

    public static byte[] decompress(final int width, final int height, final byte[] data) {
        final int pixelCount = width * height;
        final byte[] result = new byte[4 * pixelCount];

        // Several ICNS parsers advance by 4 bytes here:
        // http://code.google.com/p/icns2png/ - when the width is >= 128
        // http://icns.sourceforge.net/ - when those 4 bytes are all zero
        //
        // A scan of all .icns files on MacOS shows that
        // all 128x128 images indeed start with 4 zeroes,
        // while all smaller images don't.
        // However it is dangerous to assume
        // that 4 initial zeroes always need to be skipped,
        // because they could encode valid pixels on smaller images.
        // So always skip on 128x128, and never skip on anything else.
        int dataPos = 0;
        if (width >= 128 && height >= 128) {
            dataPos = 4;
        }

        // argb, band by band in 3 passes, with no alpha
        for (int band = 1; band <= 3; band++) {
            int remaining = pixelCount;
            int resultPos = 0;
            while (remaining > 0) {
                if ((data[dataPos] & 0x80) != 0) {
                    final int count = (0xff & data[dataPos]) - 125;
                    for (int i = 0; i < count; i++) {
                        result[band + 4 * (resultPos++)] = data[dataPos + 1];
                    }
                    dataPos += 2;
                    remaining -= count;
                } else {
                    final int count = (0xff & data[dataPos]) + 1;
                    dataPos++;
                    for (int i = 0; i < count; i++) {
                        result[band + 4 * (resultPos++)] = data[dataPos++];
                    }
                    remaining -= count;
                }
            }
        }
        return result;
    }
}