Gadgetbridge/app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/CheckSums.java

/*  Copyright (C) 2015-2017 Carsten Pfeiffer

    This file is part of Gadgetbridge.

    Gadgetbridge is free software: you can redistribute it and/or modify
    it under the terms of the GNU Affero General Public License as published
    by the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    Gadgetbridge 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 Affero General Public License for more details.

    You should have received a copy of the GNU Affero General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>. */
package nodomain.freeyourgadget.gadgetbridge.util;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;

public class CheckSums {
    public static int getCRC8(byte[] seq) {
        int len = seq.length;
        int i = 0;
        byte crc = 0x00;

        while (len-- > 0) {
            byte extract = seq[i++];
            for (byte tempI = 8; tempI != 0; tempI--) {
                byte sum = (byte) ((crc & 0xff) ^ (extract & 0xff));
                sum = (byte) ((sum & 0xff) & 0x01);
                crc = (byte) ((crc & 0xff) >>> 1);
                if (sum != 0) {
                    crc = (byte) ((crc & 0xff) ^ 0x8c);
                }
                extract = (byte) ((extract & 0xff) >>> 1);
            }
        }
        return (crc & 0xff);
    }

    //thanks http://stackoverflow.com/questions/13209364/convert-c-crc16-to-java-crc16
    public static int getCRC16(byte[] seq) {
        int crc = 0xFFFF;

        for (int j = 0; j < seq.length; j++) {
            crc = ((crc >>> 8) | (crc << 8)) & 0xffff;
            crc ^= (seq[j] & 0xff);//byte to int, trunc sign
            crc ^= ((crc & 0xff) >> 4);
            crc ^= (crc << 12) & 0xffff;
            crc ^= ((crc & 0xFF) << 5) & 0xffff;
        }
        crc &= 0xffff;
        return crc;
    }

    public static void main(String[] args) throws IOException {
        if (args == null || args.length == 0) {
            throw new IllegalArgumentException("Pass the files to be checksummed as arguments");
        }
        for (String name : args) {
            try (FileInputStream in = new FileInputStream(name)) {
                byte[] bytes = readAll(in, 1000 * 1000);
                System.out.println(name + " : " + getCRC16(bytes));
            }
        }
    }

    // copy&paste of FileUtils.readAll() to have it free from Android dependencies
    private static byte[] readAll(InputStream in, long maxLen) throws IOException {
        ByteArrayOutputStream out = new ByteArrayOutputStream(Math.max(8192, in.available()));
        byte[] buf = new byte[8192];
        int read;
        long totalRead = 0;
        while ((read = in.read(buf)) > 0) {
            out.write(buf, 0, read);
            totalRead += read;
            if (totalRead > maxLen) {
                throw new IOException("Too much data to read into memory. Got already " + totalRead);
            }
        }
        return out.toByteArray();
    }
}