Java convert 8 bit numbers to 10 bit

Question

Am reading data out of a ByteBuffer but the values I need are stored as 10 bits. ie:

1100101110 = 814

I've tried using BitSet but this stores each byte in LSB first, which causes the final two bits to be incorrect the above example turns into:

1101001111 = 815

an example using a sequence of numbers:

                     8 bit    10 bit
01 02 03 04 05 06 07 08 | 09 10 | 11 12 13 14 15 16

would end up being:

                     8 bit    10 bit
08 07 06 05 04 03 02 01 | 16 15 | 14 13 12 11 10 09

So I can manage the first byte being in either order but the final two bits are taken from the wrong end of the next byte.

my current code is as follows:

// bb stands for the ByteBuffer in use.
//length stands for the number bytes covering the 10 bit numbers
BitSet data = getBitSet(bb, length);
int totalNumbers = data.length() / 10;
int[] numbers = new int[totalNumbers];
for (int i=0; i < totalNumbers; i++){
    int start = i*10;
    int end = (i+1)*10;
    BitSet bs = data.get(start, end);
    int tenBitNumber = 0;
    for (int j = bs.nextSetBit(0); j >= 0; j = bs.nextSetBit(j+1)) {
        double power = pow(2, 9-j);
        tenBitNumber += power;
    }
    numbers[i] = tenBitNumber;
}

a worked example in Big Endian format: sequence of bytes:

11001011|10110111|00101100|11000111

which transforms when using BitSet into:

11010011|11101101|00110100|11100011

What would be the best solution? I need to read multiple 10 bit length numbers from the ByteBuffer.

Answer 1

First, let's deal with a situation when five bytes (40 bits, or 4 ten-bit numbers) are available: split the input in chunks of five bytes. Each chunk will produce a group of four 10-bit numbers:

int[] convertFive(byte a, byte b, byte c, byte d, byte e) {
    int p = ((a & 0xff) << 2) | (b & 0xc0) >>> 6;
    int q = ((b & 0x3f) << 4) | (c & 0xf0) >>> 4;
    int r = ((c & 0x0f) << 6) | (d & 0xfc) >>> 2;
    int s = ((d & 0x03) << 8) | (e & 0xff) >>> 0;
    return new int [] { p, q, r, s }; 
}

Append these four ints to the output to produce the final result. You can modify the method to append output as you go, instead of creating four-element arrays all the time.

Deal with the remaining chunk of less than five bytes in a similar way: two bytes become one 10-bit number, three bytes become two numbers, and four bytes become three numbers. If the remainder is one-byte long, the input is invalid.