如何获得两个（0〜15）个数字作为属性，以一个字节作为后备字段？

Question

I'm making a tile based 2d platformer and every byte of memory is precious. I have one byte field that can hold values from 0 to 255, but what I need is two properties with values 0~15. How can I turn one byte field into two properties like that?

Answer 1

do you mean just use the lower 4 bits for one value and the upper 4 bits for the other?

to get two values from 1 byte use...

a = byte & 15;
b = byte / 16;

setting is just the reverse as

byte = a | b * 16;

Using the shift operator is better but the compiler optimizers usually do this for you nowadays.

byte = a | (b << 4);

Answer 2

To piggy back off of sradforth's answer, and to answer your question about properties:

private byte _myByte;
public byte LowerHalf
{
    get
    {
        return (byte)(_myByte & 15);
    }
    set
    {
        _myByte = (byte)(value | UpperHalf * 16);
    }
}
public byte UpperHalf
{
    get
    {
        return (byte)(_myByte / 16);
    }
    set
    {
        _myByte = (byte)(LowerHalf | value * 16);
    }
}

Answer 3

Below are some properties and some backing store, I've tried to write them in a way that makes the logic easy to follow.

private byte HiAndLo = 0;

private const byte LoMask = 15;  // 00001111
private const byte HiMask = 240; // 11110000

public byte Lo
{
    get
    {
       // ----&&&&
       return (byte)(this.hiAndLo & LoMask);
    }

    set
    {
       if (value > LoMask) // 
       {
           // Values over 15 are too high.
           throw new OverflowException();
       }

       // &&&&0000
       // 0000----
       // ||||||||
       this.hiAndLo = (byte)((this.hiAndLo & HiMask) | value);
    }
}

public byte Hi
{
    get
    {
        // &&&&XXXX >> 0000&&&&
        return (byte)((this.hiAndLo & HiMask) >> 4);
    }

    set
    {
        if (value > LoMask)
        {
            // Values over 15 are too high.
            throw new OverflowException();
        }

        // -------- << ----0000
        //             XXXX&&&&
        //             ||||||||
        this.hiAndLo = (byte)((hiAndLo & LoMask) | (value << 4 )); 
    }
}