LM317 Programmable Current Source: How Does BCD Control 1–999mA with 4 Resistors?

"http://www.eeweb.com/blog/circuit_projects/programmable-current-source-by-lm317(Thread)":http://www.eeweb.com/blog/circuit_projects/programmable-current-source-by-lm317

I stumbled upon this thread while searching for something else, but the EE in me got the best of me and I started looking at it. It is a very simple and useful design. My question about it is this: If it is programmable from 1 to 999 mA in 1 mA intervals, then how are there only 4 resistors connected to each 10 position BCD?

The BCD switch combines the four resistor values to make the 10 needed values.

Looking at the accuracy, with the LM317T being a 1% part and the resistors being 1% the current error can be up to 2%. And the LM317T has a minimum specified load current of up to 12 mA, making the operating range no lower than 12 mA. Additionally, the up to 100 uA Adjustable Pin Current degrades the current accuracy at low current. The switch resistance (perhaps 100 m ohms) degrades the current accuracy at high current.

That's interesting. I wonder how someone could achieve the same thing replacing the BCD with say a 10 position rotary switch. Or maybe use a different IC completely?