Paul's puzzle 48 solution - 31 parts

[paul]

Here’s my solution (it’s the one in the book):

And here’s the link:

http://www.lodev.org/jstumble/?board=0e00ee0xreerxllee0l0eerrllee0l0eerrllee0l0eerrlif

Description: There’s a 4-ball cycle set up by the three bits on top. The first ball subtracts one from register A (the right one). The second ball does nothing (this is wasted in my design). The third ball adds 2 to register B. The fourth ball adds 1 to register B. (The third and fourth ball add a total of 3.) When register A underflows, the computer stops.

[menderbug]

Since the puzzle specifies that A is no greater than 5, you can drop the bottom left ramp. (Not going to make that a separate post because of how trivial the improvement is; I do have a solution at the same size which only needs 16 balls at the top though, which I might post tomorrow.)