SignalChapter 5: Half duplex

35COMMON GROUND

The task

P must receive the largest number that divides both X and Y. Each runs from 1 to 99.

Each core holds one of the two, and they talk over one wire.

The board

The board as the puzzle screen lays it out. Cyan pins read, lemon pins write, violet pins go both ways. On a narrow screen, scroll the board sideways to see all of it.

Key idea

A loop of rounds over one wire with a stop word; the gap goes back when the left value shrinks.

Hints

Each hint goes a step further than the one before. Open them one at a time.

Hint 1Show

Take the smaller from the larger until the two match. That is the answer.

Hint 2Show

When the left value must shrink, send the difference back.

Hint 3Show

Tell the left core when the two match, so it knows to send.

Solution

Reveal the Full SolutionThe complete program for COMMON GROUND. Open it only when you want the answer.

The reference program

The game's own solution. It passes every test and scores exactly par on every benchmark.

leftSC-2 standard

S: IN P0
T: OUT P1
MOV B, A
IN P1
JZ E
JG T
XCH A, B
JMP T
E: XCH A, B
OUT P2

rightSC-2 standard

S: IN P0
T: MOV B, A
IN P1
SUB B
OUT P1
JZ S
JL M
MOV A, B
JMP T
M: NEG
JMP T

CYCLES: 5926; at par
ENERGY: 50744; at par
NODES: 2; at par
INSTRUCTIONS: 21; at par