Rock Paper Scissors Lizard Spock

Verilog problems / Fun & Games

What you must build

Judge a round of the 5-way expanded rock-paper-scissors from The Big Bang Theory — each move beats exactly two of the other four, so the usual 3-way rule table isn't enough.

Engineers use “Rock Paper Scissors Lizard Spock” as a building block in fun & games. Interviewers ask for the same ports and the same corner cases this judge covers. Completing it in the browser is the same skill as writing synthesizable RTL at work, minus the EDA license.

Port contract

The judge instantiates exactly these ports. Extra ports or a different module name fail to elaborate.

How to approach this kata

This is a medium kata: you will need sequential logic or a small FSM. Decide what is registered versus combinational before you type. Reset polarity and clock edge must match the spec; the judge will fail you on the first mismatched cycle.

Hidden tests instantiate top_module, drive the ports, and compare every sample against a golden model. They do not grade coding style. They do grade X/Z, off-by-one counters, and ignoring enables. Sign in only when you want the run saved on the leaderboard — the specification below is public.

Starter shape

Copy this skeleton into the editor (or press Reset starter). Fill the body; do not rename the module.

module top_module(
  input  [2:0] p1,
  input  [2:0] p2,
  output [1:0] winner
);

  // Your code here — 0=Rock,1=Paper,2=Scissors,3=Lizard,4=Spock.
  // Each move beats exactly two others. winner: 0=tie, 1=p1 wins, 2=p2 wins.

endmodule

Why this shows up in interviews

Rock Paper Scissors Lizard Spock sits under Fun & Games (combinational, game-logic). Concept: Encode each player's move as 0=Rock, 1=Paper, 2=Scissors, 3=Lizard, 4=Spock. Every move beats exactly two others (Rock beats Scissors and Lizard; Paper beats Rock and Spock; Scissors beats Paper and Lizard; Lizard beats Spock and Paper; Spock beats Scissors and Rock) — so the cleanest implementation is a direct lookup of p1's beats-list against p2, then the reverse check if that fails. A single wrong entry in that beats-list (say, listing Paper instead of Scissors as one of Rock's victims) is a real, easy-to-make copy-paste slip — it still compiles and handles most matchups correctly, but silently flips the outcome for that one specific matchup.

A passing solution is synthesizable intent: no delays in the DUT, no initial blocks inside top_module, and no reference to testbench tasks. Use blocking assignments only in combinational always blocks; use non-blocking for registers clocked by clk.

Related problems

• Magic 8-Ball Fortune Teller — Give a "random-feeling" fortune-teller answer on every shake using a self-advancing 3-bit LFSR — a tiny, deterministic taste of pseudo-randomness in hardware.
• Hot Potato Token Passer — Pass a token around a 4-player ring every cycle until a buzzer sounds — whoever's holding it when the music stops loses, and the game freezes there.
• Reaction Game Scorer — Score a simple reflex game: a light comes on, the player presses a button, and the circuit measures how many cycles it took — flagging jumping the gun or dozing off.
• Space Invaders Collision Detector — Detect whether a bullet hits an alien: a classic range-overlap check dressed up as retro arcade collision logic.

FAQ