Posted on Dec 4, 2022 • Edited on Dec 17, 2022 • Originally published at galelmalah.com

AoC Day 2 - Rock Paper Scissors

#adventofcode #go #beginners #programming

Rock Paper Scissors 🪨🧻✂

The Elves begin to set up camp on the beach. To decide whose tent gets to be closest to the snack storage, a giant Rock Paper Scissors tournament is already in progress.

God the question's description is always pure gold...now let's jump right in.

We are given a string, our "strategy guide" that represents a rock, paper, scissors turn decision.
The string is divided into two columns, the first represents the first player's decision and the second ours.
For example:

A Y B X C Z

We are also told the following:
For player 1 - A = Rock, B = Paper, C = Scissors
For player 2 - X = Rock, Y = Paper, Z = Scissors
We are also given a score table for the outcome and the decision we made, for example, a win is 6 points and choosing paper is 2 points, etc...

Like previous questions, we'll start by parsing our input

Parsing

We can go (see what I did there?) with several options to represent our game but in my opinion, an array of tuples is the simplest option

func parse(raw string) [][]string { chunks := strings.Split(string(raw), "\n") pairs := make([][]string, len(chunks)) for i := range pairs { pairs[i] = strings.Split(chunks[i], " ") } return pairs } // example output // [ [A, Y], [B, X], [C, Z] ]

The make function allocates a piece of memory in a certain, specified size for our array

Part 1

We are asked to provide our total score if we play exactly as instructed in the strategy guide.
Let's think about this for a bit, there are several ways we can solve this, we can use a bunch of if statements or some fancy pattern matching, since go does not have pattern matching and I don't want to write a ton of if statements we will go with a hybrid approach.
We will create 3 different mappings:

Represents the points we get for our choice e.g rock, paper, or scissors
Winning state, meaning If we choose X what does the other player need to choose for us to Win
Tie state, essentially the same as .2

scores := map[string]int{ "X": 1, "Y": 2, "Z": 3, } // If I choose X(Rock) I need him to choose C(scissors) in order to win win := map[string]string{ "X": "C", "Y": "A", "Z": "B", } tie := map[string]string{ "X": "A", "Y": "B", "Z": "C", }

We don't take into account the losing state since its essentially a no-op (0 points)

Building on top of these maps and our parsing logic, we can now solve the first part with the following code

func part1(raw []byte) int { pairs := parse(string(raw)) // X Rock, Y Paper, Z Scissors scores := map[string]int{ "X": 1, "Y": 2, "Z": 3, } win := map[string]string{ "X": "C", "Y": "A", "Z": "B", } tie := map[string]string{ "X": "A", "Y": "B", "Z": "C", } score := 0 for _, pair := range pairs { his := pair[0] my := pair[1] score += scores[my] if win[my] == his { score += WINNING_POINTS } if tie[my] == his { score += TIE_POINTS } } return score } // output for part 1 based on the example is // 15 -> (8 + 1 + 6)

At each loop iteration we first add the points based on our choice score += scores[my] then we check if his move is what we need based on our player choice, to win or get a tie, and if it is we add the necessary points to our total score.

Part 2

In part two the sneaky elves switch things up a bit.
Instead of our column representing our moves, it represents the turn outcome where X = lose, Y = tie, and Z = win and we need to choose our choice accordingly.
For example, let's look at the first turn A Y, the new meaning of this pair is "player one chose Rock, and the game ended in a tie" building on this information we can create new mappings, the new mappings will be between player 1 choice and the choice player 2 need to make to get to a certain state e.g winning, losing, tie, etc...
Since it's pretty similar to part 1, we will jump right ahead and look at part 2 as a whole

func part2(raw []byte) int { var pairs = parse(string(raw)) // X Lose, Y Tie, Z Win scores := map[string]int{ "X": 1, "Y": 2, "Z": 3, } win := map[string]string{ "C": "X", "A": "Y", "B": "Z", } tie := map[string]string{ "A": "X", "B": "Y", "C": "Z", } lose := map[string]string{ "A": "Z", "B": "X", "C": "Y", } score := 0 for _, pair := range pairs { hisMove := pair[0] myMove := pair[1] // we lose if myMove == "X" { score += scores[lose[hisMove]] } // we end in a tie if myMove == "Y" { score += TIE_POINTS score += scores[tie[hisMove]] } // we win if myMove == "Z" { score += WINNING_POINTS score += scores[win[hisMove]] } } return score }