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Gal Elmalah

Posted on • Updated on • Originally published at galelmalah.com

# AoC Day 2 - Rock Paper Scissors

## Rock Paper Scissors ๐ชจ๐งปโ

Question

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:

1. Represents the points we get for our choice e.g rock, paper, or scissors
2. Winning state, meaning If we choose X what does the other player need to choose for us to Win
3. 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
}
``````

For each desired state we check what move we need to do based on player 2 choice and pass it down to the `scores` map.

That's it we are all done with paper, rock, scissors and I must admit that I didn't think it can be so confusing ๐คฃ

You can find the complete code here