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:

1
2
3

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

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

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
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18

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

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42

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

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51

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 🤣

You can find the complete code here Thanks for reading!

This post is number 3 of a 13 posts series > Learning Go.