Advent of Code 2022, Day 2: Rock Paper Scissors

Part A

Day 2 is the game Rock Paper Scissors. We have an input file with a strategy guide:

A Y
B X
C Z

The first column is your opponent move and in the second how you should play. A is for Rock, B for Paper and C for Scissors. And the same X for Rock, Y for Paper and Z for Scissors. For me it is a bit confusing to reference those values in the code, so I remapped them during reading from input file:

moves = { "A" => "R", "B" => "P", "C" => "S", "X" => "R", "Y" => "P", "Z" => "S" }

data = File.
  readlines("2.txt").
  map(&:strip).
  map { |item| item.split(" ") }.map do |(opponent, me)|
    [moves[opponent], moves[me]]
  end

What we need to do is calculate the final score. Here are the rules:

• In the first round, your opponent will choose Rock (A), and you should choose Paper (Y). This ends in a win for you with a score of 8 (2 because you chose Paper + 6 because you won).
• In the second round, your opponent will choose Paper (B), and you should choose Rock (X). This ends in a loss for you with a score of 1 (1 + 0).
• The third round is a draw with both players choosing Scissors, giving you a score of 3 + 3 = 6.

In this example, if you were to follow the strategy guide, you would get a total score of 15 (8 + 1 + 6).

moves = { "A" => "R", "B" => "P", "C" => "S", "X" => "R", "Y" => "P", "Z" => "S" }

data = File.
  readlines("2.txt").
  map(&:strip).
  map { |item| item.split(" ") }.map do |(opponent, me)|
    [moves[opponent], moves[me]]
  end

map = {
  "R" => 1,
  "P" => 2,
  "S" => 3
}

results = data.map do |(opponent, me)|
  points = 0

  if (opponent == "R" && me == "P") || (opponent == "P" && me == "S") || (opponent == "S" && me == "R")
    points += 6
  elsif opponent == me
    points += 3
  end

  points += map[me]

  points
end

puts results.sum

Part B

In Part B rules change to:

• In the first round, your opponent will choose Rock (A), and you need the round to end in a draw (Y), so you also choose Rock. This gives you a score of 1 + 3 = 4.
• In the second round, your opponent will choose Paper (B), and you choose Rock so you lose (X) with a score of 1 + 0 = 1.
• In the third round, you will defeat your opponent’s Scissors with Rock for a score of 1 + 6 = 7.

Now for the initial example you would get a total score of 12.

I am lazy so I just mapped all possible inputs, their respective scores and summed them up together:

moves = { "A" => "R", "B" => "P", "C" => "S", "X" => "R", "Y" => "P", "Z" => "S" }

strategy = {
  # Rock
  "AX" => 0 + 3,
  "AY" => 3 + 1,
  "AZ" => 6 + 2,
  # Paper
  "BX" => 0 + 1,
  "BY" => 3 + 2,
  "BZ" => 6 + 3,
  # Scissors
  "CX" => 0 + 2,
  "CY" => 3 + 3,
  "CZ" => 6 + 1
}

results = File.
  readlines("2.txt").
  map(&:strip).
  map { |item| item.split(" ") }.map do |(opponent, me)|
    strategy["#{opponent}#{me}"]
  end

puts results.sum

Just to explain. For example "CZ" => 6 + 1, opponent is playing Scissors and we need to win. We will get 6 points for the win and 1 point for playing Rock.