main.py

#

Part 1:

— Day 2: 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.

from enum import Enum
from typing import NamedTuple, TextIO

#

Rock Paper Scissors is a game between two players. Each game contains many rounds; in each round, the players each simultaneously choose one of Rock, Paper, or Scissors using a hand shape.

#

class Move(Enum):
    rock = 1
    paper = 2
    scissors = 3

#

class Round(NamedTuple):
    my_move: Move
    their_move: Move

#

Then, a winner for that round is selected: Rock defeats Scissors, Scissors defeats Paper, and Paper defeats Rock. If both players choose the same shape, the round instead ends in a draw.

winning_combos = {
    (Move.rock, Move.scissors),
    (Move.scissors, Move.paper),
    (Move.paper, Move.rock),
}

#

Appreciative of your help yesterday, one Elf gives you an encrypted strategy guide (your puzzle input) that they say will be sure to help you win. “The first column is what your opponent is going to play: A for Rock, B for Paper, and C for Scissors. The second column–” Suddenly, the Elf is called away to help with someone’s tent.

their_move_decode = {
    "A": Move.rock,
    "B": Move.paper,
    "C": Move.scissors,
}

#

The second column, you reason, must be what you should play in response: X for Rock, Y for Paper, and Z for Scissors. Winning every time would be suspicious, so the responses must have been carefully chosen.

my_move_decode = {
    "X": Move.rock,
    "Y": Move.paper,
    "Z": Move.scissors,
}

#

The winner of the whole tournament is the player with the highest score. Your total score is the sum of your scores for each round. The score for a single round is the score for the shape you selected (1 for Rock, 2 for Paper, and 3 for Scissors) plus the score for the outcome of the round (0 if you lost, 3 if the round was a draw, and 6 if you won).

#

class RoundResult(Enum):
    loss = 0
    draw = 3
    win = 6

#

Since you can’t be sure if the Elf is trying to help you or trick you, you should calculate the score you would get if you were to follow the strategy guide.

#

For example, suppose you were given the following strategy guide:

A Y
B X
C Z

This strategy guide predicts and recommends the following:

def test_part1() -> None:

#

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).

    assert score_round(decode_round("A Y")) == 8

#

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).

    assert score_round(decode_round("B X")) == 1

#

The third round is a draw with both players choosing Scissors, giving you a score of 3 + 3 = 6.

    assert score_round(decode_round("C Z")) == 6

#

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

What would your total score be if everything goes exactly according to your strategy guide?

#

Part 1 Solution:

MoveDecoder = dict[str, Move]

#

Parse and decode the round info from a strategy guide line using the given move decoder tables.

>>> decode_round("A Y")
Round(my_move=<Move.paper: 2>, their_move=<Move.rock: 1>)

def decode_round(
    round_str: str,
    their_move_decode: MoveDecoder = their_move_decode,
    my_move_decode: MoveDecoder = my_move_decode,
) -> Round:

#

    their_move_str, my_move_str = round_str.split()
    return Round(
        my_move=my_move_decode[my_move_str],
        their_move=their_move_decode[their_move_str],
    )

#

Return the result of the given RPS round.

>>> decide_round(Round(their_move=Move.rock, my_move=Move.paper))
<RoundResult.win: 6>

def decide_round(round: Round) -> RoundResult:

#

    if (round.my_move, round.their_move) in winning_combos:
        return RoundResult.win

    if (round.their_move, round.my_move) in winning_combos:
        return RoundResult.loss

    return RoundResult.draw

#

Return the total score given by this round.

def score_round(round: Round) -> int:

#

    return round.my_move.value + decide_round(round).value

#

Calculate the total score that would result if the given strategy guide was accurate.

def part1(input: TextIO) -> int:

#

Part 2:

The Elf finishes helping with the tent and sneaks back over to you. “Anyway, the second column says how the round needs to end: X means you need to lose, Y means you need to end the round in a draw, and Z means you need to win. Good luck!”

    return sum(score_round(decode_round(line.strip())) for line in input)

#

The total score is still calculated in the same way, but now you need to figure out what shape to choose so the round ends as indicated.

intended_result_decode = {
    "X": RoundResult.loss,
    "Y": RoundResult.draw,
    "Z": RoundResult.win,
}

#

The example above now goes like this:

def test_part2() -> None:

#

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.

    assert score_round(decode_round_with_result("A Y")) == 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.

    assert score_round(decode_round_with_result("B X")) == 1

#

In the third round, you will defeat your opponent’s Scissors with Rock for a score of 1 + 6 = 7.

    assert score_round(decode_round_with_result("C Z")) == 7

#

Now that you’re correctly decrypting the ultra top secret strategy guide, you would get a total score of 12.

Following the Elf’s instructions for the second column, what would your total score be if everything goes exactly according to your strategy guide?

#

Part 2 Solution:

#

I decided to solve part 2 in a way that maximizes the reuse of work from part 1. The parsing function knows the round result but throws it away since part 1’s round scoring function has to determine it anyway.

ResultDecoder = dict[str, RoundResult]

#

Parse and decode the move and result info from a strategy guide line and return the round that produces the intended result.

def decode_round_with_result(
    strat_str: str,
    their_move_decode: MoveDecoder = their_move_decode,
    intended_result_decode: ResultDecoder = intended_result_decode,
) -> Round:

#

    their_move_str, intended_result_str = strat_str.split()
    their_move = their_move_decode[their_move_str]
    intended_result = intended_result_decode[intended_result_str]

#

Consider every possible move and see which one achieves the intended result

    for my_move in Move:
        considered_round = Round(my_move=my_move, their_move=their_move)
        if decide_round(considered_round) is intended_result:
            return considered_round

    raise RuntimeError("No moves lead to the intended result")

#

Calculate the total score that would result if the strategy guide was followed using the new interpretation.

def part2(input: TextIO) -> int:

#

    return sum(score_round(decode_round_with_result(line.strip())) for line in input)


if __name__ == "__main__":

#

Print out part 1 solution

    with open("input.txt") as puzzle_input:
        print("Part 1:", part1(puzzle_input))

#

Print out part 2 solution

    with open("input.txt") as puzzle_input:
        print("Part 2:", part2(puzzle_input))