day2

AoC Day2: Rock Paper Scissors

Decode the elf’s RPS strategy and score the game. Strategy data will be in two columns like:

A Y
B X
C Z

This strategy guide predicts opponent will play Rock (A), then Paper (B), then Scissors (C). It recommends you play: * Paper (Y) two win with score of 2 for Paper + 6 for win = 8. * Rock (X) with score of 1 for Rock + 0 for loss = 1. * Scissors (Z) for score of 3 for Scissors + 3 for draw = 6. * Total Score = 15

Solution sketch:

Read the file
Replace {A,X} -> Rock, {B,Y} -> Paper, {C,Z} -> Scissors
Move scores: Rock=1, Paper=2, Scissors=3
Win scores: Loss=0, Draw=3, Win=6
Score each round, sum

Part 1

Get the data

Decode both moves to R, P, S. Keep as a two-letter string like “RP”.

RPS_CODE = str.maketrans("ABCXYZ", "RPSRPS", " ")
def rps_decode(data: str) -> str:
    return str.translate(data, RPS_CODE)

with open("../data/day2_input.txt") as f:
    data = [x.strip()
             .translate(RPS_CODE)
             for x in f.readlines()]
data[:5]

['PP', 'SS', 'SP', 'SP', 'RR']

Define the scoring

source

score_strategy

 score_strategy (strategy:list[str])

source

score_round

 score_round (moves:str)

Test using the example

assert score_round("RP") == 8   # win + 2
assert score_round("RR") == 4   # draw + 1
assert score_round("RS") == 3   # lose + 3

test = ["A Y", "B X", "C Z"]
test = [x.translate(RPS_CODE) for x in test]
assert [score_round(x) for x in test] == [8, 1, 6]
assert score_strategy(test) == 15

Run on the data

score_strategy(data)

Part 2

No wait! We had it wrong. The code is really:

X : You must lose
Y : You must draw
Z : You must win

All else is the same, so the example is now:

A Y -> RR -> Draw -> 1 + 3 = 4
B X -> PR -> Lose -> 1 + 0 = 1
C Z -> SR -> Win  -> 1 + 6 = 7
                             --
                             12

We have to change rps_decode because our moves now depend on theirs.

Redefine the coding

source

get_plan

 get_plan (strat:str)

Return dict of resposes for 1-letter strat

source

rps_decode

 rps_decode (code:str)

Convert coded ‘A X’ type string to ‘RS’ type string.

Test using example

test = ["A Y", "B X", "C Z"]
test = [rps_decode(x) for x in test]
assert [score_round(x) for x in test] == [4, 1, 7]
assert score_strategy(test) == 12

Run on data

with open("../data/day2_input.txt") as f:
    data = [rps_decode(x.strip()) for x in f.readlines()]
score_strategy(data)