Advent of Code - Day 2 - Worst Practice

Advent of Code - Day 2

Posted on December 2, 2022 @ 06:00

Posted under the Backend category

Level: beginner

Posted with the following tags: #Advent, #PHP

A classic game I used to play a lot with my son.

Advent of Code - Day 2 — Calendar icon by Kevin Sanderson from Pixabay

Today’s puzzle in short: It’s the good old Stone - Paper - Scissors game.

The input data

The input data today is simple:

Every row contains two characters separated by a space.
The first character is either A, B or C.
The second character is either X, Y or Z.

Game rules

A represents player 1’s decision: the Stone.
B represents player 1’s decision: the Paper.
C represents player 1’s decision: the Scissors.
The Stone is worth 1 point.
The Paper is worth 2 points.
The Scissors is worth 3 points.
If player 1 wins, that’s 0 point.
If it’s a draw, that’s 3 points.
If player 2 wins, that’s 6 points.

Part one

The second character in this part also represents decisions:

X represents player 2’s decision: the Stone.
Y represents player 2’s decision: the Paper.
Z represents player 2’s decision: the Scissors.

Task: how much point will player 2 get after playing all rounds?

The code

Following the rule I set up yesterday, I think in the most simple solution. So we have two players, six letters with three points assigned and also some points regarding the result of the duel. This will cause a bit of hell with the variables, so it’s better to define constants. Also need to make a 3x3 matrix for the game rules:

Stone beats scissors
Scissors beat paper
Paper beats stone

                
              
                const ROCK_1 = 'A';
const PAPER_1 = 'B';
const SCISSORS_1 = 'C';

const ROCK_2 = 'X';
const PAPER_2 = 'Y';
const SCISSORS_2 = 'Z';

const P1_WINS = 0;
const P2_WINS = 6;
const DRAW = 3;

$decisionScore = [
    ROCK_2 => 1,
    PAPER_2 => 2,
    SCISSORS_2 => 3
];

$winningMatrix = [
    ROCK_1 => [
        ROCK_2 => DRAW,
        PAPER_2 => P2_WINS,
        SCISSORS_2 => P1_WINS
    ],
    PAPER_1 => [
        ROCK_2 => P1_WINS,
        PAPER_2 => DRAW,
        SCISSORS_2 => P2_WINS
    ],
    SCISSORS_1 => [
        ROCK_2 => P2_WINS,
        PAPER_2 => P1_WINS,
        SCISSORS_2 => DRAW
    ],
];

              
            

        
      
  

You may have noticed, that the $decisionScore contains only the player 2’s decisions. It’s because we don’t care about player 1’s score in the end.

Now let’s think about the logic. Go line-by-line, split up the string into the two players’ decisions, and simply add together the decision score and the score given by the winning matrix.

    
  
    $player_2_score = 0;

if ($fn = fopen(__DIR__.'/input.txt', 'r')) {
    while (($line = fgets($fn, 1024)) !== false) {
        $line = trim($line);
        [$player_1_decision, $player_2_decision] = explode(' ', $line);
        $player_2_score += ($winningMatrix[$player_1_decision][$player_2_decision] + $decisionScore[$player_2_decision]);
    }
    fclose($fn);
}

echo $player_2_score.PHP_EOL;

  

Part two

The second character in part two represents the strategy that player 2 should follow:

X to loose the round
Y to make draw
Z to win the round

Task: how much point will player 2 get after playing all rounds?

The code

For this version we have to revert our thinking a bit. The winning matrix will become a decision matrix, and with given strategy we can easily identify the right decision. The constants will now look as follows:

    
  
    const ROCK = 'A';
const PAPER = 'B';
const SCISSORS = 'C';

const P1_SHOULD_WIN = 'X';
const END_DRAW = 'Y';
const P2_SHOULD_WIN = 'Z';

$decisionScore = [
    ROCK => 1,
    PAPER => 2,
    SCISSORS => 3
];

$duelResult = [
    P1_SHOULD_WIN => 0,
    P2_SHOULD_WIN => 6,
    END_DRAW => 3
];

$decisionMatrix = [
    ROCK => [
        P1_SHOULD_WIN => SCISSORS,
        P2_SHOULD_WIN => PAPER,
        END_DRAW => ROCK
    ],
    PAPER => [
        P1_SHOULD_WIN => ROCK,
        P2_SHOULD_WIN => SCISSORS,
        END_DRAW => PAPER
    ],
    SCISSORS => [
        P1_SHOULD_WIN => PAPER,
        P2_SHOULD_WIN => ROCK,
        END_DRAW => SCISSORS
    ],
];

  

We have one extra array compared to the previous version, and another became much simpler. The logic will be the following:

    
  
    $player_2_score = 0;

if ($fn = fopen(__DIR__.'/input.txt', 'r')) {
    while (($line = fgets($fn, 1024)) !== false) {
        $line = trim($line);
        [$player_1_decision, $player_2_strategy] = explode(' ', $line);
        $player_2_decision = $decisionMatrix[$player_1_decision][$player_2_strategy];
        $player_2_score += ($duelResult[$player_2_strategy] + $decisionScore[$player_2_decision]);
    }
    fclose($fn);
}

echo $player_2_score.PHP_EOL;

  