AoC 2021 D10: Syntax Scoring

| Problem statement | Source code | Tags: Data structures

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Part 1

Bracket matching is a first-in-last-out problem: for each closing bracket, we need to match it with the most recent unclosed opening bracket. This is akin to a stack. We push opening brackets onto the stack, and for each closing bracket, we pop the stack and check if it matches. If it doesn't match, we have found a corrupted line. If everything matches at the end, we return the remaining stack, which are the unclosed opening brackets, for part 2.

function checkLine(line: string): number | string[] {
  const stack: string[] = [];
  for (const c of line) {
    if ("([{<".includes(c)) {
      stack.push(c);
    } else {
      const last = stack.pop();
      if (
        (c === ")" && last !== "(") ||
        (c === "]" && last !== "[") ||
        (c === "}" && last !== "{") ||
        (c === ">" && last !== "<")
      ) {
        return checkerScores[c as keyof typeof checkerScores];
      }
    }
  }
  return stack;
}

Part 2

Brackets are closed in reverse order, so we can iterate the remaining stack from the end (or top, since it's a stack) to get the sequence of required closing brackets.

let lineScore = 0;
for (let i = result.length - 1; i >= 0; i--) {
  lineScore =
    lineScore * 5 +
    completionScores[result[i] as keyof typeof completionScores];
}
scores.push(lineScore);

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