aoc/2023/day_01/solution.ts

/*
--- Day 1: Trebuchet?! ---

Something is wrong with global snow production, and you've been selected to take a look. The Elves have even given you a map; on it, they've used stars to mark the top fifty locations that are likely to be having problems.

You've been doing this long enough to know that to restore snow operations, you need to check all fifty stars by December 25th.

Collect stars by solving puzzles. Two puzzles will be made available on each day in the Advent calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star. Good luck!

You try to ask why they can't just use a weather machine ("not powerful enough") and where they're even sending you ("the sky") and why your map looks mostly blank ("you sure ask a lot of questions") and hang on did you just say the sky ("of course, where do you think snow comes from") when you realize that the Elves are already loading you into a trebuchet ("please hold still, we need to strap you in").

As they're making the final adjustments, they discover that their calibration document (your puzzle input) has been amended by a very young Elf who was apparently just excited to show off her art skills. Consequently, the Elves are having trouble reading the values on the document.

The newly-improved calibration document consists of lines of text; each line originally contained a specific calibration value that the Elves now need to recover. On each line, the calibration value can be found by combining the first digit and the last digit (in that order) to form a single two-digit number.

For example:

1abc2
pqr3stu8vwx
a1b2c3d4e5f
treb7uchet

In this example, the calibration values of these four lines are 12, 38, 15, and 77. Adding these together produces 142.

Consider your entire calibration document. What is the sum of all of the calibration values?
*/

import { sum, allIndicesOf } from "../../_utils";

const extractDigits = (s: string): string => s.replaceAll(/[a-zA-Z]/g, "");
const firstLast = (s: string): number => +`${s[0]}${s[s.length - 1]}`;

export function part1_solver(lines: string[]): number {
  return lines
    .map(extractDigits)
    .map(firstLast)
    .filter((x) => !isNaN(x))
    .reduce(sum, 0);
}

/*
--- Part Two ---

Your calculation isn't quite right. It looks like some of the digits are actually spelled out with letters: one, two, three, four, five, six, seven, eight, and nine also count as valid "digits".

Equipped with this new information, you now need to find the real first and last digit on each line. For example:

two1nine
eightwothree
abcone2threexyz
xtwone3four
4nineeightseven2
zoneight234
7pqrstsixteen

In this example, the calibration values are 29, 83, 13, 24, 42, 14, and 76. Adding these together produces 281.

What is the sum of all of the calibration values?
*/

const digits = [
  "one",
  "two",
  "three",
  "four",
  "five",
  "six",
  "seven",
  "eight",
  "nine",
];

type Match = {
  digitNum: number;
  startPos: number;
};

const wordsToDigits = (s: string): string => {
  let res = "";

  // Find the starting indices of all word numbers in the string
  const matches: Match[] = digits.reduce((indices, digitStr, i) => {
    const digitNum = i + 1;
    const matchedIndices = allIndicesOf(s, digitStr);

    return [
      ...indices,
      ...matchedIndices.map((startPos) => ({ digitNum, startPos })),
    ];
  }, [] as Match[]);

  // Sort the starting indices so we process the matches in order
  matches.sort((a, b) => a.startPos - b.startPos);

  // For each match...
  //    1. Append the characters between the last match to the current match, unmodified.
  //    2. Append the numerical value of the digit string to the result.
  //    3. Update the last match value for the next iteration.
  let lastmatch = 0;
  matches.forEach((match) => {
    res = res + s.slice(lastmatch, match.startPos);
    res = res + match.digitNum;
    lastmatch = match.startPos + digits[match.digitNum - 1].length;
  });

  // In case there are no more matches, but the string contains more random characters...
  //  append the rest of them to the result.
  if (lastmatch < s.length) {
    res = res + s.slice(lastmatch, s.length);
  }

  return res;
};

export function part2_solver(lines: string[]): number {
  return part1_solver(lines.map(wordsToDigits));
}