Day 3Gear Ratios

View puzzle description on Advent of Code

This one took a fair bit of thinking, but in the end I settled on what seemed like the most obvious (naive?) solutions.

The solutions to both parts are written in a very imperative style. Usually, I tend to write — or at least think — in an imperative style initially, then go back and refactor to a more functional style once I've got a working solution and a passing test suite to work against. I didn't do that here as I'm not sure of the best way to elegantly refactor these loops — especially the while loops — into a more declarative style. At a high level the code could still be considered functional as all of the functions are pure, but the loops are a bit of a mess and miss out on that declarative magic functional programming is known for.

One thing to note is that I had an off-by-one error in part 2 which I still don't particularly understand. The below line needed tweaking to get the correct answer:

// Part 1
const endColumn = clampColumn(column + numberLength + 1);

// Part 2
const endColumn = clampColumn(column + numberLength);

I'd have expected the + 1 to be required in both cases as the endColumn variable is only used to define the search area for the special character. The way I use the variable is different between parts (a slice vs a for loop), but I still don't fully understand why the change was needed. I'll have to revisit this at some point as, at time of writing, I don't have any more time to spend on it.

import { clamp, filter, map, nth, pipe, split, sum } from 'rambda';


type LocatedNumber = [Row: number, Column: number, Number: number];


const isNumber = (x: string): boolean => !isNaN(Number(x));


const locateNumbers = (lines: string[]): LocatedNumber[] => {
  let numbers: LocatedNumber[] = [];


  for (let row = 0; row < lines.length; row++) {
    const line = lines[row];


    for (let column = 0; column < line.length; column++) {
      let currentChar = line[column];
      let prevChar = column === 0 ? '.' : line[column - 1];
      let charIdx = column;
      let numberString = '';


      if (isNumber(prevChar)) {
        continue;
      }


      while (isNumber(currentChar) && charIdx < line.length) {
        numberString += currentChar;
        charIdx++;
        currentChar = line[charIdx];
      }


      if (numberString.length > 0) {
        const number = Number(numberString);


        numbers.push([row, column, number]);
      }
    }
  }


  return numbers;
};


const validateNumber = (lines: string[]) => {
  const rowsCount = lines.length;
  const columnsCount = lines[0].length;
  const clampRow = clamp(0, rowsCount - 1);
  const clampColumn = clamp(0, columnsCount - 1);


  return ([row, column, number]: LocatedNumber): boolean => {
    const numberLength = number.toString().length;
    const startRow = clampRow(row - 1);
    const endRow = clampRow(row + 1);
    const startColumn = clampColumn(column - 1);
    const endColumn = clampColumn(column + numberLength + 1);


    let currentRow = startRow;
    let allCharacters = '';


    while (currentRow <= endRow) {
      allCharacters += lines[currentRow].slice(startColumn, endColumn);
      currentRow++;
    }


    return /[^0-9\.]/g.test(allCharacters);
  };
};


const solution = (input: string): number => {
  const lines = split('\n')(input);
  const validator = validateNumber(lines);


  return pipe(
    split('\n'),
    locateNumbers,
    filter(validator),
    map(x => x[2]),
    sum,
  )(input);
};


export default solution;