My solution for Advent of Code 2023, Day 12

aoc_2023_day_12.ts

//// Helpers written before the day started:

export function memoize<Args extends unknown[], Result>(
  func: (...args: Args) => Result,
): (...args: Args) => Result {
  const stored = new Map<string, Result>();

  return (...args) => {
    const k = JSON.stringify(args);
    if (stored.has(k)) {
      return stored.get(k)!;
    }
    const result = func(...args);
    stored.set(k, result);
    return result;
  };
}
  
export function sum(...nums: number[] | (readonly number[])[]): number {
  let tot = 0;
  for (const x of nums) {
    if (typeof x === "number") {
      tot += x;
    } else {
      for (const y of x) {
        tot += y;
      }
    }
  }
  return tot;
}

export function toInt(x: string): number {
  return parseInt(x, 10);
}



const inputFilename = process.argv[2] || "in.txt";
console.info("inputFilename = ", inputFilename);

const inputContents = readFileSync(inputFilename, { encoding: "utf8" }).trim();

const lines = inputContents.split("\n");
  
//// Code written on the day of:

const countWays = memoize((line: string, runs: readonly number[]): number => {
  if (line.length === 0) {
    if (runs.length === 0) {
      return 1;
    }
    return 0;
  }
  if (runs.length === 0) {
    for (let i = 0; i < line.length; i++) {
      if (line[i] === "#") {
        return 0;
      }
    }
    return 1;
  }

  if (line.length < sum(runs) + runs.length - 1) {
    // The line is not long enough for all runs
    return 0;
  }

  if (line[0] === ".") {
    return countWays(line.slice(1), runs);
  }
  if (line[0] === "#") {
    const [run, ...leftoverRuns] = runs;
    for (let i = 0; i < run; i++) {
      if (line[i] === ".") {
        return 0;
      }
    }
    if (line[run] === "#") {
      return 0;
    }

    return countWays(line.slice(run + 1), leftoverRuns);
  }
  // Otherwise dunno first spot, pick
  return (
    countWays("#" + line.slice(1), runs) + countWays("." + line.slice(1), runs)
  );
});

let tot = 0;
for (const line of lines) {
  const [str, numsS] = line.split(" ");
  const nums = numsS.split(",").map(toInt);

  const strExpanded = [str, str, str, str, str].join("?");
  const numsExpanded = [...nums, ...nums, ...nums, ...nums, ...nums];

  tot += countWays(strExpanded, numsExpanded);
}

console.info({ tot });