Solution in Typescript for Day 15 of Advent of Code 2020

aoc2020_day15.ts

interface NumberIndexes {
  [key: number]: number;
}

const input: string = '0,13,1,16,6,17';

function withTimings<T>(f: () => T): T {
  const startTime = new Date().getTime();
  const result = f();
  const endTime = new Date().getTime();
  console.log(`Elapsed ${endTime - startTime}ms`);
  return result;
}

// Parser
function parseStartingNumbers(input: string): number[] {
  return input.split(',')
    .map(x => parseInt(x, 10));
}

// Solution
function elementsWithIndexes(numbers: number[], preallocatedIndicesSize: number): number[] {
  const indices = (new Array(preallocatedIndicesSize)) as Array<number>;
  return numbers
    .map((element, index) => [element, index])
     .reduce((acc, [element, index]) => {
       acc[element] = index;
       return acc;
     }, indices);
}

function nthPlayNumber(previousNumbers: Array<number>, n: number) {
  const desiredIndex = n - 1;
  let elementIndices = elementsWithIndexes(previousNumbers, n);
  let currentIndex = previousNumbers.length - 1;
  let currentNumber = previousNumbers[currentIndex];
  while (currentIndex < desiredIndex) {
    const currentNumberLastIndex = elementIndices[currentNumber];
    elementIndices[currentNumber] = currentIndex;
    currentNumber = (currentNumberLastIndex === undefined) ? 0 :
      currentIndex - currentNumberLastIndex;
    currentIndex++;
  }
  return currentNumber;
}

// Display numbers
const startingNumbers = parseStartingNumbers(input);

console.log("Part 1:")
console.log(
  withTimings(
    () =>
      nthPlayNumber(startingNumbers, 2020)));

console.log("Part 2:")
console.log(
  withTimings(
    () =>
      nthPlayNumber(startingNumbers, 30000000)));

//Part 1:
//Elapsed 0ms
//234
//Part 2:
//Elapsed 586ms
//8984