import { readFileSync } from 'node:fs';

const input = readFileSync('input', 'utf-8');

const moves = input
  .split('\n')
  .filter(line => line)
  .map(line => {
    const [direction, multiplier] = line.split(' ', 2);
    return new Array(parseInt(multiplier)).fill(direction);
  })
  .flat();


function ropeSimulation(nTailKnots) {
  const positions = [new Array(nTailKnots + 1).fill().map(_ => [0, 0])];

  for (const move of moves) {
    const lastKnots = positions[positions.length - 1];
    let [x, y] = lastKnots[0];

    switch (move) {
      case 'U': y += 1; break;
      case 'D': y -= 1; break;
      case 'L': x -= 1; break;
      case 'R': x += 1; break;
      default: throw new Error('Invalid move');
    }

    const knots = [[x, y]];

    for (let n = 0; n < nTailKnots; n++) {
      ([x, y] = knots[knots.length - 1]);
      let [tx, ty] = lastKnots[n + 1];

      if (y === ty && Math.abs(x - tx) === 2) {
        // Horizontal tavel
        tx += (x - tx) / Math.abs(x - tx);
      } else if (x === tx && Math.abs(y - ty) === 2) {
        // Vertical travel
        ty += (y - ty) / Math.abs(y - ty);
      } else if (Math.abs(x - tx) === 2 || Math.abs(y - ty) === 2) {
        // Diagonal travel
        tx += (x - tx) / Math.abs(x - tx);
        ty += (y - ty) / Math.abs(y - ty);
      }

      knots.push([tx, ty]);
    }

    positions.push(knots);
  }

  return positions;
}

function countLastKnotPositions(positions) {
  // Count unique positions of tail
  return positions
    .map(position => position[position.length - 1])
    .filter((tailPosition, i, arr) =>
      arr.findLastIndex(tp =>
        tp[0] === tailPosition[0] &&
        tp[1] === tailPosition[1]
      ) === i
    )
    .length;
}

let positions = ropeSimulation(1);
const totalPositionsOneKnot = countLastKnotPositions(positions);
console.log(
  `Total unique positions of tail with 1 knot: ${totalPositionsOneKnot}`
);


positions = ropeSimulation(9);
const totalPositionsTenKnots = countLastKnotPositions(positions);
console.log(
  `Total unique positions of tail with 10 knots: ${totalPositionsTenKnots}`
);