Day 9

9/solution.mjs

@@ -0,0 +1,83 @@
+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}`
+);
+
+