Solve 2025-01

2025/01/index.ts

@@ -0,0 +1,45 @@
+import { mod } from '../../utils/numbers.ts';
+
+const size = 100;
+const initialValue = 50;
+
+function parseInput(input: string) {
+  const regex = /^([LR])(\d*)$/gm;
+  const match = input.matchAll(regex);
+
+  return Array.from(match, (m) => Number(m[2]) * (m[1] === 'L' ? -1 : 1));
+}
+
+export function solveFirst(input: string): number {
+  let zeros = 0;
+  parseInput(input).reduce((d, r) => {
+    const newDial = (d + r) % size;
+    if (newDial === 0) {
+      zeros++;
+    }
+    return newDial;
+  }, initialValue);
+
+  return zeros;
+}
+export function solveSecond(input: string): number {
+  const rots = parseInput(input);
+
+  let zeros = 0;
+  let dial = initialValue;
+
+  // brute force is easier than thinking
+  for (const rot of rots) {
+    if (rot === 0) {
+      continue;
+    }
+    for (let i = 0; i < Math.abs(rot); i++) {
+      dial = mod(dial + (rot > 0 ? 1 : -1), size);
+      if (dial === 0) {
+        zeros++;
+      }
+    }
+  }
+
+  return zeros;
+}