feat: advent of code day one

src/advent-of-code/2025/day1/main.spec.ts

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+/**
+ * @copyright NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import { readFile } from "node:fs/promises";
+import { join } from "node:path";
+import { describe, it, expect } from "vitest";
+import { partOne, partTwo } from "./main.js";
+
+const inputOne = `L68
+L30
+R48
+L5
+R60
+L55
+L1
+L99
+R14
+L82
+R0`;
+
+describe("partOne", () => {
+  it("should return the correct result", async() => {
+    expect(partOne(inputOne)).toBe(3);
+    const bigInput = await readFile(
+      join(import.meta.dirname, "inputOne.txt"),
+      "utf8",
+    );
+    expect(partOne(bigInput)).toBe(1086);
+  });
+});
+
+describe("partTwo", () => {
+  it("should return the correct result", async() => {
+    expect(partTwo(inputOne)).toBe(6);
+    const bigInput = await readFile(
+      join(import.meta.dirname, "inputOne.txt"),
+      "utf8",
+    );
+    expect(partTwo(bigInput)).toBe(6268);
+  });
+});

src/advent-of-code/2025/day1/main.ts

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+/**
+ * @copyright NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+const normalizePosition = (value: number): number => {
+  const modulus = value % 100;
+  return (modulus + 100) % 100;
+};
+const countZeroHits = (start: number, end: number): number => {
+  if (end === start) {
+    return 0;
+  }
+  if (end > start) {
+    return Math.floor(end / 100) - Math.floor(start / 100);
+  }
+  return Math.floor((start - 1) / 100) - Math.floor((end - 1) / 100);
+};
+
+/**
+ * Part one of the Advent of Code 2025 day 1 challenge.
+ * @param input - The input string.
+ * @returns The number of times the position is 0.
+ * @see https://adventofcode.com/2025/day/1
+ */
+const partOne = (input: string): number => {
+  const instructions = input.split("\n").map((line) => {
+    return [ line[0], line.slice(1) ];
+  });
+  let position = 50;
+  let count = 0;
+  for (const instruction of instructions) {
+    const [ direction, distance ] = instruction;
+    if (direction === "L") {
+      position = position - Number(distance);
+    } else {
+      position = position + Number(distance);
+    }
+    position = normalizePosition(position);
+    if (position === 0) {
+      count = count + 1;
+    }
+  }
+  return count;
+};
+
+/**
+ * Part two of the Advent of Code 2025 day 1 challenge.
+ * @param input - The input string.
+ * @returns The number of times the position passed through 0.
+ * @see https://adventofcode.com/2025/day/1
+ */
+const partTwo = (input: string): number => {
+  const instructions = input.split("\n").map((line) => {
+    return [ line[0], line.slice(1) ];
+  });
+  let position = 50;
+  let count = 0;
+  for (const instruction of instructions) {
+    const [ direction, distance ] = instruction;
+    const delta = direction === "L"
+      ? -Number(distance)
+      : Number(distance);
+    const endPosition = position + delta;
+    count = count + countZeroHits(position, endPosition);
+    position = normalizePosition(endPosition);
+  }
+  return count;
+};
+
+export { partOne, partTwo };