feat: 2025 Day 4

Author: mfulton26

2025/day/4/part/1/solve.test.ts

@@ -0,0 +1,19 @@
+import solve from "./solve.ts";
+
+import { assertEquals } from "@std/assert";
+
+Deno.test("example", () => {
+  const input = `\
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.`;
+
+  assertEquals(solve(input), 13);
+});

2025/day/4/part/1/solve.ts

@@ -0,0 +1,21 @@
+// deno-fmt-ignore
+const directions = [
+  [-1, -1], [ 0, -1], [ 1, -1],
+  [-1,  0],           [ 1,  0],
+  [-1,  1], [ 0,  1], [ 1,  1],
+];
+
+export default function solve(input: string) {
+  const grid = input.split("\n").map((line) => Array.from(line));
+  let count = 0;
+  for (let y = 0; y < grid.length; y++) {
+    for (let x = 0; x < grid[y].length; x++) {
+      if (grid[y][x] !== "@") continue;
+      const { length: adjacentCount } = directions.filter(([dx, dy]) =>
+        grid[y + dy]?.[x + dx] === "@"
+      );
+      if (adjacentCount < 4) count++;
+    }
+  }
+  return count;
+}

2025/day/4/part/2/solve.test.ts

@@ -0,0 +1,19 @@
+import solve from "./solve.ts";
+
+import { assertEquals } from "@std/assert";
+
+Deno.test("example", () => {
+  const input = `\
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.`;
+
+  assertEquals(solve(input), 43);
+});

2025/day/4/part/2/solve.ts

@@ -0,0 +1,27 @@
+// deno-fmt-ignore
+const directions = [
+  [-1, -1], [ 0, -1], [ 1, -1],
+  [-1,  0],           [ 1,  0],
+  [-1,  1], [ 0,  1], [ 1,  1],
+];
+
+export default function solve(input: string) {
+  const grid = input.split("\n").map((line) => Array.from(line));
+  let count = 0;
+  while (true) {
+    const toRemove: [number, number][] = [];
+    for (let y = 0; y < grid.length; y++) {
+      for (let x = 0; x < grid[y].length; x++) {
+        if (grid[y][x] !== "@") continue;
+        const { length: adjacentCount } = directions.filter(([dx, dy]) =>
+          grid[y + dy]?.[x + dx] === "@"
+        );
+        if (adjacentCount < 4) toRemove.push([x, y]);
+      }
+    }
+    if (toRemove.length === 0) break;
+    count += toRemove.length;
+    for (const [x, y] of toRemove) grid[y][x] = "x";
+  }
+  return count;
+}