diff --git a/2021/3.test.ts b/2021/3.test.ts
new file mode 100644
index 0000000..cb8aa69
--- /dev/null
+++ b/2021/3.test.ts
@@ -0,0 +1,23 @@
+import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";
+import { part1, part2 } from "./3.ts";
+
+const testInput = `00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010`.trim().split("\n").map((s) => s.trim());
+
+Deno.test("day 3 part 1", async () => {
+  assertEquals(await part1(testInput), 198);
+});
+
+Deno.test("day 3 part 2", async () => {
+  assertEquals(await part2(testInput), 230);
+});
diff --git a/2021/3.ts b/2021/3.ts
new file mode 100644
index 0000000..2e7551c
--- /dev/null
+++ b/2021/3.ts
@@ -0,0 +1,32 @@
+import { collectArray, inputLines, measureDuration } from "./common.ts";
+const input = await collectArray(await inputLines("3"));
+
+type SubmarineCommand = ["forward", number] | ["up", number] | ["down", number];
+
+const ASCII_ZERO = "0".charCodeAt(0);
+export function part1(input: string[]): number {
+  const width = input[0].length;
+  const counter = Array.from(Array(width), (_) => [0, 0]);
+  for (const l of input) {
+    for (let i = 0; i < width; i++) {
+      const val = l.charCodeAt(i) - ASCII_ZERO;
+      counter[i][val]++;
+    }
+  }
+  let ones = 0;
+  for (let i = 0; i < width; i++) {
+    if (counter[i][1] >= counter[i][0]) ones++;
+    ones = ones << 1;
+  }
+  ones = ones >> 1;
+  const zeroes = ones ^ (Math.pow(2, width) - 1); // xor with a mask the same size as the width
+  return ones * zeroes;
+}
+
+await measureDuration(() => console.log("Part 1", part1(input)));
+
+export function part2(input: string[]): number {
+  return 0;
+}
+
+await measureDuration(() => console.log("Part 2", part2(input)));