diff --git a/2021/1.ts b/2021/1.ts
index 277ddfd..b10ba7a 100644
--- a/2021/1.ts
+++ b/2021/1.ts
@@ -1,53 +1,52 @@
-import { readLines } from "./deps.ts";
+import { inputNumbers } from "./common.ts";
 
-// Day 1
-/*
- * This solution is relatively simple. Keep track of the last depth we measured
- * in `lastDepth` (skipping the first line of input by defaulting it to
- * `Number.MAX_VALUE`) and compare it to each line of input as we read it. Once
- * we've compared, set the `lastDepth` to the current depth for the next
+/** This solution is relatively simple. Keep track of the last depth we
+ * measured in `lastDepth` (skipping the first line of input by defaulting it
+ * to `Number.MAX_VALUE`) and compare it to each line of input as we read it.
+ * Once we've compared, set the `lastDepth` to the current depth for the next
  * iteration.
  *
  * This solution should be O(n) for compute and O(n) for memory since we
  * iterate the input once and only read a line at a time, we actually have O(1)
  * memory, but since the input is on disk, that's kind of unfair to say.
  */
-let part1Increases = 0;
-let lastDepth = Number.MAX_VALUE;
-for await (const depthString of readLines(await Deno.open("1.input"))) {
-  const depth = parseInt(depthString);
-  if (depth > lastDepth) part1Increases++;
-  lastDepth = depth;
+export async function part1(
+  input: AsyncIterableIterator<number>,
+): Promise<number> {
+  let increases = 0;
+  let lastDepth = Number.MAX_VALUE;
+  for await (const depth of input) {
+    if (depth > lastDepth) increases++;
+    lastDepth = depth;
+  }
+  return increases;
 }
-console.log("Number of depth increases (Part 1):", part1Increases);
+console.log("Part 1", await part1(await inputNumbers("1")));
 
-// Day 2
-/*
- * Since the windows we need to compare overlap, we really only need to compare
- * the values unique to each window, which will be the first of the first window
- * and the last of the last window. Since the windows have size 3, we really
- * only need to compare a value with the "three-lines-ago" value. This solution
- * otherwise is identical to the previous one.
+/** Since the windows we need to compare overlap, we really only need to
+ * compare the values unique to each window, which will be the first of the
+ * first window and the last of the last window. Since the windows have size 3,
+ * we really only need to compare a value with the "three-lines-ago" value.
+ * This solution otherwise is identical to the previous one.
  */
-let part2Increases = 0;
-const WINDOW_SIZE = 3;
-const input = readLines(await Deno.open("1.input"));
+export async function part2(
+  input: AsyncIterableIterator<number>,
+  windowSize = 3,
+): Promise<number> {
+  let increases = 0;
+  // TODO: use a proper FIFO queue data structure for this?
+  let window: number[] = await Promise.all(Array.from(
+    Array(windowSize),
+    async (_x, _i) => parseInt((await input.next()).value),
+  ));
 
-// TODO: use a proper FIFO queue data structure for this?
-// read the first `WINDOW_SIZE` values from our input in preparation
-let window: number[] = await Promise.all(Array.from(
-  Array(WINDOW_SIZE),
-  async (_x, _i) => parseInt((await input.next()).value),
-));
-
-for await (const depthString of input) {
-  const depth = parseInt(depthString);
-  // compare the current depth to the depth we read `WINDOW_SIZE` ago
-  if (depth > window[0]) part2Increases++;
-  // now we push the current depth onto the window and cut the one we just compared off
-  // TODO: if this was a proper queue, we could just push it on the queue and
-  // expect the first-in value to fall out
-  window = window.slice(1);
-  window.push(depth);
+  for await (const depth of input) {
+    if (depth > window[0]) increases++;
+    // TODO: if this was a proper queue, we could just push it on the queue and
+    // expect the first-in value to fall out
+    window = window.slice(1);
+    window.push(depth);
+  }
+  return increases;
 }
-console.log("Number of summed depth increases (Part 2):", part2Increases);
+console.log("Part 2", await part2(await inputNumbers("1")));
diff --git a/2021/common.ts b/2021/common.ts
new file mode 100644
index 0000000..c663726
--- /dev/null
+++ b/2021/common.ts
@@ -0,0 +1,72 @@
+import { ensureDir, path, readAll, readLines } from "./deps.ts";
+
+export const HOME = Deno.env.get("HOME") || "/tmp";
+export const XDG_CACHE_HOME = Deno.env.get("XDG_CACHE_HOME") ||
+  path.join(HOME, ".config");
+
+const INPUT_CACHE_DIR = path.join(XDG_CACHE_HOME, "aoc2021");
+
+async function fileExists(filePath: string) {
+  try {
+    await Deno.lstat(filePath);
+    return true;
+  } catch (e) {
+    if (e instanceof Deno.errors.NotFound) {
+      return false;
+    }
+    throw e;
+  }
+}
+
+function inputFilePath(day: string) {
+  return path.join(INPUT_CACHE_DIR, `${day}.input`);
+}
+
+async function ensureInput(day: string) {
+  const filePath = inputFilePath(day);
+  await ensureDir(INPUT_CACHE_DIR);
+  if (await fileExists(filePath)) return;
+  const cookieFile = path.join(HOME, ".advent-of-code-2021-auth-cookie");
+  let cookie = "";
+  try {
+    cookie = Deno.env.get("ADVENT_OF_CODE_2021_AUTH_COOKIE") ||
+      new TextDecoder().decode(
+        await readAll(await Deno.open(cookieFile)),
+      );
+  } catch (e) {
+    console.error(
+      `Failed to load adventofcode.com cookie from ADVENT_OF_CODE_2021_AUTH_COOKIE environment variable or from file ${cookieFile}`,
+    );
+    throw e;
+  }
+  const url = `https://adventofcode.com/2021/day/${day}/input`;
+  const response = await fetch(url, { headers: { cookie } });
+  await Deno.writeTextFile(filePath, await response.text());
+}
+
+async function fileLines(
+  filePath: string,
+): Promise<AsyncIterableIterator<string>> {
+  return readLines(await Deno.open(filePath));
+}
+
+export async function inputLines(
+  day: string,
+): Promise<AsyncIterableIterator<string>> {
+  await ensureInput(day);
+  return fileLines(inputFilePath(day));
+}
+
+async function* readStringsAsNumbers(
+  input: AsyncIterableIterator<string>,
+): AsyncIterableIterator<number> {
+  for await (const s of input) {
+    yield parseInt(s);
+  }
+}
+
+export async function inputNumbers(
+  day: string,
+): Promise<AsyncIterableIterator<number>> {
+  return readStringsAsNumbers(await inputLines(day));
+}
diff --git a/2021/deps.ts b/2021/deps.ts
index 64ea606..655af59 100644
--- a/2021/deps.ts
+++ b/2021/deps.ts
@@ -1 +1,4 @@
 export { readLines } from "https://deno.land/std@0.116.0/io/mod.ts";
+export { ensureDir } from "https://deno.land/std@0.110.0/fs/mod.ts";
+export * as path from "https://deno.land/std@0.116.0/path/mod.ts";
+export { readAll } from "https://deno.land/std@0.116.0/streams/conversion.ts";