diff --git a/2021/13/part1.mjs b/2021/13/part1.mjs
new file mode 100644
index 0000000..67b52a6
--- /dev/null
+++ b/2021/13/part1.mjs
@@ -0,0 +1,69 @@
+import fs from "fs-extra";
+import fp from "lodash/fp.js";
+import _ from "lodash";
+
+export const processInput = (input) =>
+  fs.readFile(input, "utf8").then((content) => {
+    let [coords, instructions] = content.split("\n\n");
+
+    let grid = [];
+
+    for (const [y, x] of coords
+      .split("\n")
+      .map((line) => line.split(",").map((x) => parseInt(x)))) {
+      if (!grid[x]) grid[x] = [];
+      grid[x][y] = true;
+    }
+
+    grid = [...grid].map((x) => (x ? [...x] : []));
+
+    instructions = instructions
+      .split("\n")
+      .filter((x) => x)
+      .map((line) => line.match(/([xy])=(\d+)/))
+      .map(([_, direction, level]) => [direction, parseInt(level)]);
+
+    return {
+      grid,
+      instructions,
+    };
+  });
+
+export function foldGrid(grid, direction, level) {
+  if (direction === "x") {
+    for (const row of grid) {
+      if (!row) continue;
+      const folded = row.splice(level);
+      folded.forEach((v, i) => {
+        if (i === 0) return;
+        if (!v) return;
+        row[level - i] = v;
+      });
+    }
+
+    return grid;
+  }
+
+  const folded = grid.splice(level);
+  folded.forEach((v, i) => {
+    if (i === 0) return;
+    if (!v) return;
+    grid[level - i] = _.range(
+      _.max([grid[level - i].length, folded[i].length])
+    ).map((j) => grid[level - i][j] || folded[i][j]);
+  });
+
+  return grid;
+}
+
+export function printGrid(grid) {
+  return grid
+    .map((line) => (line || []).map((x) => (x ? "#" : ".")).join(""))
+    .join("\n");
+}
+
+export function solution({ grid, instructions }) {
+  foldGrid(grid, ...instructions.shift());
+
+  return _.sum(grid.flat());
+}
diff --git a/2021/13/part2.mjs b/2021/13/part2.mjs
new file mode 100644
index 0000000..05ab8b6
--- /dev/null
+++ b/2021/13/part2.mjs
@@ -0,0 +1,11 @@
+import fs from "fs-extra";
+import fp from "lodash/fp.js";
+import _ from "lodash";
+
+import * as p1 from "./part1.mjs";
+
+export function solution({ grid, instructions }) {
+  instructions.forEach((i) => p1.foldGrid(grid, ...i));
+
+  return p1.printGrid(grid);
+}
diff --git a/2021/13/sample b/2021/13/sample
new file mode 100644
index 0000000..282114c
--- /dev/null
+++ b/2021/13/sample
@@ -0,0 +1,21 @@
+6,10
+0,14
+9,10
+0,3
+10,4
+4,11
+6,0
+6,12
+4,1
+0,13
+10,12
+3,4
+3,0
+8,4
+1,10
+2,14
+8,10
+9,0
+
+fold along y=7
+fold along x=5
diff --git a/2021/13/test.mjs b/2021/13/test.mjs
new file mode 100644
index 0000000..710bb98
--- /dev/null
+++ b/2021/13/test.mjs
@@ -0,0 +1,30 @@
+// https://adventofcode.com/2021/day/13
+
+import tap from "tap";
+import fs from "fs-extra";
+
+import * as p1 from "./part1.mjs";
+import * as p2 from "./part2.mjs";
+
+const sample = p1.processInput("sample");
+const input = p1.processInput("input");
+
+tap.test("part1", async (t) => {
+  const x = await sample;
+  t.equal(p1.solution(x), 17);
+  t.equal(p1.solution(x), 16);
+  t.equal(p1.solution(await input), 745);
+});
+
+tap.test("part2", async (t) => {
+  // t.equal(p2.solution(await sample), 0);
+  t.equal(
+    p2.solution(await input),
+    `.##..###..#..#...##.####.###...##...##..
+#..#.#..#.#.#.....#.#....#..#.#..#.#..#.
+#..#.###..##......#.###..###..#....#....
+####.#..#.#.#.....#.#....#..#.#.##.#....
+#..#.#..#.#.#..#..#.#....#..#.#..#.#..#.
+#..#.###..#..#..##..#....###...###..##..`
+  );
+});