diff --git a/days/12/main.ts b/days/12/main.ts
new file mode 100644
index 0000000..472a072
--- /dev/null
+++ b/days/12/main.ts
@@ -0,0 +1,160 @@
+import { Grid } from "@pal/datastructures";
+import { loadInput } from "utils";
+
+export function part1(input: Array<string>) {
+  let total = 0;
+  for (const region of extractRegions(input)) {
+    total += region.size * calculatePerimeter(region);
+  }
+  return total;
+}
+
+export function part2(input: Array<string>) {
+  let total = 0;
+  for (const region of extractRegions(input)) {
+    total += region.size * calculateNumberOfSides(region);
+  }
+  return total;
+}
+
+type Coord = { x: number; y: number };
+type CoordId = string;
+type Region = Set<CoordId>;
+
+function* extractRegions(input: Array<string>): Generator<Region> {
+  const grid = Grid.fromStrings(input);
+  const regionsByType = new Map<string, Set<Region>>();
+  function addToRegion(
+    x: number,
+    y: number,
+    dx: number,
+    dy: number,
+    type: string,
+    regionsOfType: Set<Region>,
+  ): Region | null {
+    const neighbourType = grid.at(x + dx, y + dy);
+    if (neighbourType === type) {
+      for (const region of regionsOfType) {
+        if (region.has(coordId({ x: x + dx, y: y + dy }))) {
+          region.add(coordId({ x, y }));
+          return region;
+        }
+      }
+    }
+    return null;
+  }
+  for (let y = 0; y < grid.height; y++) {
+    for (let x = 0; x < grid.width; x++) {
+      const type = grid.at(x, y)!;
+      let regionsOfType = regionsByType.get(type);
+      if (regionsOfType == null) {
+        regionsOfType = new Set<Region>();
+        regionsByType.set(type, regionsOfType);
+      }
+      const leftRegion = addToRegion(x, y, -1, 0, type, regionsOfType);
+      const rightRegion = addToRegion(x, y, 0, -1, type, regionsOfType);
+      if (
+        leftRegion != null && rightRegion != null && leftRegion !== rightRegion
+      ) {
+        for (const coord of rightRegion) {
+          leftRegion.add(coord);
+        }
+        regionsOfType.delete(rightRegion);
+      }
+      if (leftRegion == null && rightRegion == null) {
+        regionsOfType.add(new Set([coordId({ x, y })]));
+      }
+    }
+  }
+  for (const regions of regionsByType.values()) {
+    yield* regions;
+  }
+}
+
+function calculatePerimeter(region: Region): number {
+  let perimeter = 0;
+  for (const c of region) {
+    const { x, y } = fromCoordId(c);
+    if (region.has(coordId({ x: x - 1, y })) === false) {
+      perimeter++;
+    }
+    if (region.has(coordId({ x: x + 1, y })) === false) {
+      perimeter++;
+    }
+    if (region.has(coordId({ x, y: y - 1 })) === false) {
+      perimeter++;
+    }
+    if (region.has(coordId({ x, y: y + 1 })) === false) {
+      perimeter++;
+    }
+  }
+  return perimeter;
+}
+
+function calculateNumberOfSides(region: Region): number {
+  const verticalLeftSides = new Map<number, Array<number>>();
+  const verticalRightSides = new Map<number, Array<number>>();
+  const horizontalTopSides = new Map<number, Array<number>>();
+  const horizontalBottomSides = new Map<number, Array<number>>();
+  for (const c of region) {
+    const { x, y } = fromCoordId(c);
+    if (region.has(coordId({ x: x - 1, y })) === false) {
+      computeIfAbsent(verticalLeftSides, x, () => []).push(y);
+    }
+    if (region.has(coordId({ x: x + 1, y })) === false) {
+      computeIfAbsent(verticalRightSides, x, () => []).push(y);
+    }
+    if (region.has(coordId({ x, y: y - 1 })) === false) {
+      computeIfAbsent(horizontalTopSides, y, () => []).push(x);
+    }
+    if (region.has(coordId({ x, y: y + 1 })) === false) {
+      computeIfAbsent(horizontalBottomSides, y, () => []).push(x);
+    }
+  }
+  return [
+    ...verticalLeftSides.values(),
+    ...verticalRightSides.values(),
+    ...horizontalTopSides.values(),
+    ...horizontalBottomSides.values(),
+  ].map(getNumberOfContinousBlocks).reduce((a, b) => a + b, 0);
+}
+
+function coordId(coord: Coord): CoordId {
+  return `${coord.x},${coord.y}`;
+}
+
+function fromCoordId(coordId: CoordId): Coord {
+  const [x, y] = coordId.split(",").map(Number);
+  return { x, y };
+}
+
+function computeIfAbsent<K, V>(
+  map: Map<K, V>,
+  key: K,
+  defaultValue: () => V,
+): V {
+  let value = map.get(key);
+  if (value == null) {
+    value = defaultValue();
+    map.set(key, value);
+  }
+  return value;
+}
+
+function getNumberOfContinousBlocks(numbers: Array<number>) {
+  const sorted = numbers.toSorted();
+  let count = 1;
+  let last = sorted[0];
+  for (let i = 1; i < sorted.length; i++) {
+    if (sorted[i] !== last + 1) {
+      count++;
+    }
+    last = sorted[i];
+  }
+  return count;
+}
+
+if (import.meta.main) {
+  console.log(part1(await loadInput(12)));
+  console.log(part2(await loadInput(12)));
+}
diff --git a/days/12/main_test.ts b/days/12/main_test.ts
new file mode 100644
index 0000000..d13f334
--- /dev/null
+++ b/days/12/main_test.ts
@@ -0,0 +1,67 @@
+import { expect } from "@std/expect";
+import { describe, it } from "@std/testing/bdd";
+import { part1, part2 } from "./main.ts";
+import { loadInput } from "utils";
+
+describe("day 12 example", () => {
+  const input = [
+    "RRRRIICCFF",
+    "RRRRIICCCF",
+    "VVRRRCCFFF",
+    "VVRCCCJFFF",
+    "VVVVCJJCFE",
+    "VVIVCCJJEE",
+    "VVIIICJJEE",
+    "MIIIIIJJEE",
+    "MIIISIJEEE",
+    "MMMISSJEEE",
+  ];
+
+  it("part 1", () => {
+    expect(part1(input)).toBe(1930);
+  });
+
+  it("part 2", () => {
+    expect(part2(input)).toBe(1206);
+  });
+
+  it("part 2 small example 1", () => {
+    expect(part2([
+      "AAAAAA",
+      "AAABBA",
+      "AAABBA",
+      "ABBAAA",
+      "ABBAAA",
+      "AAAAAA",
+    ])).toBe(368);
+  });
+
+  it("part 2 small example 2", () => {
+    expect(part2([
+      "EEEEE",
+      "EXXXX",
+      "EEEEE",
+      "EXXXX",
+      "EEEEE",
+    ])).toBe(236);
+  });
+
+  it("part 2 small example 3", () => {
+    expect(part2([
+      "AAAA",
+      "BBCD",
+      "BBCC",
+      "EEEC",
+    ])).toBe(80);
+  });
+});
+
+describe("day 12 solution", () => {
+  it("part 1", async () => {
+    expect(part1(await loadInput(12))).toBe(1467094);
+  });
+
+  it("part 2", async () => {
+    expect(part2(await loadInput(12))).toBe(881182);
+  });
+});
diff --git a/days/mod.ts b/days/mod.ts
index 620d2e1..b531c80 100644
--- a/days/mod.ts
+++ b/days/mod.ts
@@ -3,7 +3,7 @@ type Solution = { part1: PartFunction; part2: PartFunction };
 
 const days = new Map<number, Solution>();
 
-for (const day of [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]) {
+for (const day of [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]) {
   days.set(day, await import(`./${day}/main.ts`));
 }
 
diff --git a/inputs b/inputs
index 94fe764..d325ce4 160000
--- a/inputs
+++ b/inputs
@@ -1 +1 @@
-Subproject commit 94fe7640ff0c710170a620f5596acd3738cf303d
+Subproject commit d325ce4a9f73afd0b033d07cc1f2fde2ccef52e9