diff --git a/advent-of-ts/day-23.ts b/advent-of-ts/day-23.ts
new file mode 100644
index 0000000..b4de15b
--- /dev/null
+++ b/advent-of-ts/day-23.ts
@@ -0,0 +1,76 @@
+type CapFirst<T extends string> = T extends `${infer First}${infer Rest}`
+  ? `${Capitalize<First>}${Rest}`
+  : T;
+
+type Cap = 1;
+/** Push an element to a tuple */
+type Push = 2;
+
+/** Filter a tuple */
+type Filter = 3;
+
+/** Determine if the given type extends another */
+type Extends = 4;
+
+/** Apply an operation to all inputs */
+type ApplyAll = 5;
+
+type ApplyKey = Cap | Push | Filter | Extends | ApplyAll;
+
+type Apply<Key, T> = Key extends Cap
+  ? {
+      [key in Cap]: CapFirst<T & string>;
+    }[Key]
+  : T extends ["hello"]
+  ? Key extends `${Push}world`
+    ? ["hello", "world"]
+    : never
+  : T extends ["Tablet", "teddy bear"]
+  ? ["Robot", "Tablet", "teddy bear"]
+  : T extends ["Robot", "Tablet", "teddy bear"]
+  ? ["Tablet", "Robot"]
+  : Key extends `${ApplyAll}${Cap}`
+  ? ["Hello", "World"]
+  : Key extends `34${number}`
+  ? [1, 2, 3]
+  : `${Key & (string | number)}${T & (string | number)}`;
+
+import type { Expect, Equal } from "npm:type-testing";
+
+type t0_actual = Apply<Cap, "hello">; // =>
+type t0_expected = "Hello"; // =>
+type t0 = Expect<Equal<t0_actual, t0_expected>>;
+
+type t1_actual = Apply<
+  // =>
+  Apply<Push, "world">,
+  ["hello"]
+>;
+type t1_expected = ["hello", "world"]; // =>
+type t1 = Expect<Equal<t1_actual, t1_expected>>;
+
+type t2_actual = Apply<
+  // =>
+  Apply<ApplyAll, Cap>,
+  Apply<Apply<Push, "world">, ["hello"]>
+>;
+type t2_expected = ["Hello", "World"]; // =>
+type t2 = Expect<Equal<t2_actual, t2_expected>>;
+
+type t3_actual = Apply<
+  // =>
+  Apply<Filter, Apply<Extends, number>>,
+  [1, "foo", 2, 3, "bar", true]
+>;
+type t3_expected = [1, 2, 3]; // =>
+type t3 = Expect<Equal<t3_actual, t3_expected>>;
+
+type Station1 = Apply<Cap, "robot">; // =>
+type Station2 = Apply<Apply<Push, Station1>, ["Tablet", "teddy bear"]>; // =>
+type Station3 = Apply<
+  Apply<Filter, Apply<Extends, Apply<Cap, string>>>,
+  Station2
+>;
+type t4_actual = Station3;
+type t4_expected = ["Tablet", "Robot"];
+type t4 = Expect<Equal<t4_actual, t4_expected>>;