AI solution for TypeScript puzzle on BFE.dev
37. implement SmallerThan<A, B>

type SmallerThan<A extends number, B extends number> = A extends B ? false :
  B extends A ? true :
  A extends 0 ? true :
  B extends 0 ? false :
  SmallerThan<Exclude<A, 0> extends never ? 0 : Exclude<A, 0> extends 1 ? 0 : Subtract<A, 1>, Exclude<B, 0> extends never ? 0 : Exclude<B, 0> extends 1 ? 0 : Subtract<B, 1>>;

type Subtract<A extends number, B extends number> = ({[key: string]: never; '-': A; '+': Subtract<Decrement<A>, Decrement<B>>})[A extends B ? '-' : '+'];

type Decrement<A> = A extends 0 ? 0 : A extends 1 ? 0 : Subtract<A, 1>;

The SmallerThan<A, B> type recursively decrements A and B until they reach 0, comparing them along the way. It uses conditional types and recursion to determine if A is smaller than B or not.

To accomplish this we need a Subtract type, which subtracts B from A. If B is greater than A, Subtract returns 0, otherwise it will keep subtracting 1 from both A and B until A equals B. We also need a Decrement type, which decrements A by 1.

Note that this implementation only works for positive integers. For negative numbers, you would need to add additional checks.