Advent of Typescript Day 13

playground.ts

// There's only so many ways you can do an accumulator in ts
type Accumulator<N extends number, Acc extends ReadonlyArray<unknown> = []> = Acc extends {
	length: N;
}
	? Acc
	: Accumulator<N, [...Acc, unknown]>;

// Make a Union of the indexes excluding 0
type IndexUnion<A extends ReadonlyArray<unknown>> = Exclude<
	{
		[K in keyof A]: K extends `${infer N extends number}` ? N : never;
	}[number],
	0
>;

// Can be used to together to hack together the range needed
type DayCounter<S extends number, E extends number> =
	| Exclude<IndexUnion<Accumulator<E>>, IndexUnion<Accumulator<S>>>
	| E;

type x = IndexUnion<Accumulator<5>>;
//   ^?

type y = IndexUnion<Accumulator<10>>;
//   ^?

type days = DayCounter<5, 10>;
//   ^?