import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<Join<['a', 'p', 'p', 'l', 'e'], '-'>, 'a-p-p-l-e'>>,
  Expect<Equal<Join<['Hello', 'World'], ' '>, 'Hello World'>>,
  Expect<Equal<Join<['2', '2', '2'], 1>, '21212'>>,
  Expect<Equal<Join<['o'], 'u'>, 'o'>>,
];

type Stringable = (string | number | bigint | boolean | null | undefined);

type JoinInternal<
  T extends Stringable[],
  U extends Stringable,
  Acc extends string
> = T extends [infer First, infer Second, ...infer Tail]
  ? First extends Stringable
    ? Second extends Stringable
      ? Tail extends Stringable[]
        ? JoinInternal<[Second, ...Tail], U, `${Acc}${First}${U}`>
        : never
      : never
    : never
  : T extends [infer Head, ...infer Tail]
    ? Head extends Stringable
      ? `${Acc}${Head}`
      : never
    : Acc
;


type Join<
  T extends Stringable[],
  U extends Stringable,
> = JoinInternal<T, U, ''>