9286 - FirstUniqueCharIndex

[prenaissance]

Find the union of unique characters and then replicate the use of IndexOf with that

type UniqueCharacters<T extends string, Acc extends string = never, Unique extends string = never> =
  T extends `${infer Head}${infer Tail}`
  ? Head extends Acc
    ? UniqueCharacters<Tail, Acc, Exclude<Unique, Head>>
    : UniqueCharacters<Tail, Acc | Head, Unique | Head>
  : Unique

type FirstUniqueCharIndex<T extends string, Prev extends string[] = [], Unique = UniqueCharacters<T>> =
  T extends `${infer Head}${infer Tail}`
  ? Head extends Unique
    ? Prev["length"]
    : FirstUniqueCharIndex<Tail, [...Prev, Head], Unique>
  : -1