[Typescript] 97. Hard Capitalize Words

Implement CapitalizeWords<T> which converts the first letter of each word of a string to uppercase and leaves the rest as-is.

For example

type capitalized = CapitalizeWords<'hello world, my friends'> // expected to be 'Hello World, My Friends'

/* _____________ Your Code Here _____________ */
type StringToUnion<T extends string> = T extends `${infer F}${infer R}` ? F | StringToUnion<R> : never;
type LowerLetterUnion = StringToUnion<'abcdefghijklmnopqrstuvwxyz'>;
type UpperLetterUnion = Uppercase<LowerLetterUnion>;

type CapitalizeWords<S extends string, Prev extends string = '', ACC extends string = ''> = S extends `${infer First}${infer REST}`
  ? Uppercase<Prev> extends UpperLetterUnion                     // only symbols and space cannnot be uppercased
    ? CapitalizeWords<REST, First, `${ACC}${First}`>                // prev is char, the follow char not need to be uppercased
    : CapitalizeWords<REST, First, `${ACC}${Uppercase<First>}`>     // pre is symbol, current char should be uppercased  
  : ACC;

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<CapitalizeWords<'foobar'>, 'Foobar'>>,
  Expect<Equal<CapitalizeWords<'FOOBAR'>, 'FOOBAR'>>,
  Expect<Equal<CapitalizeWords<'foo bar'>, 'Foo Bar'>>,
  Expect<Equal<CapitalizeWords<'foo bar hello world'>, 'Foo Bar Hello World'>>,
  Expect<Equal<CapitalizeWords<'foo bar.hello,world'>, 'Foo Bar.Hello,World'>>,
  Expect<Equal<CapitalizeWords<'aa!bb@cc#dd$ee%ff^gg&hh*ii(jj)kk_ll+mm{nn}oo|ppqq'>, 
  'Aa!Bb@Cc#Dd$Ee%Ff^Gg&Hh*Ii(Jj)Kk_Ll+Mm{Nn}Oo|PpQq'>>,
  Expect<Equal<CapitalizeWords<''>, ''>>,
]