type IsAlpha<S extends string> =
  Uppercase<S> extends Lowercase<S>
  ? false // if uppercase and lowercase version is same, it is not an alphabet.
  : true
type CapitalizeWords<S extends string, InWord=false, Acc extends string=""> =
  S extends `${infer S1}${infer S2}`
  ? InWord extends true
    ? IsAlpha<S1> extends true
      ? CapitalizeWords<S2,true,`${Acc}${S1}`>
      : CapitalizeWords<S2,false,`${Acc}${S1}`>
    : IsAlpha<S1> extends true
      ? CapitalizeWords<S2,true,`${Acc}${Uppercase<S1>}`>
      : CapitalizeWords<S2,false,`${Acc}${S1}`>
  : Acc

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<''>, ''>>,
]