type Delimiter = ' ' | '.' | ','

type UppercaseAfterDelim<S extends string> = S extends `${infer T}${infer U}`
  ? T extends Delimiter
    // 先頭が区切り文字の場合
    ? U extends `${infer V}${infer W}`
      // 区切り文字の後ろに文字がある場合
      ? `${T}${Uppercase<V>}${UppercaseAfterDelim<W>}`
      // 区切り文字の後ろに文字がない場合
      : S
    // 先頭が区切り文字でない場合
    : `${T}${UppercaseAfterDelim<U>}`
  : ''

type CapitalizeWords<S extends string> = S extends `${infer T}${infer U}`
  ? `${Uppercase<T>}${UppercaseAfterDelim<U>}`
  : ''

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<''>, ''>>,
]