no - 4260 AllCombinations

[rimo030]

• playground

type StringToUnion<S extends string> = S extends `${infer F}${infer R}`
  ? F | StringToUnion<R>
  : ''

type AllCombinations<S extends string, U = StringToUnion<S>, K = U> = U extends string
  ? `${U}` | `${U}${AllCombinations<'' ,Exclude<K, U>>}`
  : never

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

type cases = [
  Expect<Equal<AllCombinations<''>, ''>>,
  Expect<Equal<AllCombinations<'A'>, '' | 'A'>>,
  Expect<Equal<AllCombinations<'AB'>, '' | 'A' | 'B' | 'AB' | 'BA'>>,
  Expect<Equal<AllCombinations<'ABC'>, '' | 'A' | 'B' | 'C' | 'AB' | 'AC' | 'BA' | 'BC' | 'CA' | 'CB' | 'ABC' | 'ACB' | 'BAC' | 'BCA' | 'CAB' | 'CBA'>>,
  Expect<Equal<AllCombinations<'ABCD'>, '' | 'A' | 'B' | 'C' | 'D' | 'AB' | 'AC' | 'AD' | 'BA' | 'BC' | 'BD' | 'CA' | 'CB' | 'CD' | 'DA' | 'DB' | 'DC' | 'ABC' | 'ABD' | 'ACB' | 'ACD' | 'ADB' | 'ADC' | 'BAC' | 'BAD' | 'BCA' | 'BCD' | 'BDA' | 'BDC' | 'CAB' | 'CAD' | 'CBA' | 'CBD' | 'CDA' | 'CDB' | 'DAB' | 'DAC' | 'DBA' | 'DBC' | 'DCA' | 'DCB' | 'ABCD' | 'ABDC' | 'ACBD' | 'ACDB' | 'ADBC' | 'ADCB' | 'BACD' | 'BADC' | 'BCAD' | 'BCDA' | 'BDAC' | 'BDCA' | 'CABD' | 'CADB' | 'CBAD' | 'CBDA' | 'CDAB' | 'CDBA' | 'DABC' | 'DACB' | 'DBAC' | 'DBCA' | 'DCAB' | 'DCBA'>>,
]

[rimo030]

바로 이전에 작성한 #110 을 이용해 다음과 같이 풀었었다.

type StringToArray<S extends string> = S extends `${infer F}${infer R}`
  ? [F, ...StringToArray<R>]
  : []

type Combination<T extends string[], U = T[number], K = U> = K extends string
  ? `${K}` | `${K}${Combination<[], Exclude<U, K>>}`
  : never

type AllCombinations<S extends string> = '' | Combination<StringToArray<S>> 

불필요하게 배열로 바꾸는 작업을 빼고 바로 문자열을 유니온으로 바꾸어 작성하면 더 간단하다.

type StringToUnion<S extends string> = S extends `${infer F}${infer R}`
  ? F | StringToUnion<R>
  : ''

type AllCombinations<S extends string, U = StringToUnion<S>, K = U> = U extends string
  ? `${U}` | `${U}${AllCombinations<'' ,Exclude<K, U>>}`
  : never