02257-medium-minusone.ts

// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'

type cases = [
  Expect<Equal<MinusOne<1>, 0>>,
  Expect<Equal<MinusOne<55>, 54>>,
  Expect<Equal<MinusOne<3>, 2>>,
  Expect<Equal<MinusOne<100>, 99>>,
  Expect<Equal<MinusOne<1101>, 1100>>,
  Expect<Equal<MinusOne<0>, -1>>,
  Expect<Equal<MinusOne<9_007_199_254_740_992>, 9_007_199_254_740_991>>,
]


// ============= Your Code Here =============
type ParseInt<T extends string> = T extends `${infer Digit extends number}` ? Digit : never
type TestParseInt = ParseInt<'123'> // 123

type ReverseString<S extends string> = S extends `${infer First}${infer Rest}` ? `${ReverseString<Rest>}${First}` : ''
type TestReverseString = ReverseString<'123'> // 321

type RemoveLeadingZeros<S extends string> = S extends '0' ? S : S extends `${'0'}${infer R}` ? RemoveLeadingZeros<R> : S
type TestRemoveLeadingZeros = RemoveLeadingZeros<'000123'> // 123

type InternalMinusOne<
  S extends string
> = S extends `${infer Digit extends number}${infer Rest}` ?
    Digit extends 0 ?
      `9${InternalMinusOne<Rest>}` :
    `${[9, 0, 1, 2, 3, 4, 5, 6, 7, 8][Digit]}${Rest}`:
  never
type TestInternalMinusOne = InternalMinusOne<'123'> // "023" 
  
type MinusOne<T extends number> = T extends 0 ? -1 :  ParseInt<RemoveLeadingZeros<ReverseString<InternalMinusOne<ReverseString<`${T}`>>>>>
type TestMinusOne = MinusOne<123> // 122
type test = MinusOne<9007199254740992>