type LUT = [9, 0, 1, 2, 3, 4, 5, 6, 7, 8]

type Reverse<T extends string, O extends string = ""> =
  T extends `${infer L}${infer R}`
  ? Reverse<R, `${L}${O}`>
  : T extends string
  ? `${T}${O}`
  : O

type MinusOne_<T extends string> =
  Reverse<T> extends `${infer Digit extends number}${infer Rest}`
  ? Digit extends 0
  ? `${MinusOne_<Reverse<Rest>>}9`
  : `${Reverse<Rest>}${LUT[Digit]}`
  : never

type TrimZero<T extends string> =
  T extends `${infer H extends number}${infer R}`
  ? R extends ""
  ? `${H}`
  : H extends 0
  ? TrimZero<R>
  : T
  : T

type ParseInt<T extends string> = T extends `${infer N extends number}` ? N : never

type MinusOne<T extends number> = ParseInt<TrimZero<MinusOne_<`${T}`>>>

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