type A = {
  '0': []; '1': [unknown]; '2': [unknown, unknown]; '3': [unknown, unknown, unknown];
  '4': [unknown, unknown, unknown, unknown]; '5': [unknown, unknown, unknown, unknown, unknown];
  '6': [unknown, unknown, unknown, unknown, unknown, unknown];
  '7': [unknown, unknown, unknown, unknown, unknown, unknown, unknown];
  '8': [unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown];
  '9': [unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown];
};

type Times10<T extends readonly unknown[]> = [
  ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T, ...T
]

type BuildFromStr<S extends string, D extends unknown[] = []> =
  S extends `${infer B extends keyof A}${infer C}` ? BuildFromStr<C, [...Times10<D>, ...A[B]]> : D;

type MinusOne<T extends number> = BuildFromStr<`${T}`> extends [infer _, ...infer Rest] ? Rest["length"] : never

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>>,
]