300 - String to Number

[acwink]

// your answers
type NumberMap = {
  "0": 0;
  "1": 1;
  "2": 2;
  "3": 3;
  "4": 4;
  "5": 5;
  "6": 6;
  "7": 7;
  "8": 8;
  "9": 9;
};
// 两数相加
type Add<
  A extends number,
  B extends number,
  CountA extends 0[] = [],
  CountB extends 0[] = []
> = CountA["length"] extends A
  ? CountB["length"] extends B
    ? [...CountA, ...CountB]["length"]
    : Add<A, B, CountA, [0, ...CountB]>
  : CountB["length"] extends B
  ? Add<A, B, [0, ...CountA], CountB>
  : Add<A, B, [0, ...CountA], [0, ...CountB]>;

// 两数相乘
// 2 x 5 = 2 + 2 + 2 + 2 
type Mul<
  A extends number,
  B extends number,
  Res extends number = 0,
  Cnt extends 0[] = []
> = Cnt["length"] extends B ? Res : Mul<A, B, Add<A, Res>, [0, ...Cnt]>;

type ToNumber<
  T extends string,
  Res extends number = 0
> = T extends `${infer F}${infer R}`
  ? F extends keyof NumberMap
    ? ToNumber<R, Add<Mul<Res, 10>, NumberMap[F]>>
    : never
  : Res;