300 - String to Number

[Vampirelee]

思路：

1. 需要解析 0-9 的数字，定义联合类型 NumberUnion

S extends `${infer L extends NumberUnion}${infer Rest}` 
// 这样L便可以推断成数字了(0-9)。

2. 需要实现加法 AddNumber， 利用数组实现

type AddNumber<N extends number, M extends number> = number extends N
  ? number extends M
    ? 0
    : 0
  : [...NumberToArr<N>, ...NumberToArr<M>]["length"] & number;

3. 需要实现 乘10 MultiplyTen， 利用数组实现，写得比较死板

type MultiplyTen<N extends number> = number extends N
  ? 0
  : [
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>
    ]["length"] & number;

// 单个数字类型
type NumberUnion = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
// 将数字转为相同大小的数组  NumberToArr<3>  // 结果是[1,1,1]
type NumberToArr<N extends number, Count extends 1[] = []> = Equal<
  N,
  Count["length"]
> extends true
  ? Count
  : NumberToArr<N, [...Count, 1]>;

// 将两个数字相加 AddNumber<1, 2> // 3
type AddNumber<N extends number, M extends number> = number extends N
  ? number extends M
    ? 0
    : 0
  : [...NumberToArr<N>, ...NumberToArr<M>]["length"] & number;

// 将某个数字乘10   MultiplyTen<10> // 100 
type MultiplyTen<N extends number> = number extends N
  ? 0
  : [
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>,
      ...NumberToArr<N>
    ]["length"] &
      number;

type ToNumber<
  S extends string,
  Res extends number = 0
> = S extends `${infer L extends NumberUnion}${infer Rest}`
  ? ToNumber<Rest, AddNumber<L, MultiplyTen<Res>>>
  : S extends ""
  ? Res
  : never;