「重学TS 2.0 」TS 练习题第三十二题 about awesome-typescript HOT 9 OPEN

type RepeatString<T extends string, C extends number, A extends any[] = [], R extends string = ""> = A["length"] extends C
  ? R
  : RepeatString<T, C, [...A, ''], `${R}${T}`;

type S0 = RepeatString<"a", 0>; // ''
type S1 = RepeatString<"a", 2>; // 'aa'
type S2 = RepeatString<"ab", 3>; // 'ababab'

type Repeat<T, C extends number, A extends any[] = []> = A["length"] extends C
  ? A
  : Repeat<T, C, [...A, T]>;

type Join<T extends string[], splitor extends string = ""> = T extends [infer F, ...infer R]
  ? R extends string[]
    ? F extends string
      ? `${F}${splitor}${Join<R, splitor>}`
      : ""
    : F
  : "";

type RepeatString<T extends string, C extends number> = Join<Repeat<T, C>, "">;

type S0 = RepeatString<"a", 0>; // ''
type S1 = RepeatString<"a", 2>; // 'aa'
type S2 = RepeatString<"ab", 3>; // 'ababab'

type Push<T extends any[], V> =  [...T, V];

type RepeatString<
  T extends string,
  C extends number,
  R extends string = '',
  A extends any[] = [],
> = A['length'] extends C ? R: (RepeatString<T, C, `${R}${T}`, Push<A, T>>)

type S01 = RepeatString<"a", 0>; // ''
type S11 = RepeatString<"a", 2>; // 'aa'
type S21 = RepeatString<"ab", 3>; // 'ababab'
type S22 = RepeatString<"abc", 3>; // 'abcabcabc'

type RepeatString<
  T extends string,
  C extends number,
  S extends any[] = [],     //  用于判断是否递归完毕
  U extends string = ''    //  用于累加记录已遍历过的字符串
> = S['length'] extends C ? U : RepeatString<T, C, [...S, 1], `${U}${T}`>

type RepeatString<T extends string, C extends number> = JoinStrArray<
  Repeat<T, C>,
  ""
>;

type S02 = RepeatString<"a", 0>; // ''
type S12 = RepeatString<"a", 2>; // 'aa'
type S22 = RepeatString<"ab", 3>; // 'ababab'

type _RepeatString<
  T extends string,
  C extends number,
  S extends string,
  Count extends string[]
> = Count["length"] extends C
  ? S
  : _RepeatString<T, C, `${S}${T}`, [...Count, T]>;

type RepeatString<T extends string, C extends number> = _RepeatString<
  T,
  C,
  "",
  []
>;

// 将RepeatString拆分成两步: 1. 将其转变成目标数组   2. 将数组拼接成为字符串
// 1. 通过RepeatStr将字符串和数量转变成为数组
// 如: <'ab', 2> => ['ab', 'ab']
type RepeatStr<S extends string, N extends number, R extends string[] = []> = R['length'] extends N ? R : RepeatStr<S, N, [...R, S]>
// 2. 创建一个Join帮助类型, 将重复的数组变成字符串
// 如: ['ab', 'ab'] => 'abab'
type Join<T extends string[]> = T extends [f: infer F, ...r: infer R] ? R extends string[] ? F extends string ? `${F}${Join<R>}` : '' : '' : ''

// 3. 组合上面的两个类型即可
type RepeatString<
  T extends string,
  C extends number,
> = Join<RepeatStr<T, C>>