Run-length encoding

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题目

Given a string sequence of a letters f.e. AAABCCXXXXXXY. Return run-length encoded string 3AB2C6XY. Also make a decoder for that string.

解答

namespace RLE {
  type EncodeHelper<
    S extends string,
    P extends string = "",
    L extends 1[] = []
  > = S extends `${infer F}${infer R}`
    ? F extends P
      ? EncodeHelper<R, P, [1, ...L]>
      : P extends ""
      ? EncodeHelper<R, F, [1]>
      : L["length"] extends 1
      ? `${P}${EncodeHelper<R, F, [1]>}`
      : `${L["length"]}${P}${EncodeHelper<R, F, [1]>}`
    : P extends ""
    ? ""
    : L["length"] extends 1
    ? P
    : `${L["length"]}${P}`;
  export type Encode<S extends string> = EncodeHelper<S>;

  type DecodeHelper<
    S extends string,
    L extends string = ""
  > = S extends `${infer F}${infer R}`
    ? F extends `${number}`
      ? DecodeHelper<R, `${L}${F}`>
      : `${Repeat<F, L extends "" ? "1" : L>}${DecodeHelper<R, "">}`
    : "";
  type Repeat<
    S extends string,
    L extends string,
    C extends 1[] = []
  > = `${C["length"]}` extends L ? "" : `${S}${Repeat<S, L, [1, ...C]>}`;
  export type Decode<S extends string> = DecodeHelper<S>;
}