TypeScriptの型で巨大な数を扱う

bigint.ts

// 再帰消費数が 10*桁数 程度
// 適当にテーブルを容易すれば定数倍の改善ができるので必要ならやる

type Repeat<T, N, Acc extends any[] = []> = Acc['length'] extends N ? Acc : Repeat<T, N, [T, ...Acc]>

type EncodeAux<D, Acc extends any[] = []> = `${Acc['length']}` extends D ? Acc['length'] : EncodeAux<D, [0, ...Acc]> 
type Encode<T extends number | string> =
  T extends number ? Encode<`${T}`> :
  T extends `${infer D}${infer Rest}` ? [EncodeAux<D>, ...Encode<Rest>] :
  []

type Decode<T, A extends any[] = []> =
  T extends [infer D, ...(infer Rest)] ? (
    Decode<Rest, [...A, ...A, ...A, ...A, ...A, ...A, ...A, ...A, ...A, ...A, ...Repeat<0, D>]>
  ) :
  T extends [] ? A['length'] :
  never