任意進数のエンコード、デコード

baseN.js

class BaseN {
  table = undefined
  /** @type {bigint} */
  n = undefined
  constructor(chars){
    this.table = new Map([...Object.entries(chars).map(([k,v])=>[v,BigInt(k)]),...Object.entries(chars).map(([k,v])=>[BigInt(k),v])])
    this.n = BigInt(chars.length)
  }
  /** @param {arrayBuffer} buf */
  enc(buf){
    const u8 = new Uint8Array(buf)
    let int = 0n
    u8.forEach( (x,i) => int += BigInt(x * 256 ** (u8.length-1-i)) )
    let res = ""
    while (1){
        const [x,y] = [int%this.n,int/=this.n]
        res = this.table.get(x) + res
        if (!y)return res
    }
  }

  /** @param {string} value */
  dec(value){
    const u8 = []
    let int = 0n;
    [...value].forEach( (x,i) => int += this.table.get(x) * this.n ** BigInt(value.length-1-i) )
    while (1){
        const [x,y] = [int%256n,int/=256n] 
        u8.unshift(Number(x))
        if (!y)return new Uint8Array(u8).buffer
    }
  }
}