BigNumber Base62 Encoding

base62-bignumber.ts

import * as assert from 'assert';
import BigNumber from 'bignumber.js';

class Base62 {
  private readonly base: BigNumber = new BigNumber(62);
  private readonly charset: string[] = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('');

  public encode(integer: string): string {
    if (Number(integer) === 0) {
      return '0';
    }

    let num: BigNumber = new BigNumber(integer);
    let str: string[] = [];

    while (num.gt(0)) {
      str = [this.charset[num.mod(this.base).toNumber()], ...str];
      num = num.div(this.base).integerValue(BigNumber.ROUND_FLOOR);
    }

    return str.join('');
  }

  public decode(str: string): string {
    return str.split('').reverse().reduce(
      (prev: BigNumber, char: string, i: number) =>
        this.base.pow(i).times(this.charset.indexOf(char)).plus(prev),
      new BigNumber(0)).toFixed();
  }
}

const base62: Base62 = new Base62();

// Test
const numbers: string[] = [
  '0', '1', '2', '3', '4', '5', '1234567890', '9876543210',
  '616580508813361172', '616580508813361200', '793548328091516928',
  '24809234902840923840294829048204', '90275980235738905734980573490857'];

for (const numstr of numbers) {
  assert.strictEqual(numstr, base62.decode(base62.encode(numstr)));
}