service.ts

import { Injectable, Inject } from '@angular/core';

@Injectable()
export class SecretService {
  
 constructor( @Inject('ENcRYPTI0n_k3y') private key: string) {}
 //public api
 encode(secret: string) {
   return SecretService._encodeSecret(this.key, secret);
 }
 
 //private api
 static _mapToKeyChar(key: string, char: str, index: number) {
   const len = key.length;
   const dist = index > len-1 ? index - len : len - index;
   return key[dist];
 }
 
 static _encodeSecret(key: string, secret: string) {
   return secret
   .split('')
   .reverse()
   .map((char, index, str) => {
      return SecretService._mapToKeyChar(key, char, index); 
   })
   .join('');
 }
 
}