code: function hash(css, algorithm, trim) {
      return crypto
          .createHash(algorithm)
          .update(css)
          .digest("hex")
          .substr(0, trim);
  }
docs: /*
  a function to get hash value for an given content with desired string length.
  input: ('a{}', 'sha', 10)   output: a1b2c3d4e5