code: function entitytag (entity) {
    if (entity.length === 0) {
      // fast-path empty
      return '"0-2jmj7l5rSw0yVb/vlWAYkK/YBwk"'
    }
  
    // compute hash of entity
    var hash = crypto
      .createHash('sha1')
      .update(entity, 'utf8')
      .digest('base64')
      .substring(0, 27)
  
    // compute length of entity
    var len = typeof entity === 'string'
      ? Buffer.byteLength(entity, 'utf8')
      : entity.length
  
    return '"' + len.toString(16) + '-' + hash + '"'
  }
docs: Generate an entity tag.
  
  @param {Buffer|string} entity
  @return {string}
  @private