/** This module defines a express.Router() instance
 * - supporting filter=key1,key2
 * - it sets res.locals.filter to a string "key1 key2"
 * - calls next with error if a filter=key is given, but key does not exist (not raised on empty item arrays!)
 *
 *  Note: it expects to be called before any data is fetched from DB
 *  Note: it sets an Error-Object to next with error.status set to HTTP status code 400
 *
 * @author Johannes Konert
 * @licence CC BY-SA 4.0
 *
 * @module restapi/filter-middleware-mongo
 * @type {Factory function returning an Router}
 * @param Schema {Object} a MongooseSchema.path value or similar object with attributes representing the valid keys
 * @param suppressId {Boolean} if true, the _id is not returned implicitly
 */

// remember: in modules you have 3 variables given by CommonJS
// 1.) require() function
// 2.) module.exports
// 3.) exports (which is module.exports)
"use strict";

const express = require('express');
const logger = require('debug')('me2:filterware');

/**
 * private helper function to filter Objects by given keys
 * @param keys {Array} the keys from GET parameter filter
 * @param schema [Object} containing the keys as attributes that are allowed
 * @returns {Object or Error} either the filtered items or an Error object
 */
const limitFilterToSchema = (keys, schema) => {
  if (!keys || !schema) {  // empty arrays evaluate to false
    return undefined; // means no filter at all
  }

  let error = null;
  const result = [];
  // now for each given filter=key1,key2in the array check that the schema allows this key and store it in result
  keys.forEach((key) => {
    if (schema.hasOwnProperty(key)) {
      result.push(key);
    } else {
      error = new Error('given key for filter does not exist in ressource: ' + key);
    }
  });
  return error ? error : result
};

/**
 * closure function as factory returning the  router
 *
 * @param Schema {Object} a MongooseSchema.path value or similar object with attributes representing the valid keys
 * @param suppressId {Boolean} if true, the _id is not returned implicitly
 * @returns {Router}
 */

const createFilterRouter = (schema, supressID) => {
  const router = express.Router();
  // the exported router with handler
  router.use((req, res, next) => {
    const filterString = req.query.filter;
    let filterKeys = [];
    let err = null;

    if (filterString !== undefined) {
      filterKeys = filterString.split(',');
      filterKeys.forEach((item, index, array) => {
        array[index] = item.trim();
      });
      filterKeys = filterKeys.filter((item) => {
        return item.length > 0;
      });
      if (filterKeys.length === 0) {
        err = new Error('given filter does not contain any keys');
        err.status = 400;
      } else {
        const result = limitFilterToSchema(filterKeys, schema);
        if (result instanceof Error) {
          err = result;
          err.status = 400;
        } else {
          res.locals.filter = result.join(' '); // create a string with space as seperator
          if (supressID) {
            res.locals.filter = '-_id ' + res.locals.filter;
          }
        }
      }
    }
    if (err) {
      logger(err);
      next(err)
    } else {
      if (res.locals.filter) {
        logger('Successfully set filter to ' + res.locals.filter);
      }
      next()
    }
  });
  return router;
};

module.exports = createFilterRouter;