/** This module defines a express.Router() instance
 * - supporting offset=<number> and limit=<number>*
 * - calls next with error if a impossible offset and/or limit value is given
 *
 *  Note: it expects to be called BEFORE any data fetched from DB
 *  Note: it sets an object { limit: 0, skip: 0 } with the proper number values in req.locals.limitskip
 *  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/limitoffset-middleware-mongo
 * @type {Router}
 */

// 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 router = require('express').Router();
const logger = require('debug')('me2:offsetlimit');


// the exported router with handler
router.use((req, res, next) => {
  let offset = undefined;
  let limit = undefined;
  const offsetString = req.query.offset;
  const limitString = req.query.limit;
  let err = null;


  if (offsetString) {
    if (!isNaN(offsetString)) {
      offset = parseInt(offsetString);
      if (offset < 0) {
        err = new Error('offset is negative')
      }
    }
    else {
      err = new Error('given offset is not a valid number ' + offsetString);
    }
  }
  if (limitString) {
    if (!isNaN(limitString)) {
      limit = parseInt(limitString);
      if (limit < 1) {
        err = new Error('limit is zero or negative')
      }
    }
    else {
      err = new Error('given limit is not a valid number ' + limitString);
    }
  }
  if (err) {
    logger('problem occurred with limit/offset values');
    err.status = 400;
    next(err)
  } else {
    res.locals.limitskip = {}; // mongoDB uses parameter object for skip/limit
    if (limit) res.locals.limitskip.limit = limit;
    if (offset) res.locals.limitskip.skip = offset;
    next()
  }
});

module.exports = router;