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const _ = require('lodash')
const axios = require('axios')
const cheerio = require('cheerio')

/// ////////////////////////////////////////////////////////////////////////////
// UTILITY FUNCTIONS
/// ////////////////////////////////////////////////////////////////////////////

/**
 * Compose function arguments starting from right to left
 * to an overall function and returns the overall function
 */
const compose = (...fns) => (arg) => {
  return _.flattenDeep(fns).reduceRight((current, fn) => {
    if (_.isFunction(fn)) return fn(current)
    throw new TypeError('compose() expects only functions as parameters.')
  }, arg)
}

/**
 * Compose async function arguments starting from right to left
 * to an overall async function and returns the overall async function
 */
const composeAsync = (...fns) => (arg) => {
  return _.flattenDeep(fns).reduceRight(async (current, fn) => {
    if (_.isFunction(fn)) return fn(await current)
    throw new TypeError('compose() expects only functions as parameters.')
  }, arg)
}

/**
 * Enforces the scheme of the URL is https
 * and returns the new URL
 */
const enforceHttpsUrl = (url) =>
  _.isString(url) ? url.replace(/^(https?:)?\/\//, 'https://') : null

/**
 * Strips number of all non-numeric characters
 * and returns the sanitized number
 */
const sanitizeNumber = (number) =>
  _.isString(number)
    ? number.replace(/[^0-9-.]/g, '')
    : _.isNumber(number)
    ? number
    : null

/**
 * Filters null values from array
 * and returns an array without nulls
 */
const withoutNulls = (arr) => (_.isArray(arr) ? arr.filter((val) => !_.isNull(val)) : [])

/**
 * Transforms an array of ({ key: value }) pairs to an object
 * and returns the transformed object
 */
const arrayPairsToObject = (arr) => arr.reduce((obj, pair) => ({ ...obj, ...pair }), {})

/**
 * A composed function that removes null values from array of ({ key: value }) pairs
 * and returns the transformed object of the array
 */
const fromPairsToObject = compose(arrayPairsToObject, withoutNulls)

/**
 * Handles the request(Promise) when it is fulfilled
 * and sends a JSON response to the HTTP response stream(res).
 */
const sendResponse = (res) => async (request) => {
  return await request
    .then((data) => res.json({ status: 'success', data }))
    .catch((e) => res.json(e))
}

/**
 * Loads the html string returned for the given URL
 * and sends a Cheerio parser instance of the loaded HTML
 */
const fetchHtmlFromUrl = async (url) => {
  return await axios
    .get(enforceHttpsUrl(url))
    .then((response) => cheerio.load(response.data))
    .catch((error) => {
      error.status = (error.response && error.response.status) || 500
      throw error
    })
}

/// ////////////////////////////////////////////////////////////////////////////
// HTML PARSING HELPER FUNCTIONS
/// ////////////////////////////////////////////////////////////////////////////

/**
 * Fetches the inner text of the element
 * and returns the trimmed text
 */
const fetchElemInnerText = (elem) => (elem.text && elem.text().trim()) || null

/**
 * Fetches the specified attribute from the element
 * and returns the attribute value
 */
const fetchElemAttribute = (attribute) => (elem) =>
  (elem.attr && elem.attr(attribute)) || null

/**
 * Extract an array of values from a collection of elements
 * using the extractor function and returns the array
 * or the return value from calling transform() on array
 */
const extractFromElems = (extractor) => (transform) => (elems) => ($) => {
  const results = elems.map((i, element) => extractor($(element))).get()
  return _.isFunction(transform) ? transform(results) : results
}

/**
 * A composed function that extracts number text from an element,
 * sanitizes the number text and returns the parsed integer
 */
const extractNumber = compose(parseInt, sanitizeNumber, fetchElemInnerText)

/**
 * A composed function that extracts url string from the element's attribute(attr)
 * and returns the url with https scheme
 */
const extractUrlAttribute = (attr) => compose(enforceHttpsUrl, fetchElemAttribute(attr))

module.exports = {
  compose,
  composeAsync,
  enforceHttpsUrl,
  sanitizeNumber,
  withoutNulls,
  arrayPairsToObject,
  fromPairsToObject,
  sendResponse,
  fetchHtmlFromUrl,
  fetchElemInnerText,
  fetchElemAttribute,
  extractFromElems,
  extractNumber,
  extractUrlAttribute,
}