HORIZON PROJECT

/*!

 * fresh

 * Copyright(c) 2012 TJ Holowaychuk

 * Copyright(c) 2016-2017 Douglas Christopher Wilson

 * MIT Licensed

 */

'use strict'

/**

 * RegExp to check for no-cache token in Cache-Control.

 * @private

 */

var CACHE_CONTROL_NO_CACHE_REGEXP = /(?:^|,)\s*?no-cache\s*?(?:,|$)/

/**

 * Module exports.

 * @public

 */

module.exports = fresh

/**

 * Check freshness of the response using request and response headers.

 *

 * @param {Object} reqHeaders

 * @param {Object} resHeaders

 * @return {Boolean}

 * @public

 */

function fresh (reqHeaders, resHeaders) {

  // fields

  var modifiedSince = reqHeaders['if-modified-since']

  var noneMatch = reqHeaders['if-none-match']

  // unconditional request

  if (!modifiedSince && !noneMatch) {

    return false

  }

  // Always return stale when Cache-Control: no-cache

  // to support end-to-end reload requests

  // https://tools.ietf.org/html/rfc2616#section-14.9.4

  var cacheControl = reqHeaders['cache-control']

  if (cacheControl && CACHE_CONTROL_NO_CACHE_REGEXP.test(cacheControl)) {

    return false

  }

  // if-none-match

  if (noneMatch && noneMatch !== '*') {

    var etag = resHeaders['etag']

    if (!etag) {

      return false

    }

    var etagStale = true

    var matches = parseTokenList(noneMatch)

    for (var i = 0; i < matches.length; i++) {

      var match = matches[i]

      if (match === etag || match === 'W/' + etag || 'W/' + match === etag) {

        etagStale = false

        break

      }

    }

    if (etagStale) {

      return false

    }

  }

  // if-modified-since

  if (modifiedSince) {

    var lastModified = resHeaders['last-modified']

    var modifiedStale = !lastModified || !(parseHttpDate(lastModified) <= parseHttpDate(modifiedSince))

    if (modifiedStale) {

      return false

    }

  }

  return true

}

/**

 * Parse an HTTP Date into a number.

 *

 * @param {string} date

 * @private

 */

function parseHttpDate (date) {

  var timestamp = date && Date.parse(date)

  // istanbul ignore next: guard against date.js Date.parse patching

  return typeof timestamp === 'number'

    ? timestamp

    : NaN

}

/**

 * Parse a HTTP token list.

 *

 * @param {string} str

 * @private

 */

function parseTokenList (str) {

  var end = 0

  var list = []

  var start = 0

  // gather tokens

  for (var i = 0, len = str.length; i < len; i++) {

    switch (str.charCodeAt(i)) {

      case 0x20: /*   */

        if (start === end) {

          start = end = i + 1

        }

        break

      case 0x2c: /* , */

        list.push(str.substring(start, end))

        start = end = i + 1

        break

      default:

        end = i + 1

        break

    }

  }

  // final token

  list.push(str.substring(start, end))

  return list

}