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+/*!
+ * 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
+}