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    'use strict';
    
    var parseUrl = require('url').parse;
    
    var DEFAULT_PORTS = {
      ftp: 21,
      gopher: 70,
      http: 80,
      https: 443,
      ws: 80,
      wss: 443,
    };
    
    var stringEndsWith = String.prototype.endsWith || function(s) {
      return s.length <= this.length &&
        this.indexOf(s, this.length - s.length) !== -1;
    };
    
    /**
     * @param {string|object} url - The URL, or the result from url.parse.
     * @return {string} The URL of the proxy that should handle the request to the
     *  given URL. If no proxy is set, this will be an empty string.
     */
    function getProxyForUrl(url) {
      var parsedUrl = typeof url === 'string' ? parseUrl(url) : url || {};
      var proto = parsedUrl.protocol;
      var hostname = parsedUrl.host;
      var port = parsedUrl.port;
      if (typeof hostname !== 'string' || !hostname || typeof proto !== 'string') {
        return '';  // Don't proxy URLs without a valid scheme or host.
      }
    
      proto = proto.split(':', 1)[0];
      // Stripping ports in this way instead of using parsedUrl.hostname to make
      // sure that the brackets around IPv6 addresses are kept.
      hostname = hostname.replace(/:\d*$/, '');
      port = parseInt(port) || DEFAULT_PORTS[proto] || 0;
      if (!shouldProxy(hostname, port)) {
        return '';  // Don't proxy URLs that match NO_PROXY.
      }
    
      var proxy =
        getEnv('npm_config_' + proto + '_proxy') ||
        getEnv(proto + '_proxy') ||
        getEnv('npm_config_proxy') ||
        getEnv('all_proxy');
      if (proxy && proxy.indexOf('://') === -1) {
        // Missing scheme in proxy, default to the requested URL's scheme.
        proxy = proto + '://' + proxy;
      }
      return proxy;
    }
    
    /**
     * Determines whether a given URL should be proxied.
     *
     * @param {string} hostname - The host name of the URL.
     * @param {number} port - The effective port of the URL.
     * @returns {boolean} Whether the given URL should be proxied.
     * @private
     */
    function shouldProxy(hostname, port) {
      var NO_PROXY =
        (getEnv('npm_config_no_proxy') || getEnv('no_proxy')).toLowerCase();
      if (!NO_PROXY) {
        return true;  // Always proxy if NO_PROXY is not set.
      }
      if (NO_PROXY === '*') {
        return false;  // Never proxy if wildcard is set.
      }
    
      return NO_PROXY.split(/[,\s]/).every(function(proxy) {
        if (!proxy) {
          return true;  // Skip zero-length hosts.
        }
        var parsedProxy = proxy.match(/^(.+):(\d+)$/);
        var parsedProxyHostname = parsedProxy ? parsedProxy[1] : proxy;
        var parsedProxyPort = parsedProxy ? parseInt(parsedProxy[2]) : 0;
        if (parsedProxyPort && parsedProxyPort !== port) {
          return true;  // Skip if ports don't match.
        }
    
        if (!/^[.*]/.test(parsedProxyHostname)) {
          // No wildcards, so stop proxying if there is an exact match.
          return hostname !== parsedProxyHostname;
        }
    
        if (parsedProxyHostname.charAt(0) === '*') {
          // Remove leading wildcard.
          parsedProxyHostname = parsedProxyHostname.slice(1);
        }
        // Stop proxying if the hostname ends with the no_proxy host.
        return !stringEndsWith.call(hostname, parsedProxyHostname);
      });
    }
    
    /**
     * Get the value for an environment variable.
     *
     * @param {string} key - The name of the environment variable.
     * @return {string} The value of the environment variable.
     * @private
     */
    function getEnv(key) {
      return process.env[key.toLowerCase()] || process.env[key.toUpperCase()] || '';
    }
    
    exports.getProxyForUrl = getProxyForUrl;