var Q = require("q");
  var MimeParse = require("mimeparse");
  
  exports.negotiate = negotiate;
  function negotiate(request, types, header) {
      var keys = Object.keys(types);
      var accept = request.headers[header || "accept"] || "*";
      var best = MimeParse.bestMatch(keys, accept);
      return types[best];
  }
  
  /// branch on HTTP method
  /**
   * @param {Object * App} methods
   * @param {App} notAllowed (optional)
   * @returns {App}
   */
  exports.Method = function (methods, methodNotAllowed) {
      var keys = Object.keys(methods);
      if (!methodNotAllowed)
          methodNotAllowed = exports.methodNotAllowed;
      return function (request, response) {
          var method = request.method;
          if (Object.has(keys, method)) {
              return Object.get(methods, method)(request, response);
          } else {
              return methodNotAllowed(request, response);
          }
      };
  };
  
  var Negotiator = function (requestHeader, responseHeader, respond) {
      return function (types, notAcceptable) {
          var keys = Object.keys(types);
          if (!notAcceptable)
              notAcceptable = exports.notAcceptable;
          return function (request, response) {
              var header = requestHeader;
              if (typeof header === "function") {
                  header = requestHeader(request);
              }
              var accept = request.headers[requestHeader] || "*";
              var type = MimeParse.bestMatch(keys, accept);
              request.terms = request.terms || {};
              request.terms[responseHeader] = type;
              if (Object.has(keys, type)) {
                  return Q.when(types[type](request, response), function (response) {
                      if (
                          respond !== null &&
                          response &&
                          response.status === 200 &&
                          response.headers
                      ) {
                          response.headers[responseHeader] = type;
                      }
                      return response;
                  });
              } else {
                  return notAcceptable(request, response);
              }
          };
      };
  };
  
  /// branch on HTTP content negotiation
  /**
   * Routes based on content negotiation, between the request's `accept`
   * header and the application's list of possible content types.
   *
   * @param {Object * App} types mapping content types to apps that can
   * handle them.
   * @param {App} notAcceptable
   * @returns {App}
   */
  exports.ContentType = Negotiator("accept", "content-type");
  exports.Language = Negotiator("accept-language", "language");
  exports.Charset = Negotiator("accept-charset", "charset");
  exports.Encoding = Negotiator("accept-encoding", "encoding");
  exports.Host = Negotiator(function (request) {
      return (request.headers.host || "*") + ":" + request.port;
  }, "host", null);
  
  // Branch on a selector function based on the request
  exports.Select = function (select) {
      return function (request, response) {
          return Q.when(select(request, response), function (app) {
              return app(request, response);
          });
      };
  };