// Helpers
// -------

// Shared empty constructor function to aid in prototype-chain creation.
var ctor = function(){};

// Helper function to correctly set up the prototype chain, for subclasses.
// Similar to `goog.inherits`, but uses a hash of prototype properties and
// class properties to be extended.
var inherits = function(parent, protoProps, staticProps) {
  var child;

  // The constructor function for the new subclass is either defined by you
  // (the "constructor" property in your `extend` definition), or defaulted
  // by us to simply call `super()`.
  if (protoProps && protoProps.hasOwnProperty('constructor')) {
    child = protoProps.constructor;
  } else {
    child = function(){ return parent.apply(this, arguments); };
  }

  // Inherit class (static) properties from parent.
  _.extend(child, parent);

  // Set the prototype chain to inherit from `parent`, without calling
  // `parent`'s constructor function.
  ctor.prototype = parent.prototype;
  child.prototype = new ctor();

  // Add prototype properties (instance properties) to the subclass,
  // if supplied.
  if (protoProps) _.extend(child.prototype, protoProps);

  // Add static properties to the constructor function, if supplied.
  if (staticProps) _.extend(child, staticProps);

  // Correctly set child's `prototype.constructor`.
  child.prototype.constructor = child;

  // Set a convenience property in case the parent's prototype is needed later.
  child.__super__ = parent.prototype;

  return child;
};

// Helper function to get a URL from a Model or Collection as a property
// or as a function.
var getUrl = function(object) {
  if (!(object && object.url)) return null;
  return _.isFunction(object.url) ? object.url() : object.url;
};

// Throw an error when a URL is needed, and none is supplied.
var urlError = function() {
  throw new Error('A "url" property or function must be specified');
};

// Wrap an optional error callback with a fallback error event.
var wrapError = function(onError, originalModel, options) {
  return function(model, resp) {
    var resp = model === originalModel ? resp : model;
    if (onError) {
      onError(model, resp, options);
    } else {
      model.trigger('error', model, resp, options);
    }
  };
};