/**
 * @ngdoc object
 * @name ui.router.util.$resolve
 *
 * @requires $q
 * @requires $injector
 *
 * @description
 * Manages resolution of (acyclic) graphs of promises.
 */
$Resolve.$inject = ['$q', '$injector'];
function $Resolve(  $q,    $injector) {
  
  var VISIT_IN_PROGRESS = 1,
      VISIT_DONE = 2,
      NOTHING = {},
      NO_DEPENDENCIES = [],
      NO_LOCALS = NOTHING,
      NO_PARENT = extend($q.when(NOTHING), { $$promises: NOTHING, $$values: NOTHING });
  

  /**
   * @ngdoc function
   * @name ui.router.util.$resolve#study
   * @methodOf ui.router.util.$resolve
   *
   * @description
   * Studies a set of invocables that are likely to be used multiple times.
   * <pre>
   * $resolve.study(invocables)(locals, parent, self)
   * </pre>
   * is equivalent to
   * <pre>
   * $resolve.resolve(invocables, locals, parent, self)
   * </pre>
   * but the former is more efficient (in fact `resolve` just calls `study` 
   * internally).
   *
   * @param {object} invocables Invocable objects
   * @return {function} a function to pass in locals, parent and self
   */
  this.study = function (invocables) {
    if (!isObject(invocables)) throw new Error("'invocables' must be an object");
    
    // Perform a topological sort of invocables to build an ordered plan
    var plan = [], cycle = [], visited = {};
    function visit(value, key) {
      if (visited[key] === VISIT_DONE) return;
      
      cycle.push(key);
      if (visited[key] === VISIT_IN_PROGRESS) {
        cycle.splice(0, cycle.indexOf(key));
        throw new Error("Cyclic dependency: " + cycle.join(" -> "));
      }
      visited[key] = VISIT_IN_PROGRESS;
      
      if (isString(value)) {
        plan.push(key, [ function() { return $injector.get(value); }], NO_DEPENDENCIES);
      } else {
        var params = $injector.annotate(value);
        forEach(params, function (param) {
          if (param !== key && invocables.hasOwnProperty(param)) visit(invocables[param], param);
        });
        plan.push(key, value, params);
      }
      
      cycle.pop();
      visited[key] = VISIT_DONE;
    }
    forEach(invocables, visit);
    invocables = cycle = visited = null; // plan is all that's required
    
    function isResolve(value) {
      return isObject(value) && value.then && value.$$promises;
    }
    
    return function (locals, parent, self) {
      if (isResolve(locals) && self === undefined) {
        self = parent; parent = locals; locals = null;
      }
      if (!locals) locals = NO_LOCALS;
      else if (!isObject(locals)) {
        throw new Error("'locals' must be an object");
      }       
      if (!parent) parent = NO_PARENT;
      else if (!isResolve(parent)) {
        throw new Error("'parent' must be a promise returned by $resolve.resolve()");
      }
      
      // To complete the overall resolution, we have to wait for the parent
      // promise and for the promise for each invokable in our plan.
      var resolution = $q.defer(),
          result = resolution.promise,
          promises = result.$$promises = {},
          values = extend({}, locals),
          wait = 1 + plan.length/3,
          merged = false;
          
      function done() {
        // Merge parent values we haven't got yet and publish our own $$values
        if (!--wait) {
          if (!merged) merge(values, parent.$$values); 
          result.$$values = values;
          result.$$promises = true; // keep for isResolve()
          resolution.resolve(values);
        }
      }
      
      function fail(reason) {
        result.$$failure = reason;
        resolution.reject(reason);
      }
      
      // Short-circuit if parent has already failed
      if (isDefined(parent.$$failure)) {
        fail(parent.$$failure);
        return result;
      }
      
      // Merge parent values if the parent has already resolved, or merge
      // parent promises and wait if the parent resolve is still in progress.
      if (parent.$$values) {
        merged = merge(values, parent.$$values);
        done();
      } else {
        extend(promises, parent.$$promises);
        parent.then(done, fail);
      }
      
      // Process each invocable in the plan, but ignore any where a local of the same name exists.
      for (var i=0, ii=plan.length; i<ii; i+=3) {
        if (locals.hasOwnProperty(plan[i])) done();
        else invoke(plan[i], plan[i+1], plan[i+2]);
      }
      
      function invoke(key, invocable, params) {
        // Create a deferred for this invocation. Failures will propagate to the resolution as well.
        var invocation = $q.defer(), waitParams = 0;
        function onfailure(reason) {
          invocation.reject(reason);
          fail(reason);
        }
        // Wait for any parameter that we have a promise for (either from parent or from this
        // resolve; in that case study() will have made sure it's ordered before us in the plan).
        forEach(params, function (dep) {
          if (promises.hasOwnProperty(dep) && !locals.hasOwnProperty(dep)) {
            waitParams++;
            promises[dep].then(function (result) {
              values[dep] = result;
              if (!(--waitParams)) proceed();
            }, onfailure);
          }
        });
        if (!waitParams) proceed();
        function proceed() {
          if (isDefined(result.$$failure)) return;
          try {
            invocation.resolve($injector.invoke(invocable, self, values));
            invocation.promise.then(function (result) {
              values[key] = result;
              done();
            }, onfailure);
          } catch (e) {
            onfailure(e);
          }
        }
        // Publish promise synchronously; invocations further down in the plan may depend on it.
        promises[key] = invocation.promise;
      }
      
      return result;
    };
  };
  
  /**
   * @ngdoc function
   * @name ui.router.util.$resolve#resolve
   * @methodOf ui.router.util.$resolve
   *
   * @description
   * Resolves a set of invocables. An invocable is a function to be invoked via 
   * `$injector.invoke()`, and can have an arbitrary number of dependencies. 
   * An invocable can either return a value directly,
   * or a `$q` promise. If a promise is returned it will be resolved and the 
   * resulting value will be used instead. Dependencies of invocables are resolved 
   * (in this order of precedence)
   *
   * - from the specified `locals`
   * - from another invocable that is part of this `$resolve` call
   * - from an invocable that is inherited from a `parent` call to `$resolve` 
   *   (or recursively
   * - from any ancestor `$resolve` of that parent).
   *
   * The return value of `$resolve` is a promise for an object that contains 
   * (in this order of precedence)
   *
   * - any `locals` (if specified)
   * - the resolved return values of all injectables
   * - any values inherited from a `parent` call to `$resolve` (if specified)
   *
   * The promise will resolve after the `parent` promise (if any) and all promises 
   * returned by injectables have been resolved. If any invocable 
   * (or `$injector.invoke`) throws an exception, or if a promise returned by an 
   * invocable is rejected, the `$resolve` promise is immediately rejected with the 
   * same error. A rejection of a `parent` promise (if specified) will likewise be 
   * propagated immediately. Once the `$resolve` promise has been rejected, no 
   * further invocables will be called.
   * 
   * Cyclic dependencies between invocables are not permitted and will caues `$resolve`
   * to throw an error. As a special case, an injectable can depend on a parameter 
   * with the same name as the injectable, which will be fulfilled from the `parent` 
   * injectable of the same name. This allows inherited values to be decorated. 
   * Note that in this case any other injectable in the same `$resolve` with the same
   * dependency would see the decorated value, not the inherited value.
   *
   * Note that missing dependencies -- unlike cyclic dependencies -- will cause an 
   * (asynchronous) rejection of the `$resolve` promise rather than a (synchronous) 
   * exception.
   *
   * Invocables are invoked eagerly as soon as all dependencies are available. 
   * This is true even for dependencies inherited from a `parent` call to `$resolve`.
   *
   * As a special case, an invocable can be a string, in which case it is taken to 
   * be a service name to be passed to `$injector.get()`. This is supported primarily 
   * for backwards-compatibility with the `resolve` property of `$routeProvider` 
   * routes.
   *
   * @param {object} invocables functions to invoke or 
   * `$injector` services to fetch.
   * @param {object} locals  values to make available to the injectables
   * @param {object} parent  a promise returned by another call to `$resolve`.
   * @param {object} self  the `this` for the invoked methods
   * @return {object} Promise for an object that contains the resolved return value
   * of all invocables, as well as any inherited and local values.
   */
  this.resolve = function (invocables, locals, parent, self) {
    return this.study(invocables)(locals, parent, self);
  };
}

angular.module('ui.router.util').service('$resolve', $Resolve);

