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• Can't get FTP to work on centOS 5.6

  - by josi

  Hi guys I have been trying for a few hours to install and get FTP to work... I did yum install ftp and yum install vsftpd They all installed and are running but when I try to use filezilla or some other client I just can't connect....I've tried connecting on port 21 and port 990 ....nothing! These are my iptables # Firewall configuration written by system-config-securitylevel # Manual customization of this file is not recommended. *filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :RH-Firewall-1-INPUT - [0:0] -A INPUT -j RH-Firewall-1-INPUT -A FORWARD -j RH-Firewall-1-INPUT -A RH-Firewall-1-INPUT -i lo -j ACCEPT -A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT -A RH-Firewall-1-INPUT -p 50 -j ACCEPT -A RH-Firewall-1-INPUT -p 51 -j ACCEPT -A RH-Firewall-1-INPUT -p esp -j ACCEPT -A RH-Firewall-1-INPUT -p ah -j ACCEPT -A RH-Firewall-1-INPUT -d 224.0.0.251 -p udp -m udp --dport 5353 -j ACCEPT -A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m tcp --dport 631 -j ACCEPT -A RH-Firewall-1-INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 21 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 25 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 53 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 110 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 990 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 465 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 646 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 993 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 995 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 3306 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 10009 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 7778 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 5000 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 25566 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 8765 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 8192 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 8123 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 20 -j ACCEPT -A RH-Firewall-1-INPUT -p udp -m state --state NEW -m udp --dport 23877 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 9091 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 51413 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 10011 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 30033 -j ACCEPT -A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT -A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m tcp --dport 631 -j ACCEPT -A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT -A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited COMMIT Any help would be much appreciated! If I do lsof -i :21 without the "." it shows nothing. [root@ks3000420 ~]# lsof -i :21 . COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME bash 9964 root cwd DIR 8,1 4096 483329 . bash 11608 root cwd DIR 8,1 4096 483329 . bash 13550 root cwd DIR 8,1 4096 483329 . vi 14117 root cwd DIR 8,1 4096 483329 . sftp-serv 15261 root cwd DIR 8,1 4096 483329 . sftp-serv 15477 root cwd DIR 8,1 4096 483329 . bash 19074 root cwd DIR 8,1 4096 483329 . lsof 19100 root cwd DIR 8,1 4096 483329 . lsof 19101 root cwd DIR 8,1 4096 483329 .

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• Session State with MVP and Application Controller patterns

  - by Graham Bunce

  Hi, I've created an MVP (passive view) framework for development and decided to go for an Application Controller pattern to manage the navigation between views. This is targeted at WinForms, ASP.NET and WPF interfaces. Although I'm not 100% convinced that these view technologies really swappable, that's my aim at the moment so my MVP framework is quite lightweight. What I'm struggling to fit in is the concept of a "Business Conversation" that needs state information to be either (a) maintained for the lifetime of the View or, more likely, (b) maintained across several views for the lifetime of a use case (business conversation). I want state management to be part of the framework as I don't want developers to worry about it. All they need to do is to "start" a conversation, "Register" objects and the framework does the rest until the "end" a conversation. Has anybody got any thoughts (patterns) to how to fit this into MVP? I was thinking it may be part of the Application Controller responsibility (delegating to a Conversation Manager object) as it knows about current state in order to send the user to the next view.... but then I thought it may be up to the Presenter to start and end the conversation so then it comes down the presenters to manage conversations and the objects registered for the that conversation. Unfortunately that means presenters can't be used in different conversations... so that idea doesn't seem right. As you can see, I don't think there is an easy answer (and I've looked for a while). So anybody else got any thoughts?

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• Knowing state of game in real time

  - by evthim

  I'm trying to code a tic tac toe game in java and I need help figuring out how to efficiently and without freezing the program check if someone won the game. I'm only in the design stages now, I haven't started programming anything but I'm wondering how would I know at all times the state of the game and exactly when someone wins? Response to MarkR: (note: had to place comment here, it was too long for comment section) It's not a homework problem, I'm trying to get more practice programming GUI's which I've only done once as a freshman in my second introductory programming course. I understand I'll have a 2D array. I plan to have a 2D integer array where x would equal 1 and o would equal 0. However, won't it take too much time if I check after every move if someone won the game? Is there a way or a data structure or algorithm I can use so that the program will know the state (when I say state I mean not just knowing every position on the board, the int array will take care of that, I mean knowing that user 1 will win if he places x on this block) of the game at all times and thus can know automatically when someone won?

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• State changes in entities or components

  - by GriffinHeart

  I'm having some trouble figuring how to deal with state management in my entities. I don't have trouble with Game state management, like pause and menus, since these are not handled as an entity component system; just with state in entities/components. Drawing from Orcs Must Die as an example, I have my MainCharacter and Trap entities which only have their components like PositionComponent, RenderComponent, PhysicsComponent. On each update the Entity will call update on its components. I also have a generic EventManager with listeners for different event types. Now I need to be able to place the traps: first select the trap and trap position then place the trap. When placing a trap it should appear in front of the MainCharacter, rendered in a different way and following it around. When placed it should just respond to collisions and be rendered in the normal way. How is this usually handled in component based systems? (This example is specific but can help figure out the general way to deal with entities states.)

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• Unable to make the session state request to the session state server

  - by Angry_IT_Guru

  For about 4-5 months now, I seem to be having this sporadic issue--mainly during our busiest time of the day between 10:30-11:45AM, where all my Windows 2003 web servers in a Microsoft NLB cluster start throwing session state server errors. A sample error is below. System.Web.HttpException: Unable to make the session state request to the session state server. Please ensure that the ASP.NET State service is started and that the client and server ports are the same. If the server is on a remote machine, please ensure that it accepts remote requests by checking the value of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aspnet_state\Parameters\AllowRemoteConnection. If the server is on the local machine, and if the before mentioned registry value does not exist or is set to 0, then the state server connection string must use either 'localhost' or '127.0.0.1' as the server name. at System.Web.SessionState.OutOfProcSessionStateStore.MakeRequest(StateProtocolVerb verb, String id, StateProtocolExclusive exclusiveAccess, Int32 extraFlags, Int32 timeout, Int32 lockCookie, Byte[] buf, Int32 cb, Int32 networkTimeout, SessionNDMakeRequestResults& results) at System.Web.SessionState.OutOfProcSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) at System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Now I'm using ASP.NET State service on a centralized back-end Windows 2003 server that all servers communicate to. I was originally using SQL Server state for a couple years as well prior to having this issue. The problem with SQL wqas that when the issue occurred, it created a blocking situation which essentially impacted all users across all servers. The product company recommended that I use the standard ASP.NET State service as that was what they technically supported. Why this would make a difference is beyond me -- but I had no choice but to try it! I have attempted to create multiple application pools, adding additional servers, chaning TCP/IP timeout from 20 to 30 seconds, and even calling Microsoft ASP.NET product support, with very little success. I even recommended that they review whether they are using read-only session state instead of read/write per page request -- as I understand that this basically causes every page to make round-trips to state server even if state isn't being used on the page. Unfortunately, the application is developed by our product company and they insist that it is something with my environment because other clients do not have these sort of issues. However, I've talked to other clients and they tell me when they've seen issues like they, they've basically had to create another web farm. This issue almost seems like I've simply reached some architectural limit within the application... Microsoft's position on the issue is that the session state needs to be reduced and the returncode being reported back from the state server indicates buffers are full. To better understand the scope of issues (rather than wait for customers to call and complain), I installed ELMAH and configured it to send me e-mails when unhandled exceptions occur. I basically get 500-1000 e-mails during the time period of high activity! If any one has any other ideas I could try or better ways to troubleshoot, I'd appreciate it.

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• Finite state machine in C++

  - by Electro

  So, I've read a lot about using FSMs to do game state management, things like what and FSM is, and using a stack or set of states for building one. I've gone through all that. But I'm stuck at writing an actual, well-designed implementation of an FSM for that purpose. Specifically, how does one cleanly resolve the problem of transitioning between states, (how) should a state be able to use data from other states, and so on. Does anyone have any tips on designing and writing a implementation in C++, or better yet, code examples?

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• Throwing exception from a property when my object state is invalid

  - by Rumi P.

  Microsoft guidelines say: "Avoid throwing exceptions from property getters", and I normally follow that. But my application uses Linq2SQL, and there is the case where my object can be in invalid state because somebody or something wrote nonsense into the database. Consider this toy example: [Table(Name="Rectangle")] public class Rectangle { [Column(Name="ID", IsPrimaryKey = true, IsDbGenerated = true)] public int ID {get; set;} [Column(Name="firstSide")] public double firstSide {get; set;} [Column(Name="secondSide")] public double secondSide {get; set;} public double sideRatio { get { return firstSide/secondSide; } } } Here, I could write code which ensures that my application never writes a Rectangle with a zero-length side into the database. But no matter how bulletproof I make my own code, somebody could open the database with a different application and create an invalid Rectangle, especially one with a 0 for secondSide. (For this example, please forget that it is possible to design the database in a way such that writing a side length of zero into the rectangle table is impossible; my domain model is very complex and there are constraints on model state which cannot be expressed in a relational database). So, the solution I am gravitating to is to change the getter to: get { if(firstSide > 0 && secondSide > 0) return firstSide/secondSide; else throw new System.InvalidOperationException("All rectangle sides should have a positive length"); } The reasoning behind not throwing exceptions from properties is that programmers should be able to use them without having to make precautions about catching and handling them them. But in this case, I think that it is OK to continue to use this property without such precautions: if the exception is thrown because my application wrote a non-zero rectangle side into the database, then this is a serious bug. It cannot and shouldn't be handled in the application, but there should be code which prevents it. It is good that the exception is visibly thrown, because that way the bug is caught. if the exception is thrown because a different application changed the data in the database, then handling it is outside of the scope of my application. So I can't do anything about it if I catch it. Is this a good enough reasoning to get over the "avoid" part of the guideline and throw the exception? Or should I turn it into a method after all? Note that in the real code, the properties which can have an invalid state feel less like the result of a calculation, so they are "natural" properties, not methods.

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• Modeling player mechanics with a finite state machine

  - by K..

  I have three states standing walking jumping When I press D standing transitions to walking. The velocity will be set to a defined value and the player moves. When I release D walking transitions back to standing, which sets the velocity back to 0. When I press W and the state is walking it transitions to jumping, but when the player hits the ground, it goes back to standing. jumping has a transition land that always leads to standing because a state doesn't know about its previous states. Since standing sets a velocity of 0 the player stops walking, when he hits the ground. How do I prevent this?

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• Implementing a State Machine in Angular.js to control routing

  - by ldn_tech_exec

  Can anyone help me with integrating a state machine to control routing? What's the best method to do this? Create a service? I need to basically intercept every $location request, run the state machine and let it figure out what the next $location.path should be. Think of the problem like a bank of questions that get added and removed over time. The user visits once in a while, passes in the user's answers object to the statemachine, and the statemachine figures out which question to load. This is my pseudocode, but i need to figure out where to put this or what event I can hook into to make sure all route requests are passed through the machine. Do I need a specific stateMachine controller? Do I create a service? Where do I use the service? Do I need to override $locationProvider? $scope.user.answers = [{ id: 32, answer: "whatever" }, { id:33, answer: "another answer" }] $scope.questions = [{ id:32, question:"what is your name?", path:"/question/1" },{ id:34, question:"how old are you?", path:"/question/2" }] var questions = $scope.questions; angular.forEach(questions, function(question) { if(question.id !exist in $scope.user.answers.id) { $location.path = question.path break; }); Thanks

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• Code Golf: Finite-state machine!

  - by Adam Matan

  Finite state machine A deterministic finite state machine is a simple computation model, widely used as an introduction to automata theory in basic CS courses. It is a simple model, equivalent to regular expression, which determines of a certain input string is Accepted or Rejected. Leaving some formalities aside, A run of a finite state machine is composed of: alphabet, a set of characters. states, usually visualized as circles. One of the states must be the start state. Some of the states might be accepting, usually visualized as double circles. transitions, usually visualized as directed arches between states, are directed links between states associated with an alphabet letter. input string, a list of alphabet characters. A run on the machine begins at the starting state. Each letter of the input string is read; If there is a transition between the current state and another state which corresponds to the letter, the current state is changed to the new state. After the last letter was read, if the current state is an accepting state, the input string is accepted. If the last state was not an accepting state, or a letter had no corresponding arch from a state during the run, the input string is rejected. Note: This short descruption is far from being a full, formal definition of a FSM; Wikipedia's fine article is a great introduction to the subject. Example For example, the following machine tells if a binary number, read from left to right, has an even number of 0s: The alphabet is the set {0,1}. The states are S1 and S2. The transitions are (S1, 0) -> S2, (S1, 1) -> S1, (S2, 0) -> S1 and (S2, 1) -> S2. The input string is any binary number, including an empty string. The rules: Implement a FSM in a language of your choice. Input The FSM should accept the following input: <States> List of state, separated by space mark. The first state in the list is the start state. Accepting states begin with a capital letter. <transitions> One or more lines. Each line is a three-tuple: origin state, letter, destination state) <input word> Zero or more characters, followed by a newline. For example, the aforementioned machine with 1001010 as an input string, would be written as: S1 s2 S1 0 s2 S1 1 S1 s2 0 S1 s2 1 s2 1001010 Output The FSM's run, written as <State> <letter> -> <state>, followed by the final state. The output for the example input would be: S1 1 -> S1 S1 0 -> s2 s2 0 -> S1 S1 1 -> S1 S1 0 -> s2 s2 1 -> s2 s2 0 -> S1 ACCEPT For the empty input '': S1 ACCEPT For 101: S1 1 -> S1 S1 0 -> s2 s2 1 -> s2 REJECT For '10X': S1 1 -> S1 S1 0 -> s2 s2 X REJECT Prize A nice bounty will be given to the most elegant and short solution. Reference implementation A reference Python implementation will be published soon.

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• Turtle Graphics as a Haskell Monad

  - by iliis

  I'm trying to implement turtle graphis in Haskell. The goal is to be able to write a function like this: draw_something = do fordward 100 right 90 forward 100 ... and then have it produce a list of points (maybe with additional properties): > draw_something (0,0) 0 -- start at (0,0) facing east (0 degrees) [(0,0), (0,100), (-100,100), ...] I have all this working in a 'normal' way, but I fail to implement it as a Haskell Monad and use the do-notation. The basic code: data State a = State (a, a) a -- (x,y), angle deriving (Show, Eq) initstate :: State Float initstate = State (0.0,0.0) 0.0 -- constrain angles to 0 to 2*pi fmod :: Float -> Float fmod a | a >= 2*pi = fmod (a-2*pi) | a < 0 = fmod (a+2*pi) | otherwise = a forward :: Float -> State Float -> [State Float] forward d (State (x,y) angle) = [State (x + d * (sin angle), y + d * (cos angle)) angle] right :: Float -> State Float -> [State Float] right d (State pos angle) = [State pos (fmod (angle+d))] bind :: [State a] -> (State a -> [State a]) -> [State a] bind xs f = xs ++ (f (head $ reverse xs)) ret :: State a -> [State a] ret x = [x] With this I can now write > [initstate] `bind` (forward 100) `bind` (right (pi/2)) `bind` (forward 100) [State (0.0,0.0) 0.0,State (0.0,100.0) 0.0,State (0.0,100.0) 1.5707964,State (100.0,99.99999) 1.5707964] And get the expected result. However I fail to implement this as an instance of Monad. instance Monad [State] where ... results in `State' is not applied to enough type arguments Expected kind `*', but `State' has kind `* -> *' In the instance declaration for `Monad [State]' And if I wrap the list in a new object data StateList a = StateList [State a] instance Monad StateList where return x = StateList [x] I get Couldn't match type `a' with `State a' `a' is a rigid type variable bound by the type signature for return :: a -> StateList a at logo.hs:38:9 In the expression: x In the first argument of `StateList', namely `[x]' In the expression: StateList [x] I tried various other versions but I never got it to run as I'd like to. What am I doing wrong? What do I understand incorrectly?

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• What methods should save/load a game state

  - by vedi

  There are a lot of articles about how to save a state of a game and they are pretty good. But I have one conceptual misunderstanding where should I save the state? My game has number of screens and pair of them are MainMenuScreen and MainSceneScreen these are inherited from Screen class. MainMenuScreen is shown at start of the game the MainSceneScreen little later. What is the problem? I navigated to MainSceneScreen, forced Android to stop the application (I change a language settings on the device to achieve it, please let me know if I'm wrong). After that I select the application again and I can see MainMenuScreen is shown. But I want MainSceneScreen to be shown. I suppose I should override resume method. But what class I should override? I have class PsGame that extends Game class of libgdx. I put breakpoints to its resume method and it turned out that method was not called. I investigated the problem and I've found little strange code in onResume method of AndroidApplication class of libgdx: if (!firstResume) graphics.resume(); else firstResume = false; My debugger said firstResume was true and didn't go to *graphics.resume()*line. Sorry for a lot of words but could you answer following question: What did I do wrong? What methods should I override? Thank you in advance.

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• Sharing ASP.NET State databases between multiple apps

  - by MikeWyatt

  Is it better for a collection of ASP.NET web apps to share the same session database, or should each one have its own? If there is no significant difference, having a single database would be preferable due to easier maintenance. Background My team has an assortment of ASP.NET web apps, all written in either Monorail 1.1 or ASP.NET MVC 1.0. Each app currently uses a dedicated session state database. I'm working on adding a new site to that list, and am debating whether I should create another new session database, or just share an existing one with another app.

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• Haskell maps returning a monad

  - by sabauma

  The lookup function in Data.Map and Data.IntMap currently return values wrapped in Maybe with the type signature lookup :: Ord k => k -> Map k a -> Maybe a It used to have the more general type of lookup :: (Monad m, Ord k) => k -> Map k a -> m a I realize the former likely reduces the need of extra type specification, but the latter would make it much more general and allow lookup to be used in list comprehensions. Is there any way to mimic this behavior with the newer version, or would I have to use an older version of the library?

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• Java: How to manage UDP client-server state

  - by user92947

  I am trying to write a Java application that works similar to MapReduce. There is a server and several workers. Workers may come and go as they please and the membership to the group has a soft-state. To become a part of the group, the worker must send a UDP datagram to the server, but to continue to be part of the group, the worker must send the UDP datagram to the server every 5 minutes. In order to accommodate temporary errors, a worker is allowed to miss as many as two consecutive periodic UDP datagrams. So, the server must keep track of the current set of workers as well as the last time each worker had sent a UDP datagram. I've implemented a class called WorkerListener that implements Runnable and listens to UDP datagrams on a particular UDP port. Now, to keep track of active workers, this class may maintain a HashSet (or HashMap). When a datagram is received, the server may query the HashSet to check if it is a new member. If so, it can add the new worker to the group by adding an entry into the HashSet. If not, it must reset a "timer" for the worker, noting that it has just heard from the corresponding worker. I'm using the word timer in a generic sense. It doesn't have to be a clock of sorts. Perhaps this could also be implemented using int or long variables. Also, the server must run a thread that continuously monitors the timers for the workers to see that a client that times out on two consecutive datagram intervals, it is removed from the HashSet. I don't want to do this in the WorkerListener thread because it would be blocking on the UDP datagram receive() function. If I create a separate thread to monitor the worker HashSet, it would need to be a different class, perhaps WorkerRegistrar. I must share the HashSet with that thread. Mutual exclusion must also be implemented, then. My question is, what is the best way to do this? Pointers to some sample implementation would be great. I want to use the barebones JDK implementation, and not some fancy state maintenance API that takes care of everything, because I want this to be a useful demonstration for a class that I am teaching. Thanks

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• RK4 Bouncing a Ball

  - by Jonathan Dickinson

  I am trying to wrap my head around RK4. I decided to do the most basic 'ball with gravity that bounces' simulation. I have implemented the following integrator given Glenn Fiedler's tutorial: /// <summary> /// Represents physics state. /// </summary> public struct State { // Also used internally as derivative. // S: Position // D: Velocity. /// <summary> /// Gets or sets the Position. /// </summary> public Vector2 X; // S: Position // D: Acceleration. /// <summary> /// Gets or sets the Velocity. /// </summary> public Vector2 V; } /// <summary> /// Calculates the force given the specified state. /// </summary> /// <param name="state">The state.</param> /// <param name="t">The time.</param> /// <param name="acceleration">The value that should be updated with the acceleration.</param> public delegate void EulerIntegrator(ref State state, float t, ref Vector2 acceleration); /// <summary> /// Represents the RK4 Integrator. /// </summary> public static class RK4 { private const float OneSixth = 1.0f / 6.0f; private static void Evaluate(EulerIntegrator integrator, ref State initial, float t, float dt, ref State derivative, ref State output) { var state = new State(); // These are a premature optimization. I like premature optimization. // So let's not concentrate on that. state.X.X = initial.X.X + derivative.X.X * dt; state.X.Y = initial.X.Y + derivative.X.Y * dt; state.V.X = initial.V.X + derivative.V.X * dt; state.V.Y = initial.V.Y + derivative.V.Y * dt; output = new State(); output.X.X = state.V.X; output.X.Y = state.V.Y; integrator(ref state, t + dt, ref output.V); } /// <summary> /// Performs RK4 integration over the specified state. /// </summary> /// <param name="eulerIntegrator">The euler integrator.</param> /// <param name="state">The state.</param> /// <param name="t">The t.</param> /// <param name="dt">The dt.</param> public static void Integrate(EulerIntegrator eulerIntegrator, ref State state, float t, float dt) { var a = new State(); var b = new State(); var c = new State(); var d = new State(); Evaluate(eulerIntegrator, ref state, t, 0.0f, ref a, ref a); Evaluate(eulerIntegrator, ref state, t + dt * 0.5f, dt * 0.5f, ref a, ref b); Evaluate(eulerIntegrator, ref state, t + dt * 0.5f, dt * 0.5f, ref b, ref c); Evaluate(eulerIntegrator, ref state, t + dt, dt, ref c, ref d); a.X.X = OneSixth * (a.X.X + 2.0f * (b.X.X + c.X.X) + d.X.X); a.X.Y = OneSixth * (a.X.Y + 2.0f * (b.X.Y + c.X.Y) + d.X.Y); a.V.X = OneSixth * (a.V.X + 2.0f * (b.V.X + c.V.X) + d.V.X); a.V.Y = OneSixth * (a.V.Y + 2.0f * (b.V.Y + c.V.Y) + d.V.Y); state.X.X = state.X.X + a.X.X * dt; state.X.Y = state.X.Y + a.X.Y * dt; state.V.X = state.V.X + a.V.X * dt; state.V.Y = state.V.Y + a.V.Y * dt; } } After reading over the tutorial I noticed a few things that just seemed 'out' to me. Notably how the entire simulation revolves around t at 0 and state at 0 - considering that we are working out a curve over the duration it seems logical that RK4 wouldn't be able to handle this simple scenario. Never-the-less I forged on and wrote a very simple Euler integrator: static void Integrator(ref State state, float t, ref Vector2 acceleration) { if (state.X.Y > 100 && state.V.Y > 0) { // Bounce vertically. acceleration.Y = -state.V.Y * t; } else { acceleration.Y = 9.8f; } } I then ran the code against a simple fixed-time step loop and this is what I got: 0.05 0.20 0.44 0.78 1.23 1.76 ... 74.53 78.40 82.37 86.44 90.60 94.86 99.23 103.05 105.45 106.94 107.86 108.42 108.76 108.96 109.08 109.15 109.19 109.21 109.23 109.23 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 ... As I said, I was expecting it to break - however I am unsure of how to fix it. I am currently looking into keeping the previous state and time, and working from that - although at the same time I assume that will defeat the purpose of RK4. How would I get this simulation to print the expected results?

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• Change players state and controls in-game

  - by Samurai Fox

  I'm using Unity 3D Let's say the player is an ice cube. You control it like a normal player. On press of a button, ice transforms (with animation) into water. You control it completely different than the ice cube. Another great example would be: Player is human being and has normal FPS controls. On press of a button human transforms into birds and now has completely different controls. Now, my question is, what would be easier and better: make one object with animation transition and to stay in that state of anim. until button is pressed again make two object: ice and water. Ice has an animation of turning into water. So replace ice (with animation) with water object And if anyone knows this one too: how to switch between 2 different types of player controls.

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• Implementing a "state-machine" logic for methods required by an object in C++

  - by user827992

  What I have: 1 hypothetical object/class + other classes and related methods that gives me functionality. What I want: linking this object to 0 to N methods in realtime on request when an event is triggered Each event is related to a single method or a class, so a single event does not necessarily mean "connect this 1 method only" but can also mean "connect all the methods from that class or a group of methods" Avoiding linked lists because I have to browse the entire list to know what methods are linked, because this does not ensure me that the linked methods are kept in a particular order (let's say an alphabetic order by their names or classes), and also because this involve a massive amount of pointers usage. Example: I have an object Employee Jon, Jon acquires knowledge and forgets things pretty easily, so his skills may vary during a period of time, I'm responsible for what Jon can add or remove from his CV, how can I implement this logic?

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• Can an object oriented program be seen as a Finite State Machine?

  - by Peretz

  This might be a philosophical/fundamental question, but I just want to clarify it. In my understanding a Finite State Machine is a way of modeling a system in which the system's output will not only depend on the current inputs, but also the current state of the system. Additionally, as the name suggests it, a finite state machine can be segmented in a finite N number of states with its respective state and behavior. If this is correct, shouldn't every single object with data and function members be a state in our object oriented model, making any object oriented design a finite state machine? If that is not the interpretation of a FSM in object design, what exactly people mean when they implement a FSM in software? am I missing something? Thanks

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• Deferred contexts and inheriting state from the immediate context

  - by dreijer

  I took my first stab at using deferred contexts in DirectX 11 today. Basically, I created my deferred context using CreateDeferredContext() and then drew a simple triangle strip with it. Early on in my test application, I call OMSetRenderTargets() on the immediate context in order to render to the swap chain's back buffer. Now, after having read the documentation on MSDN about deferred contexts, I assumed that calling ExecuteCommandList() on the immediate context would execute all of the deferred commands as "an extension" to the commands that had already been executed on the immediate context, i.e. the triangle strip I rendered in the deferred context would be rendered to the swap chain's back buffer. That didn't seem to be the case, however. Instead, I had to manually pull out the immediate context's render target (using OMGetRenderTargets()) and then set it on the deferred context with OMSetRenderTargets(). Am I doing something wrong or is that the way deferred contexts work?

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• Appropriate uses of Monad `fail` vs. MonadPlus `mzero`

  - by jberryman

  This is a question that has come up several times for me in the design code, especially libraries. There seems to be some interest in it so I thought it might make a good community wiki. The fail method in Monad is considered by some to be a wart; a somewhat arbitrary addition to the class that does not come from the original category theory. But of course in the current state of things, many Monad types have logical and useful fail instances. The MonadPlus class is a sub-class of Monad that provides an mzero method which logically encapsulates the idea of failure in a monad. So a library designer who wants to write some monadic code that does some sort of failure handling can choose to make his code use the fail method in Monad or restrict his code to the MonadPlus class, just so that he can feel good about using mzero, even though he doesn't care about the monoidal combining mplus operation at all. Some discussions on this subject are in this wiki page about proposals to reform the MonadPlus class. So I guess I have one specific question: What monad instances, if any, have a natural fail method, but cannot be instances of MonadPlus because they have no logical implementation for mplus? But I'm mostly interested in a discussion about this subject. Thanks! EDIT: One final thought occured to me. I recently learned (even though it's right there in the docs for fail) that monadic "do" notation is desugared in such a way that pattern match failures, as in (x:xs) <- return [] call the monad's fail. It seems like the language designers must have been strongly influenced by the prospect of some automatic failure handling built in to haskell's syntax in their inclusion of fail in Monad.

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• Resolving a Forward Declaration Issue Involving a State Machine in C++

  - by hypersonicninja

  I've recently returned to C++ development after a hiatus, and have a question regarding implementation of the State Design Pattern. I'm using the vanilla pattern, exactly as per the GoF book. My problem is that the state machine itself is based on some hardware used as part of an embedded system - so the design is fixed and can't be changed. This results in a circular dependency between two of the states (in particular), and I'm trying to resolve this. Here's the simplified code (note that I tried to resolve this by using headers as usual but still had problems - I've omitted them in this code snippet): #include <iostream> #include <memory> using namespace std; class Context { public: friend class State; Context() { } private: State* m_state; }; class State { public: State() { } virtual void Trigger1() = 0; virtual void Trigger2() = 0; }; class LLT : public State { public: LLT() { } void Trigger1() { new DH(); } void Trigger2() { new DL(); } }; class ALL : public State { public: ALL() { } void Trigger1() { new LLT(); } void Trigger2() { new DH(); } }; // DL needs to 'know' about DH. class DL : public State { public: DL() { } void Trigger1() { new ALL(); } void Trigger2() { new DH(); } }; class HLT : public State { public: HLT() { } void Trigger1() { new DH(); } void Trigger2() { new DL(); } }; class AHL : public State { public: AHL() { } void Trigger1() { new DH(); } void Trigger2() { new HLT(); } }; // DH needs to 'know' about DL. class DH : public State { public: DH () { } void Trigger1() { new AHL(); } void Trigger2() { new DL(); } }; int main() { auto_ptr<LLT> llt (new LLT); auto_ptr<ALL> all (new ALL); auto_ptr<DL> dl (new DL); auto_ptr<HLT> hlt (new HLT); auto_ptr<AHL> ahl (new AHL); auto_ptr<DH> dh (new DH); return 0; } The problem is basically that in the State Pattern, state transitions are made by invoking the the ChangeState method in the Context class, which invokes the constructor of the next state. Because of the circular dependency, I can't invoke the constructor because it's not possible to pre-define both of the constructors of the 'problem' states. I had a look at this article, and the template method which seemed to be the ideal solution - but it doesn't compile and my knowledge of templates is a rather limited... The other idea I had is to try and introduce a Helper class to the subclassed states, via multiple inheritance, to see if it's possible to specify the base class's constructor and have a reference to the state subclasse's constructor. But I think that was rather ambitious... Finally, would a direct implmentation of the Factory Method Design Pattern be the best way to resolve the entire problem?

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• Has anyone ever encountered a Monad Transformer in the wild?

  - by martingw

  In my area of business - back office IT for a financial institution - it is very common for a software component to carry a global configuration around, to log it's progress, to have some kind of error handling / computation short circuit... Things that can be modelled nicely by Reader-, Writer-, Maybe-monads and the like in Haskell and composed together with monad transformers. But there seem to some drawbacks: The concept behind monad transformers is quite tricky and hard to understand, monad transformers lead to very complex type signatures, and they inflict some performance penalty. So I'm wondering: Are monad transformers best practice when dealing with those common tasks mentioned above?

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• State Changes in a Component Based Architecture [closed]

  - by Maxem

  I'm currently working on a game and using the naive component based architecture thingie (Entities are a bag of components, entity.Update() calls Update on each updateable component), while the addition of new features is really simple, it makes a few things really difficult: a) multithreading / currency b) networking c) unit testing. Multithreading / Concurrency is difficult because I basically have to do poor mans concurrency (running the entity updates in separate threads while locking only stuff that crashes (like lists) and ignoring the staleness of read state (some states are already updated, others aren't)) Networking: There are no explicit state changes that I could efficiently push over the net. Unit testing: All updates may or may not conflict, so automated testing is at least awkward. I was thinking about these issues a bit and would like your input on these changes / idea: Switch from the naive cba to a cba with sub systems that work on lists of components Make all state changes explicit Combine 1 and 2 :p Example world update: statePostProcessing.Wait() // ensure that post processing has finished Apply(postProcessedState) state = new StateBag() Concurrently( () => LifeCycleSubSystem.Update(state), // populates the state bag () => MovementSubSystem.Update(state), // populates the state bag .... }) statePostProcessing = Future(() => PostProcess(state)) statePostProcessing.Start() // Tick is finished, the post processing happens in the background So basically the changes are (consistently) based on the data for the last tick; the post processing can a) generate network packages and b) fix conflicts / remove useless changes (example: entity has been destroyed - ignore movement etc.). EDIT: To clarify the granularity of the state changes: If I save these post processed state bags and apply them to an empty world, I see exactly what has happened in the game these state bags originated from - "Free" replay capability. EDIT2: I guess I should have used the term Event instead of State Change and point out that I kind of want to use the Event Sourcing pattern

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• Threading extra state through a parser in Scala

  - by Travis Brown

  I'll give you the tl;dr up front I'm trying to use the state monad transformer in Scalaz 7 to thread extra state through a parser, and I'm having trouble doing anything useful without writing a lot of t m a -> t m b versions of m a -> m b methods. An example parsing problem Suppose I have a string containing nested parentheses with digits inside them: val input = "((617)((0)(32)))" I also have a stream of fresh variable names (characters, in this case): val names = Stream('a' to 'z': _*) I want to pull a name off the top of the stream and assign it to each parenthetical expression as I parse it, and then map that name to a string representing the contents of the parentheses, with the nested parenthetical expressions (if any) replaced by their names. To make this more concrete, here's what I'd want the output to look like for the example input above: val target = Map( 'a' -> "617", 'b' -> "0", 'c' -> "32", 'd' -> "bc", 'e' -> "ad" ) There may be either a string of digits or arbitrarily many sub-expressions at a given level, but these two kinds of content won't be mixed in a single parenthetical expression. To keep things simple, we'll assume that the stream of names will never contain either duplicates or digits, and that it will always contain enough names for our input. Using parser combinators with a bit of mutable state The example above is a slightly simplified version of the parsing problem in this Stack Overflow question. I answered that question with a solution that looked roughly like this: import scala.util.parsing.combinator._ class ParenParser(names: Iterator[Char]) extends RegexParsers { def paren: Parser[List[(Char, String)]] = "(" ~> contents <~ ")" ^^ { case (s, m) => (names.next -> s) :: m } def contents: Parser[(String, List[(Char, String)])] = "\\d+".r ^^ (_ -> Nil) | rep1(paren) ^^ ( ps => ps.map(_.head._1).mkString -> ps.flatten ) def parse(s: String) = parseAll(paren, s).map(_.toMap) } It's not too bad, but I'd prefer to avoid the mutable state. What I want Haskell's Parsec library makes adding user state to a parser trivially easy: import Control.Applicative ((*>), (<$>), (<*)) import Data.Map (fromList) import Text.Parsec paren = do (s, m) <- char '(' *> contents <* char ')' h : t <- getState putState t return $ (h, s) : m where contents = flip (,) [] <$> many1 digit <|> (\ps -> (map (fst . head) ps, concat ps)) <$> many1 paren main = print $ runParser (fromList <$> paren) ['a'..'z'] "example" "((617)((0)(32)))" This is a fairly straightforward translation of my Scala parser above, but without mutable state. What I've tried I'm trying to get as close to the Parsec solution as I can using Scalaz's state monad transformer, so instead of Parser[A] I'm working with StateT[Parser, Stream[Char], A]. I have a "solution" that allows me to write the following: import scala.util.parsing.combinator._ import scalaz._, Scalaz._ object ParenParser extends ExtraStateParsers[Stream[Char]] with RegexParsers { protected implicit def monadInstance = parserMonad(this) def paren: ESP[List[(Char, String)]] = (lift("(" ) ~> contents <~ lift(")")).flatMap { case (s, m) => get.flatMap( names => put(names.tail).map(_ => (names.head -> s) :: m) ) } def contents: ESP[(String, List[(Char, String)])] = lift("\\d+".r ^^ (_ -> Nil)) | rep1(paren).map( ps => ps.map(_.head._1).mkString -> ps.flatten ) def parse(s: String, names: Stream[Char]) = parseAll(paren.eval(names), s).map(_.toMap) } This works, and it's not that much less concise than either the mutable state version or the Parsec version. But my ExtraStateParsers is ugly as sin—I don't want to try your patience more than I already have, so I won't include it here (although here's a link, if you really want it). I've had to write new versions of every Parser and Parsers method I use above for my ExtraStateParsers and ESP types (rep1, ~>, <~, and |, in case you're counting). If I had needed to use other combinators, I'd have had to write new state transformer-level versions of them as well. Is there a cleaner way to do this? I'd love to see an example of a Scalaz 7's state monad transformer being used to thread state through a parser, but Scala 6 or Haskell examples would also be useful.

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