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Planet Generation Plans




Hey all!

This time I'm going to write a little about my plans for generating planets. This entry won’t go into any specific details of the terrain generation but will be just a very high level overview of the basic framework. I don't think any of the ideas in here are new.. the procedural generation community are a very clever bunch indeed! I'm always amazed how when an idea pops into my head I'm able to find an existing article about it that goes into fine detail on the subject. Anyway let's crack on!

Heightmaps & Voxels

When it comes to generating terrain an easy first choice is to generate height maps and build the terrain mesh from that. You can get some good looking results quickly by overlaying some noise generation and tweaking things here and there.


Though these can give some nice results they have limitations. For example tunnels and overhanging terrain can’t be represented in a simple height map. You also have to choose a projection scheme to map a 2D height map onto a sphere.


There’s another approach to terrain generation using voxels and that’s what I’m aiming to use. With voxels you can define all sorts of complex terrain and you can define what’s hiding under the ground too - whether it's seams of coal, ore or underground rivers. Many games use voxel terrains to great effect such as the infamous Minecraft. Sign me up!

In Minecraft the voxels are arranged in a grid, keeping everything nice and simple.. until you want to model a whole planet. Then you’d get something like this:


Though that does look really cool, I don’t want my worlds to be great big cubes floating in space, so what do I do? There are all sorts of solutions to building a spherical world from a voxel grid, but they seem to be full of difficult space warping, caveats and rendering complications. I’d rather not deal with these kinds of complications, so I’m going to try a different approach.

Instead of arranging the voxels in a grid I’m planning to arrange them as points on a geodesic sphere like this (imagine the vertices are voxel points):


It’s like taking the space warping issues you’d need to do at the cubes edges and spreading it evenly across the entire surface of the sphere. Instead of it becoming a difficult edge case it becomes a constant low level annoyance and keeps things consistent at all times. It will make the mesh generation a little more fun later too ;)

Voxel Planet Generation

The plan is to use an icosahedron as a starting point. Each vertex here is a voxel. This can be considered the lowest possible Level Of Detail (LOD) of a planet.


The space is chopped up into tetrahedrons from the planet surface into its core. There is an extra vertex at the centre of the planet for this.


These tetrahedrons can be subdivided through to the core of the planet as required.


These illustrations are somewhat misleading though as this isn’t just generating a spherical mesh, but a bunch of voxels that could be empty space. The voxel points (vertices) hold information about what's actually in the space they occupy, whether it’s atmosphere, rock, magma etc. It is the job of the terrain generator to define what a voxel contains. I'm going to keep a clear divide between the planet voxel generation code and the terrain generation code this way.

I have some uncertainties on how to best manage the subdivisions of the planets voxels as required, but I’ll bash it out in a prototype.

Dynamic Changes

The game also needs to be able to make permanent changes to the terrain during play. Large explosions should create craters and I want them to persist. To do this I will need to be address the voxels and save state changes about them. I'm not 100% sure on the approach for this yet either. One train of thought is to basically create an algorithm for the voxel generation that that is able to assign each possibly generated vertex a unique 64bit number. That would have no waste and allow the maximum number of voxels, and some napkin math makes me believe it would have good detail on earth sized planets. Another approach could be some kind of hash of their XYZ or polar coordinates, though that will be more wasteful so it’d bring the total addressable voxels way below what a 64bit unique id could theoretically hold.

Ok that’s enough for now!





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I've had some success applying the usual cube-> sphere transformations to a cube where each face is a voxel shell with some pre-determined thickness. It's not terribly elegant, but it's easy to get up and running. Your tetrahedral approach might avoid some of the distortion and apparent grid - interested to see how it turns out.

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Very interesting.  I'll be curious to see how you realise the plan.

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Looks neat. In fact I'm working on almost the exact same thing :)  However I'm not sure you can divide tetrahedrons evenly. You can get four new ones on the corners and then you get this sort of odd center space.  In your picture it looks to me like you divided up the faces of the original tetrahedrons which you can do easily, however chopping up the insides might be tricky, however if you figure out a good way to do it, it would be way cool.

I decided to use prisms, just because you can make an octree out of them like you can a cube.  The down side is you can't go all the way to the center of your sphere and also the inner ones are a bit smaller than the outer ones. But if you build your geometry around some reasonable band of the world it shouldn't be a huge problem, and also it gets rid of the problem of geometry looking different in different corners of the world like you would get if you generated your world in a simple subdivided cube.

I had an old version working a few years back but it just used height-maps applied to a subdivided icosahedron based simplex-noise. However it did let you walk around a huge planet without generating any data in advance.

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19 minutes ago, Gnollrunner said:

However I'm not sure you can divide tetrahedrons evenly. You can get four new ones on the corners and then you get this sort of odd center space.

That odd sort of space in the centre is an octahedron. Decomposing the octahedorn is tricky, but not impossible

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1 hour ago, swiftcoder said:

That odd sort of space in the centre is an octahedron. Decomposing the octahedorn is tricky, but not impossible

Somehow the prisms still seem simpler to deal with though.

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You guys are spot on about dividing up the space. I didn't realise at first that tetrahedrons don't tessellate in 3D space, it would have been great if they did. The picture of the sphere where you can see inside is quite misleading isn't it! Gnollrunner I'm playing with some options now, which includes some with prisms, seems I may be going down the same route you did :) Do you have any links related to what you did?

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1 hour ago, nukomod said:

You guys are spot on about dividing up the space. I didn't realise at first that tetrahedrons don't tessellate in 3D space, it would have been great if they did. The picture of the sphere where you can see inside is quite misleading isn't it! Gnollrunner I'm playing with some options now, which includes some with prisms, seems I may be going down the same route you did :) Do you have any links related to what you did?

I really don't, I'm just kind of winging it.  I'm basically doing the same thing as marching cubes except with prisms. The hard part is really the LOD transitions.  I think surface nets or dual contouring does that better but then you get other issues you need to deal with. I haven't found any magic bullets. I'm using octrees with chunking such that all cells in a chunk that have any geometry must be at the same level AND neighboring chunks can at most have one level difference. Even with that, the edge conditions are a pain. I worked out an algorithm that seems to work but I'm still debugging all the cases. I have table look ups with pointers to little subroutines that handle various cases. It's a lot of code, but I'm tying to avoid a lot of ifs.

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On 6/19/2018 at 6:03 PM, nukomod said:

You guys are spot on about dividing up the space. I didn't realise at first that tetrahedrons don't tessellate in 3D space, it would have been great if they did. The picture of the sphere where you can see inside is quite misleading isn't it! Gnollrunner I'm playing with some options now, which includes some with prisms, seems I may be going down the same route you did :) Do you have any links related to what you did?

Yeah, I was doing the same kind of problem (I started with an octohedron, but...) and found the same problem. I don't have my notes at the moment, but I seem to remember splitting lengths into three (or maybe four?) rather than two and thus getting a vertex in the center, which IIRC solved it. Maybe you can experiment along those lines, I'd love to know what you end up with in either case!

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You could consider an more complex cube->sphere thingy, similar to the results a hexahedral meshing algorithm would produce, see this image on the left. The only advancement is that the interior does not tesselate to infinity, if you want destruction of whole planets for example. Increasing scale issue on the surface remains the same.


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5 hours ago, JoeJ said:

You could consider an more complex cube->sphere thingy, similar to the results a hexahedral meshing algorithm would produce, see this image on the left. The only advancement is that the interior does not tesselate to infinity, if you want destruction of whole planets for example. Increasing scale issue on the surface remains the same.


The first one looks similar to what I'm doing, except you start out with a cube so you end up with hexahedrons instead of prisms. In my case I won't have destruction of whole planets (no Death Star in my game :P) so I don't worry about the middle. You can easily use marching cubs, surface nets etc with this, but you sill have the same issues of LOD transition, chunking and so forth which in my experience is the hardest part.

I think if I wanted to support full destruction I might be tempted to just forget about tying the make voxel orientation uniform around the world and just build everything with regular matrix.

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Posted (edited)

Thinking of it my proposal is pretty stupid - using a simple subdividing cube grid projected to sphere has the same advantages at the core and the same number of singularities at the surface. (Ooops :) )


Edit: But there may be advantages to support Minecraft alike building eventually? Not sure - my world looks flat if i look outside the window and i'm happy with that :)

Edited by JoeJ

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Just got a look at my old handwritten notes (dear lord in heaven....). I completely forgot that I started experimenting on something you miiiight have a use for. Basically, treating existing objects as point clouds and converting anything near the player to visible graphics via a standard marching cubes algorithm. If there are a lot of gold points in a section, that section is depicted as a gold thingie, and so on.

Just a thought. I'll go into my padded cell now and count the silence.....

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as far as topology goes one method I've found that looks sort of neat is as follows:

1. create a random line (for sphere, hemisphere), random side of the line raise +1, other side of line -1.
2. repeat procecess until desired shape is achieved. (100-1000+)

 Problem with this though is that sometimes it leads to really unround, lopsided planets sometimes. Other than that it can look pretty dynamic. I did not come up with this idea, though, and I don't remember where I got it. Also may need to smooth or scale.

Edited by h8CplusplusGuru

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4 hours ago, h8CplusplusGuru said:

that looks like it exactly 😃

I implemented this years ago. The main problem I have with it is, it doesn't easily let you just generate local terrain at one point on the planet.  While it's procedural, you end up having to store everything rather than just keeping the stuff you need that's around the camera.  For me simplex noise works a lot better since you can easily throw away and regenerate local data as needed to whatever level you want, and also you can customize your functions locally in infinite ways. 

I suppose you could use it as a base function for which you store your terrain at a predetermined level and then apply simplex-nose stuff on top of it to fill in the details.

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      static void Main(string[] args) { // First we need to create the state machine. // Note that I'm using the abstract IStateMachine instead of a concrete class. IStateMachine machine = GetMachine(); // We have a machine, now run it. while(!machine.IsComplete()) { // Print out our current state Console.WriteLine("Currently in " + machine.CurrentState); machine.CurrentState.Run(); // Print out our possible transitions Console.WriteLine("\nAvailable choices are:"); string[] transitions = machine.PossibleTransitions(); foreach (string item in transitions) { Console.WriteLine(" " + item); } // Request a transition from the user Console.WriteLine("\nWhat do you want to do?"); string nextState = Console.ReadLine(); machine.Advance(nextState); } // And we're done! // Run our final node as a special case since the above loop won't do it. Console.WriteLine("Currently in " + machine.CurrentState); machine.CurrentState.Run(); // Finish off. Console.WriteLine("\n\nPress any key to continue."); Console.ReadKey(true); } That is all we need, and we've got the framework for a text-based dungeon explorer game.
      A Boring State Machine
      Just to prove out the above code I created a simple machine. Because it doesn't do anything fun or exciting I called it the BoringMachine.
      In this machine, I chose to make a single state and then caused the state to modify itself as the machine moves around it. This state machine implements the interface I described above, but that interface requires you to state the transition name. In this case, I just ignore the name of the state and advance along a fixed route. Remember that this is just a proof of concept of the state machine runner, it doesn't do anything fancy. It is like the state machine above with states labelled 0, 1, 2, and 3 --- it doesn't look like much yet.
      class BoringMachine : IStateMachine { BoringMachineState mState = new BoringMachineState(); public override IState CurrentState { get { return mState; } } public override string[] PossibleTransitions() { // For this simple example, forward it on to the state return mState.ListTransitions(); } public override bool Advance(string nextState) { Console.WriteLine("I'm a boring state machine. I don't care what you entered. Advancing state."); return mState.Advance(); } public override bool IsComplete() { // For the simple example, forward it on to the state return mState.IsComplete(); } } class BoringMachineState : IState { #region Members internal enum SameOldStates { Enter, DoStuff, Exiting, Done } SameOldStates mState = SameOldStates.Enter; #endregion #region IState overrides public override string GetName() { return mState.ToString(); } public override void Run() { // Do nothing. This is the game logic. } #endregion #region Helper functions public bool IsComplete() { return mState == SameOldStates.Done; } public string[] ListTransitions() { List result = new List(); switch (mState) { case SameOldStates.Enter: result.Add("DoStuff"); break; case SameOldStates.DoStuff: result.Add("Exiting"); break; case SameOldStates.Exiting: result.Add("Done"); break; case SameOldStates.Done: result.Add("Done"); break; } return result.ToArray(); } public bool Advance() { switch (mState) { case SameOldStates.Enter: mState = SameOldStates.DoStuff; break; case SameOldStates.DoStuff: mState = SameOldStates.Exiting; break; case SameOldStates.Exiting: mState = SameOldStates.Done; break; case SameOldStates.Done: // Do nothing. break; } return true; } #endregion } At this point, I can feed that machine into the runner and it works. Sometimes that is the biggest accomplishment.
      Let's Make a Game
      Now that I have proved that the state machine runner is up to the task, I create another state machine implementation. This is one of the nice things about writing to an interface: I can create many variations easily. I'll call this one FunMachine. It derives from IState and implements that simple interface. The state machine will represent a map to explore. Each state will represent the room. Here's that code:
      class FunMachineState : IState { string mName; string mDescription; List mNeighbors = new List(); public List Neighbors { get { return mNeighbors; } } /// /// Initializes a new instance of the FunnerState class. /// ///Name to display for this state ///Text to display for this state public FunMachineState(string mName, string mDescription) { this.mName = mName; this.mDescription = mDescription; } #region IState Overrides public override string GetName() { return mName; } public override void Run() { // We don't do any fancy stuff, just print out where we are Console.WriteLine(); Console.WriteLine(mDescription); } #endregion }  
      Most of the State Machine
      Remember that we can implement a state machine in many ways.

      In this example, the machine will point to the current state, and during transitions will move to the next state. This source listing is going to include everything EXCEPT the state machine constructor. I'll go over that in more detail in the next section. The machine implements the same IStateMachine interface.
      It also adds three machine-specific values: A list of states, a link to the current state, and a link to the exit state (which we will eventually remove). Two things to note are how it gets the list of possible transitions and how it advances. We look at the names of the current node's neighbors --- in this case, the name of a room on the map. We advance by making sure we can only travel to neighboring rooms. That avoids the exploit of typing "move Exit" and winning the game in the first step.
      class FunMachine : IStateMachine { List mStates; FunMachineState mCurrent; FunMachineState mExit; /// CONSTRUCTOR NOT SHOWN #region IStateMachine Overrides public override IState CurrentState { get { return mCurrent; } } public override string[] PossibleTransitions() { List result = new List(); foreach (FunMachineState state in mCurrent.Neighbors) { result.Add(state.GetName()); } return result.ToArray(); } public override bool Advance(string nextState) { foreach (FunMachineState state in mCurrent.Neighbors) { if (nextState == state.GetName()) { mCurrent = state; return true; } } System.Console.WriteLine("Invalid state."); return false; } public override bool IsComplete() { return mCurrent == mExit; } #endregion }
      Again, this should seem straightforward if you've been following along. We've got the state machine in just a few lines of code.
      On to the Constructor
      This state machine is still hard-coded. In order to generate the maze, we need to manually construct each room (aka FSM state, or vertex, or node). We also need to construct each transition (aka edge) of the state machine.
      public FunMachine() { // Create all the fun states in our mini-world FunMachineState entryHall = new FunMachineState("Grand Entrance", "You are standing in a grand entrance of a castle.\nThere are tables and chairs, but nothing you can interact with."); FunMachineState staircase = new FunMachineState("Grand Staircase", "The staircase is made from beautiful granite."); FunMachineState eastWing = new FunMachineState("East Wing", "This wing is devoted to bedrooms."); FunMachineState westWing = new FunMachineState("West Wing", "This wing is devoted to business."); FunMachineState bedroomA = new FunMachineState("Master Suite", "This is the master suite. What a fancy room."); FunMachineState bedroomB = new FunMachineState("Prince Bob's Room", "The prince has an extensive library on his wall.\nHe also has more clothes than most males know what to do with."); FunMachineState bedroomC = new FunMachineState("Princess Alice's Room", "The princess has filled her room with a small compur lab.\nShe spends her days playing games and writing code."); FunMachineState workroomA = new FunMachineState("Study", "This is the study. It has many books."); FunMachineState workroomB = new FunMachineState("Bathroom", "Every home needs one"); FunMachineState workroomC = new FunMachineState("Do Not Enter", "I warned you not to enter.\nYou are in a maze of twisty little passages, all alike."); FunMachineState passage = new FunMachineState("Twisty Passage", "You are in a maze of twisty little passages, all alike"); mExit = new FunMachineState("Outside", "You have successfully exited the castle."); // Hook up doors. entryHall.Neighbors.Add(staircase); entryHall.Neighbors.Add(mExit); staircase.Neighbors.Add(eastWing); staircase.Neighbors.Add(westWing); staircase.Neighbors.Add(entryHall); eastWing.Neighbors.Add(bedroomA); eastWing.Neighbors.Add(bedroomB); eastWing.Neighbors.Add(bedroomC); eastWing.Neighbors.Add(staircase); bedroomA.Neighbors.Add(eastWing); bedroomB.Neighbors.Add(eastWing); bedroomC.Neighbors.Add(eastWing); westWing.Neighbors.Add(workroomA); westWing.Neighbors.Add(workroomB); westWing.Neighbors.Add(workroomC); workroomA.Neighbors.Add(westWing); workroomB.Neighbors.Add(westWing); // Trap of doom. workroomC.Neighbors.Add(passage); passage.Neighbors.Add(passage); // Add them to the collection mStates = new List(); mStates.Add(entryHall); mStates.Add(staircase); mStates.Add(eastWing); mStates.Add(westWing); mStates.Add(bedroomA); mStates.Add(bedroomB); mStates.Add(bedroomC); mStates.Add(workroomA); mStates.Add(workroomB); mStates.Add(workroomC); mStates.Add(passage); mStates.Add(mExit); // Finally set my starting point mCurrent = entryHall; }  
      This creates a fun little graph. In case you're having trouble visualizing it, the picture version looks like this:

      It has rooms. It has a text adventure with a death trap room and a route to victory. It is starting to look like a real game.
      Run the Example Code
      If you haven't gone through the executable yet, now would be a great time to do it. You can explore this little text-based world. It doesn't have a plot, it doesn't have inventory items or any action other than moving, but it demonstrates a part of the game.
      Making it Data Driven
      That term shows up a lot in games: Data Driven.
      What does it mean? So far if I wanted to make changes I needed to modify the source code, recompile, and test the app. For a small castle map and a single developer, this is not a hard thing. But what happens when the app grows?
      Let's imagine I hire someone to design my levels. That person is not a programmer. I don't want them touching my C# files, and I don't want to teach them to read and write C#. So what should I do?
      Simple: I create a save file that contains all the information describing the level. I can allow non-programmers to work on my game by loading data at runtime. Data-driven means level designers can modify rooms, put different things in different locations, and otherwise improve the game without touching the code. It means another programmer can implement game objects like ropes and bottles and water without touching the game code. It means that if we were making a graphical game, artists could create new art, modellers could create new models, and animators could modify animations, all without touching the code. Data-driven means that only programmers need to touch the code. Everyone else uses data to modify the game. Fancy engines will implement ways to reload data while the game is running. Designers and artists and animators and modellers can iterate on their work much faster, potentially saving months of development time.
      Making My Dungeon Data Driven
      The states are only slightly modified from last time. Last time I stored the state's name, description, and neighbors.
      This time I add a unique name key and a set of flags. The flags indicate if the node is the Enter node (there is only one) or if the node is an exit node. Next I added the functions ReadXml and WriteXml. These two functions save and load my five elements (unique name, flags, visible name, description, and neighbors) into an XML file. Because it is basically free I chose to implement them using the IXmlSerializable interface. Someday when I'm feeling ambitious I can extend future components to also use the C# serialization routines to automatically handle my data. Since the state machine will need to create these objects from XML, I create a second constructor that takes an XmlReader and pass that on to the ReadXml function.
      Finally, I added some accessors and mutators (get and set functions) to help out the state machine.
      public class SavedMachineState : IState, IXmlSerializable { #region Members [Flags] public enum StateFlags { None = 0, Enter = 1, Exit = 2, } public string mKey; StateFlags mFlags; string mName; string mDescription; List mNeighbors = new List(); public List Neighbors { get { return mNeighbors; } } #endregion #region Constructors /// /// Manual constructor for default maze /// ///unique name for the stateFlags ///flags to indicate enter nodes and exit nodes ///name to show to the user ///text to show for the description ///unique keys for neighboring rooms, seperated by commas and not spaces public SavedMachineState(string uniqueKey, StateFlags flags, string name, string description, string neighbors) { mKey = uniqueKey; mFlags = flags; mName = name; mDescription = description; mNeighbors.AddRange(neighbors.Split(',')); } /// /// Constructor to create an object from a save file /// ///xml stream to read from public SavedMachineState(XmlReader reader) { ReadXml(reader); } #endregion #region Helper Functions public bool IsStartState { get { return (mFlags & StateFlags.Enter) != StateFlags.None; } } public bool IsExitState { get { return (mFlags & StateFlags.Exit) != StateFlags.None; } } public string Key { get { return mKey; } } public bool IsMyName(string nameToTest) { //TODO: Add shortcuts to names. For example, allow "Great Hall", "Hall", etc. if (nameToTest.ToLower() == mName.ToLower()) return true; if (nameToTest.ToLower() == mKey.ToLower()) return true; return false; } #endregion #region IState Overrides public override string GetName() { return mName; } public override void Run() { // We don't do any fancy stuff, just print out where we are Console.WriteLine(); Console.WriteLine(mDescription); } #endregion #region IXmlSerializable Members public System.Xml.Schema.XmlSchema GetSchema() { return null; } public void ReadXml(System.Xml.XmlReader reader) { reader.ReadStartElement(); mKey = reader.ReadElementContentAsString("UniqueName",""); string flagString = reader.ReadElementContentAsString("Flags",""); mFlags = (StateFlags)Enum.Parse(typeof(StateFlags), flagString); mName = reader.ReadElementContentAsString("VisibleName", ""); mDescription = reader.ReadElementContentAsString("Description", ""); string neighborsString = reader.ReadElementContentAsString("Neighbors", ""); mNeighbors.AddRange(neighborsString.Split(',')); reader.ReadEndElement(); } public void WriteXml(System.Xml.XmlWriter writer) { writer.WriteElementString("UniqueName", mKey); writer.WriteElementString("Flags", mFlags.ToString()); writer.WriteElementString("VisibleName", mName); writer.WriteElementString("Description", mDescription); string neighbors = String.Join(",",Neighbors.ToArray()); writer.WriteElementString("Neighbors",neighbors); } #endregion }  
      If that was the first exposure to the code it might be a little intimidating. But since we've been slowly adding to it over time, you should see that it is only a minor incremental change.
      State Machine Changes
      Changes to the state machine were a little more dramatic. The exit node information is now contained in the data, so I can drop the mExit state I mentioned earlier. For convenience, I moved the map construction code from a constructor to its own function: GenerateDefaultMap(). It allows us to generate and save a map when bootstrapping the toolchain. The constructor calls ImportFromXml() instead. If that fails we generate the default map, save a copy with ExportToXML(), and then reload our newly created map. ExportToXML() creates an XML writer, loops through the states, and writes each state out using the WriteXml() function. ImportFromXML() creates an XML reader and reads the file in through the corresponding ReadXml() function.
      Here's the modified code:
      public class SavedMachine : IStateMachine, IXmlSerializable { #region Members List mStates = new List(); SavedMachineState mCurrent; #endregion #region Constructor /// /// Initializes a new instance of the FunnerMachine class. /// public SavedMachine() { try { ImportFromXML(); } catch (Exception ex) { mStates.Clear(); } if (mStates.Count == 0) { GenerateDefaultMap(); ImportFromXML(); } // Find the entry state for (int i = 0; i < mStates.Count; i++) { if (mStates.IsStartState) { mCurrent = mStates; break; } } if (mCurrent == null) { Console.WriteLine("\n\nERROR! NO ENTRY STATE DEFINED."); throw new Exception("No entry state defined in this state machine. Cannot continue."); } } #endregion #region Helper Functions private void GenerateDefaultMap() { mStates.Clear(); // Create all the fun states in our mini-world mStates.Add(new SavedMachineState("entryHall", SavedMachineState.StateFlags.Enter, "Grand Entrance", "You are standing in a grand enterance of a castle.\nThere are tables and chairs, but nothing you can interact with.", "staircase,outside")); mStates.Add(new SavedMachineState("staircase", SavedMachineState.StateFlags.None, "Grand Staircase", "The staircase is made from beautiful granite.", "eastWing,westWing,entryHall")); mStates.Add(new SavedMachineState("eastWing", SavedMachineState.StateFlags.None, "East Wing", "This wing is devoted to bedrooms.", "bedroomA,bedroomB,bedroomC,staircase")); mStates.Add(new SavedMachineState("westWing", SavedMachineState.StateFlags.None, "West Wing", "This wing is devoted to business.", "workroomA,workroomB,workroomC")); mStates.Add(new SavedMachineState("bedroomA", SavedMachineState.StateFlags.None, "Master Suite", "This is the master suite. What a fancy room.", "eastWing")); mStates.Add(new SavedMachineState("bedroomB", SavedMachineState.StateFlags.None, "Prince Bob's Room", "The prince has an extensive library on his wall.\nHe also has more clothes than most males know what to do with.", "eastWing")); mStates.Add(new SavedMachineState("bedroomC", SavedMachineState.StateFlags.None, "Princess Alice's Room", "The princess has filled her room with a small compur lab.\nShe spends her days playing games and writing code.", "eastWing")); mStates.Add(new SavedMachineState("workroomA", SavedMachineState.StateFlags.None, "Study", "This is the study. It has many books.", "westWing")); mStates.Add(new SavedMachineState("workroomB", SavedMachineState.StateFlags.None, "Bathroom", "Every home needs one", "westWing")); mStates.Add(new SavedMachineState("workroomC", SavedMachineState.StateFlags.None, "Do Not Enter", "I warned you not to enter.\nYou are in a maze of twisty little passages, all alike.", "passage")); mStates.Add(new SavedMachineState("passage", SavedMachineState.StateFlags.None, "Twisty Passage", "You are in a maze of twisty little passages, all alike", "passage")); mStates.Add(new SavedMachineState("outside", SavedMachineState.StateFlags.Exit, "Outside", "You have successfully exited the castle.", "")); ExportToXML(); } public void ExportToXML() { XmlWriterSettings settings = new XmlWriterSettings(); settings.Indent = true; settings.OmitXmlDeclaration = true; settings.NewLineHandling = NewLineHandling.Entitize; using (XmlWriter writer = XmlWriter.Create("GameRooms.xml",settings)) { writer.WriteStartDocument(); writer.WriteStartElement("SavedMachine"); WriteXml(writer); writer.WriteEndElement(); writer.WriteEndDocument(); } } public void ImportFromXML() { XmlReaderSettings settings = new XmlReaderSettings(); settings.IgnoreWhitespace = true; XmlReader reader = XmlReader.Create("GameRooms.xml", settings); ReadXml(reader); } #endregion #region IStateMachine Overrides public override IState CurrentState { get { return mCurrent; } } public override string[] PossibleTransitions() { List result = new List(); foreach (string state in mCurrent.Neighbors) { result.Add(state); } return result.ToArray(); } public override bool Advance(string nextState) { foreach (SavedMachineState state in mStates) { if(state.IsMyName(nextState) && mCurrent.Neighbors.Contains(state.Key)) { mCurrent = state; return true; } } System.Console.WriteLine("Cannot do that."); return false; } public override bool IsComplete() { return mCurrent.IsExitState; } #endregion #region IXmlSerializable Members public System.Xml.Schema.XmlSchema GetSchema() { return null; } public void ReadXml(XmlReader reader) { bool isEmpty = reader.IsEmptyElement; reader.ReadStartElement(); if (isEmpty) return; while (reader.NodeType == XmlNodeType.Element) { if (reader.Name == "Room") { mStates.Add(new SavedMachineState(reader)); } else throw new XmlException("Unexpected node: " + reader.Name); } reader.ReadEndElement(); } public void WriteXml(XmlWriter writer) { foreach (SavedMachineState state in mStates) { writer.WriteStartElement("Room"); state.WriteXml(writer); writer.WriteEndElement(); } } #endregion } Not too bad for an incremental change.
      Run the Game and Generate Bootstrap Data
      Now when I run the game it attempts to load the save file. It cannot find one so it generates a new GameRooms.xml data file. Then it plays the same dungeon explorer code as before. Since I am bootstrapping my tools, I need to jump straight into the generated xml:
      entryHall Enter Grand Entrance You are standing in a grand enterance of a castle. There are tables and chairs, but nothing you can interact with. staircase,outside staircase None Grand Staircase The staircase is made from beautiful granite. eastWing,westWing,entryHall eastWing None East Wing This wing is devoted to bedrooms. bedroomA,bedroomB,bedroomC,staircase westWing None West Wing This wing is devoted to business. workroomA,workroomB,workroomC bedroomA None Master Suite This is the master suite. What a fancy room. eastWing bedroomB None Prince Bob's Room The prince has an extensive library on his wall. He also has more clothes than most males know what to do with. eastWing bedroomC None Princess Alice's Room The princess has filled her room with a small compur lab. She spends her days playing games and writing code. eastWing workroomA None Study This is the study. It has many books. westWing workroomB None Bathroom Every home needs one westWing workroomC None Do Not Enter I warned you not to enter. You are in a maze of twisty little passages, all alike. passage passage None Twisty Passage You are in a maze of twisty little passages, all alike passage outside Exit Outside You have successfully exited the castle.  
      I can look it over and verify that it is our original map, saved out in XML format. To prove that the system actually works, we can make some minor modifications to the map:
      This requires a tiny modification to the entrance hall (pointing the neighbor to 'courtyard') and making three new rooms. A few seconds in a text editor, copy/paste, a little wordsmithing, and I get this addition to the save file:
      courtyard None Courtyard The courtyard is decorated with many large trees and several marble benches. entryHall,townGate townGate None Town Gate You arrive at the gate of the town. Ahh, to be home again.\n\nNOTICE: The guards will not let you return to the castle if you leave. courtyard,village village Exit Quaint Village You return to your village. You won't soon forget your experiences in the castle. I fire up the game, and can quickly verify that I have altered the map just by changing data. Someone could now write a simple tool that allows a map designer to visualize dungeons. Or they could just let the map designer work with the raw XML files. Either way, the map is now data driven --- it can be modified entirely by data without any work from the programmer.
      Always Room for Improvement
      There are many more things we could do with this simple example. The first thing I would do is create objects that can be placed in rooms. These are simple state machines in themselves. For example, I could have a bucket object that has several states: Empty, Water, WaterAndFish. Events could be added (again with a simple state machine) that tracks your progress through a quest and grants points as you complete objectives. Those are nice things to have in a game, but in this case, they don't add anything beyond what was just demonstrated. That can be left as an exercise for the reader.
      A More Complex Set of Machines
      Let's move on to a more complex topic. Every game I've worked on has used an AI system of some form. There are actors and objects, and the actors do something. Often the actors use objects. For this demo, I created the following structure:

      The game container is a playing field. It contains a collection of GameObject instances. The playing field gets regular updates at about 30 frames per second, and each update gets passed along to the individual objects. There are two types of game objects: Pets and Toys. These to objects work together using activities. Now to go over each of these in detail.
      The Base GameObject Class
      A game object fits in with the state machines. They serve as both state machines AND as state nodes. Remember from earlier that a state machine is a concept. The implementation details don't matter when the concept is intact. It has an Update(), which means to run the current state, and also to advance the current state if necessary. We'll expand on this a little later. The GameObject represents any object we can place on our game board. They have an owner (in this case, the playing field). They have a location. They have an image. For convenience they have a ToString() override that makes things look nicer when I view them in a property grid. Also during development, they have evolved to have a PushToward() and a MaxSpeed() method. These would probably be integrated into a physics system or a collision system, but for now, this is their best natural fit.
      public abstract class GameObject { public PlayingField Owner { get; set; } /// /// Initializes a new instance of the GameObject class. /// public GameObject(PlayingField owner) { Owner = owner; } /// /// Update the object /// ///seconds since last update. /// seconds is the easiest scale for the individual settings public abstract void Update(float seconds); /// /// Location on the playing field to draw the actor /// public PointF Location { get; set; } /// /// What to draw on the playing field /// public abstract Image Image { get; } /// /// Push the game object toward a location. Default behavior is to not move. /// ///Location to push toward ///Seconds that have passed in this movement public virtual void PushToward(PointF destination, float seconds) { return; } /// /// Get the maximim speed of this game object. Default behavior is not to move. /// /// public virtual float MaxSpeed() { return 0; } /// /// Simplified name for the object to display in the property browser /// /// Shorter name public override string ToString() { string classname = base.ToString(); int index = classname.LastIndexOf('.'); string shortname = classname.Substring(index+1); return shortname; } }  
      All game objects need to implement the interface.
      Pets and Motives
      The basic pet class is pretty simple. A pet is a game object (so it gets everything above), plus it also gets an activity and a collection of motives. The motives are nothing more than a wrapper for the pet's status. In this case, we are only tracking fun and energy. (Note for comparison in The Sims3 there are 8 visible motives - hunger, social, bladder, hygiene, energy, and fun.) When a pet is created we default them to the Idle activity and initialize their Motives. We have a default update behavior to run whatever activity we are currently doing, or if we aren't doing anything to create a new idle activity and do that instead. We'll also implement what it means to push a pet.
      public class MotiveBase { public float Fun { get; set; } public float Energy { get; set; } } public abstract class Pet : GameObject { public MotiveBase Motives { get; set; } public Activities.Activity Activity { get; set; } /// /// Initializes a new instance of the Pet class. /// public Pet(PlayingField owner) : base(owner) { Activity = new Activities.Idle(this, null); Motives = new MotiveBase(); } /// /// Allow a pet to do something custom on their update /// /// protected virtual void OnUpdate(float seconds) { return; } public override void Update(float seconds) { if (Activity == null) { Activity = new Activities.Idle(this, null); } Activity.Update(seconds); } public override void PushToward(System.Drawing.PointF destination, float seconds) { base.PushToward(destination, seconds); // TODO: Someday accumulate force and make a physics system. Just bump it the correct direction. // TODO: Create a vector class someday float xDiff = destination.X - Location.X; float yDiff = destination.Y - Location.Y; float magnitude = (float)Math.Sqrt(xDiff * xDiff) + (float)Math.Sqrt(yDiff * yDiff); if (magnitude > (MaxSpeed() * seconds)) { float scale = (MaxSpeed() * seconds) / magnitude; xDiff *= scale; yDiff *= scale; } Location = new PointF(xDiff + Location.X, yDiff + Location.Y); } } There are a few TODO: markers in the code, but the interface work. Remember the Dependency Inversion Principle mentioned before: Program against an interface or abstract base class, don't program against the individual concrete classes.
      Finally, we get to create a concrete class. A puppy. Note that we're just pulling the values from saved resources so a designer and artist can modify them later.
      class Puppy : Pet { /// /// Initializes a new instance of the GameObject class. /// public Puppy(PlayingField owner) : base(owner) { } public override System.Drawing.Image Image { get { return FSM_Puppies.Properties.Resources.Puppy; } } public override float MaxSpeed() { return FSM_Puppies.Properties.Settings.Default.Pet_Puppy_MaxSpeed; } } Yup, all that work and we only get a tiny little concrete class. That is actually a very good thing. It means that when we want to extend it later for different kinds of puppies, kitties, horses, and other pets, we only need to add a tiny bit of code to hook up the new actors with their new data.
      A toy is also a game object, so it can behave as a state machine and as a state node, as appropriate. A toy has a default activity associated with it. When a pet attempts to use a toy they will get this default activity (aka behavior tree) and start running it. A toy is also responsible for computing the interest level in the object. For now, these will just be hard-coded formulas inside each toy object. Later on, these could be a more complex series of interactions but for this system it is adequate. Here's the Toy abstract class:
      public abstract class Toy : GameObject { /// /// Initializes a new instance of the Toy class. /// public Toy(PlayingField owner) : base(owner) { } public abstract Activities.Activity DefaultActivity(Pets.Pet actor, GameObject target); public abstract float Interest(Pets.Pet pet); public override void Update(float seconds) { // Note that most toys do nothing of themselves. They are driven by their activities. return; } }  
      Two Toys
      Now we'll create two concrete classes for toys. First, a sleeping mat. The interest of the sleeping mat is only based on energy. It has an image to draw. The default activity is to sleep on the mat.
      class SleepingMat : Toy { /// /// Initializes a new instance of the SleepingMat class. /// public SleepingMat(PlayingField owner) : base(owner) { } public override FSM_Puppies.Game.Activities.Activity DefaultActivity(Pets.Pet actor, GameObject target) { return new Activities.SleepOnMat(actor, this); } public override System.Drawing.Image Image { get { return FSM_Puppies.Properties.Resources.SleepingMat; } } public override float Interest(FSM_Puppies.Game.Pets.Pet pet) { return MaxEnergy() - pet.Motives.Energy; } }  
      Second, a ball to kick around. The interest is only based on fun, although it probably should include an energy component. It has an image to draw, and the default activity is to chase the ball.
      class Ball : Toy { /// /// Initializes a new instance of the Ball class. /// public Ball(PlayingField owner) : base(owner) { } public override Image Image { get { return FSM_Puppies.Properties.Resources.Ball; } } public override Activities.Activity DefaultActivity(Pets.Pet actor, GameObject target) { return new Activities.ChaseBall(actor, target); } public override float Interest(FSM_Puppies.Game.Pets.Pet pet) { return MaxFun() - pet.Motives.Fun; } } Now we move on to the activities that drive the system.
      Activities Are Both Glue and Oil
      Activities serve as glue to the system. They are the interactions between actors and objects. Without them there wouldn't be much of a connection between the two. Activities also serve as the oil that keeps the parts moving smoothly. They are constantly moving. They change themselves, and they change the actors they work with, and they can change the objects they work with. A more complex example of a food bowl could change the actor by modifying hunger, and also change the target by reducing the amount of food in the bowl.
      So here is our activity base class. An activity has an Actor and a Target. I intentionally limited Actors to be pets. I could have allowed any object to interact with any object, but that doesn't quite make sense in practice. We don't really want a food bowl to interact with a chew toy, or a ball to interact with a sleeping mat. We DO want to allow a pet to be a target allowing default activities to play social events. For example, pets could dance together or sniff each other or do whatever groups of pets do together. We allow an Update event on the activity base. This update is run by the pet earlier. We pass that on through the OnUpdate callback in each activity. If the activity returns true then we know it is complete and the pet needs to find something new to do.
      Finally, we have a magical function, FindBestActivity() that needs to live somewhere in the code. I could have created another class for it, but for now, this is the best spot.  This FindBestActivity is the magic that makes the AI do fun things. In this example, it is only 35 lines. We loop over all the toys in the game world and see how interesting they are. Then we take the best interaction and return a new instance of it. If we fail we just return the idle activity. For a game like The Sims, there are potentially tens of thousands of objects to choose from, and each object can have many activities associated with it. Finding the best activity among them all is a complex job. The theory behind it is no different: Find the best activity, and create an instance of it.
      public abstract class Activity { public Pets.Pet Actor { get; set; } public GameObject Target { get; set; } /// /// Initializes a new instance of the Activity class. /// public Activity(Pets.Pet actor, GameObject target) { Actor = actor; Target = target; } /// /// Update this activity state /// /// elapsed time /// true if the activity is complete public abstract bool OnUpdate( float seconds ); /// /// Update this activity state /// /// elapsed time public void Update(float seconds) { if(OnUpdate(seconds)) { Actor.Activity = new Idle(Actor, null); } } /// /// Utility function to locate the best next activity for the actor. /// /// public static Activity FindBestActivity(Pets.Pet actor) { // Look for a toy to play with... if (actor.Owner != null && actor.Owner.GameObjects != null) { List candidates = new List(); foreach (GameObject obj in actor.Owner.GameObjects) { Toys.Toy t = obj as Toys.Toy; if (t != null) { candidates.Add(t); } } if (candidates.Count > 0) { float bestScore = float.MinValue; Toys.Toy bestToy = null; foreach (Toys.Toy t in candidates) { float myscore = t.Interest(actor); if(myscore>bestScore) { bestScore = myscore; bestToy = t; } } return bestToy.DefaultActivity(actor, bestToy); } } return new Idle(actor, null); } public override string ToString() { string classname = base.ToString(); int index = classname.LastIndexOf('.'); string shortname = classname.Substring(index + 1); return shortname; } } Now we know what an activity is. Basically just another state machine.
      Idle Activity
      We'll start with the idle activity. It has an idle time. After enough time has passed we look for something new to do. This new activity will replace our current idle activity. If we don't find anything interesting to do we can just sit there, slowly dropping our fun and our energy. Since this is C# we don't need to schedule cleanup and deletion of our own idle activity which simplifies our code quite a lot.
      class Idle : Activity { float mTimeInIdle = 0; public Idle(Pets.Pet actor, GameObject target) : base(actor, target) { } public override bool OnUpdate(float seconds) { mTimeInIdle += seconds; if (mTimeInIdle >= FSM_Puppies.Properties.Settings.Default.Activity_Idle_WaitingTime) { Actor.Activity = FindBestActivity(Actor); } // Sitting there idle isn't much fun and slowly decays energy. This encourages us to pick up other activiites. Actor.Motives.Fun += FSM_Puppies.Properties.Settings.Default.Activity_Idle_Fun * seconds; Actor.Motives.Energy += FSM_Puppies.Properties.Settings.Default.Activity_Idle_Energy * seconds; // Always return false because idle is never finished. It auto-replaces if it can find something. return false; } }  
      ChaseBall Activity
      This is actually TWO activities. It is a miniature state machine within itself. Chasing a ball has one component "RunToObject", and then a second component where they actually kick the ball. It isn't a difficult state machine, just two states, and can be represented directly with a simple if statement. That is a good time for this reminder:
      So every update we attempt to run to the ball object. If we succeeded in running to the object, we kick the ball a random distance. We also bump fun a little bit whenever they kick the ball. An activity's return result indicates when it is complete. We'll return true only when our fun is maxed out. We might want to have a second exit condition when energy runs low, but that is for later.
      class ChaseBall : Activity { RunToObject mRto; /// /// Initializes a new instance of the ChaseBall class. /// public ChaseBall(Pets.Pet actor, GameObject target) : base(actor, target) { mRto = new RunToObject(actor, target); } public override bool OnUpdate(float seconds) { // When they kick the ball, move it to a new location and continue our activity. if( mRto.OnUpdate(seconds)) { float kickDistance = FSM_Puppies.Properties.Settings.Default.Activity_ChaseBall_KickDistance; // Get a random number with +/- kick distance float newX = Target.Location.X + (((float)Target.Owner.Rng.NextDouble()*(2*kickDistance))-kickDistance); float newY = Target.Location.Y + (((float)Target.Owner.Rng.NextDouble()*(2*kickDistance))-kickDistance); PointF randomLocation = new PointF(newX,newY); Target.Location = randomLocation; Actor.Motives.Fun += FSM_Puppies.Properties.Settings.Default.Toy_Ball_Fun; if(Actor.Motives.Fun > 100) return true; } return false; } }  
      RunToObject Activity
      Next, we'll look at how they run to an object. It is pretty simple. If we are close enough (another designer-adjustable value) then they have made it to the object and we return true. If they are not there yet we push them toward the object, drop their energy, and return false (we aren't done with the activity yet).
      class RunToObject : Activity { /// /// Initializes a new instance of the RunToObject class. /// public RunToObject(Pets.Pet actor, GameObject target) : base(actor, target) { } public override bool OnUpdate(float seconds) { // Are we there yet? // And why didn't PointF implement operator-() ? PointF offset = new PointF( Target.Location.X - Actor.Location.X, Target.Location.Y - Actor.Location.Y); float distanceSquared = offset.X * offset.X + offset.Y * offset.Y; float closeEnough = FSM_Puppies.Properties.Settings.Default.Activity_RunToObject_CloseEnough; float closeEnoughSquared = closeEnough * closeEnough; if (distanceSquared < closeEnoughSquared) return true; Actor.PushToward(Target.Location, seconds); Actor.Motives.Energy += FSM_Puppies.Properties.Settings.Default.Activity_RunToObject_Energy * seconds; return false; } }  
      Sleeping On the Mat
      Just like chasing a ball, we start out by running to the object. So first we call RunToObject. If it succeeds (meaning we finally got there), then we start resting. We bump the motives, return true or false based on our energy status.
      class SleepOnMat : Activity { RunToObject mRto; /// /// Initializes a new instance of the SleepOnMat class. /// public SleepOnMat(Pets.Pet actor, GameObject target) : base(actor, target) { mRto = new RunToObject(actor, target); } public override bool OnUpdate(float seconds) { // Route to the sleeping mat if(mRto.OnUpdate(seconds)) { // Now that we are on the mat, just sit here and increase our motives. Actor.Motives.Energy += FSM_Puppies.Properties.Settings.Default.Toy_SleepingMat_Energy; Actor.Motives.Fun += FSM_Puppies.Properties.Settings.Default.Toy_SleepingMat_Fun; if (Actor.Motives.Energy > 100) return true; } return false; } }  
      Proof of Concept is Complete
      And... we're done. Go ahead and play with the sample now. Drop multiple play mats, multiple balls, and multiple puppies. Watch them run around chasing balls, and when they get tired watch them run over and sleep on the mat. Pick them from the list on the left-hand pane to view details in the little property grid. It is not much, but it is enough to see a sandbox game starting to grow. I showed my children and they immediately made a list of new objects to add. Add a kitty! Add a teeter-totter! Add a tetherball! (Really?) Repeat this around thirty times with new objects, add a few motives, and you can have your own self-running simulated pet world. Focus on a few of them to create minigames, and drop some spawn points in the world. Repeat until the game is complete.
      But I Wanted a First Person Shooter
      That is solved very easily: Rename "Pets" to "Monsters", and "Puppy" to "Grunt". Rename "Toys" to "WayPoint", "Ball" to "Flamethrower", and "SleepingMat" to "SpawnPoint". Finally, rename activities as appropriate.
      Wrapping It Up
      So we learned that state machines are the least powerful of the automata in computer science. We also learned that they are used everywhere in our daily lives, and can do incredible things for games. We can use state machines to run different code based on state. We can use state machines to represent arbitrary meshes. These meshes can include things like maps and connectivity grids. We don't need to hard code the state machines. When you load them from data you can get substantially more use out of them. Designers and artists and modellers and animators and producers can all modify the game without touching the code. We can build complex systems such as AI behavior trees out of a nested tree of very simple state machines.
      How Else Can I Apply This
      Most games apply these concepts all over the place. One of the most common jobs in the industry is Gameplay Engineer, or GPE. All they do is create the exact types of items we have made in this series of tutorials.
      For example, in Littlest PetShop I spent months adding new behaviors and actions. They ranged from 'in the world' behaviors like those above to minigames such as hide and seek where pets would run behind bushes to hide, then run up to the camera and celebrate with the player when they were found. In The Sims every one of the tens of thousands of objects in the game required scripting. You need a pile of clothes, then you need some of these scripts: Behaviors and interactions to create the piles of clothes; interactions to drop the piles of clothes and pick them up again; interactions for a washing machine; interactions for a dryer; interactions for the maid to come clean up; and on and on and on. Or if you are into faced paced games, you need behaviors and actions for your main characters to know to attack or run away, or decide if they should attack towers or players or minions. You need to create objects to pick up, weapons, inventories, and more. All of these behave the same way that we demonstrated above. It uses a common interface for objects, and the programmer fills out the details. These little activities and behaviors grow into complex ecosystems powering most games.
      So there you go. A text-based dungeon crawler prototype and a sandbox world prototype, both filled with state machines and written in just a few hours. Creating objects and behaviors like this describes the daily life of a gameplay engineer. Chances are good that if you get a programming career in the industry you will spend a few years of your life writing code much like that above. Remembering that it is a simple state machine can help avoid many nasty bugs and can simplify your life greatly.
      Thanks for reading.
      Source code can be found here: StateMachineTutorialsV5.zip

      Updates: 2013-03-26 Fix a typo in the map images.
      [Wayback Machine Archive]

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