8 min read

Structuring the flow of logic in a game can be challenging. If you’re not careful, you quickly end up with a scattered collection of state variables and conditionals that is difficult to wrap your head around.

In my past two game projects, I found it helpful to structure my game flow as a stack of states. In this article, I’ll give a quick overview of this technique and some examples of what makes it useful. The example code is written in D, but it should be pretty easy to apply in any language.

Stacking States for Isolation

Stacking states provides a nice way to isolate chunks of game logic from one another. I leveraged this while making damage_control, a game reminiscent of the Arcade/SNES title Rampart.

In it, a match is divided into rounds, and each round passes through a series of phases. First you place some turrets in your territory, then you fire at your opponent, and then you try to repair the damage done during the firing phase. Before each phase, a banner scrolls across the screen telling the player what phase they are in.


Here’s the logic that sets up a new round (simplified from the original source for clarity):

game.states.push(
new ShowBanner("Place Turrets", game),
new PlaceTurrets(game),

new ShowBanner("Fire!", game),
new Fire(game, _currentRound),

new ShowBanner("Rebuild!", game),
new PlaceWalls(game),

new StatsSummary(game));

Because all of the states can be stacked up at once within a single function, none of the states have to be aware what state comes next.

For example, PlaceWalls doesn’t have to know to show a stats summary when it ends; it just pops itself off the stack when done and lets the next state kick in.

The code shown above resides in the StartRound state, which sits at the bottom of the state stack. Once all the phases for the current round are popped, we once again enter StartRound and push a new set of states.

The flow of states looks like this (the right side represents the top of the stack, or the active state):

StartRound
StartRound | StatsSummary | PlaceWalls | ShowBanner | Fire | ShowBanner | PlaceTurrets | ShowBanner
StartRound | StatsSummary | PlaceWalls | ShowBanner | Fire | ShowBanner | PlaceTurrets
StartRound | StatsSummary | PlaceWalls | ShowBanner | Fire | ShowBanner
StartRound | StatsSummary | PlaceWalls | ShowBanner | Fire
StartRound | StatsSummary | PlaceWalls | ShowBanner
StartRound | StatsSummary | PlaceWalls
StartRound | StatsSummary
StartRound
StartRound | StatsSummary | PlaceWalls | ShowBanner | Fire | ShowBanner | PlaceTurrets | ShowBanner
... and so on ...

I’ll provide another example at the end of the article, but first I’ll discuss the implementation.

The State

interface State(T) {
void enter(T);
void exit(T);
void run(T);
}

T is a generic type here, and represents whatever kind of object the states will operate on. For example, it might be a Game object that provides access to game entities, resources, input devices, and more.

At any given time, you have a single active state;run is executed once for each update loop of the game.

enter is called whenever a state becomes active, before the first call to run. This allows the state to perform any preparation it needs before it begins its normal flow of logic. Similarly, exit allows a state to perform some sort of tear-down before it becomes inactive. Note that enter and exit are not equivalent to a constructor and destructor; we will see later that a single state may enter and exit multiple times during its life.

As an example, let’s take the PlaceWalls state from earlier. enter might start a timer for how long the state should last, run would process input from the player to move and place pieces, and exit would mark off areas that the player had enclosed.

The Stack

The StateStack itself is pretty straightforward as well. It only needs to support three operations:

  • push : place a state on top of the stack.
  • pop : remove the state on top of the stack.
  • run : cause the state on top of the stack to process its object.

The only bit of trickiness comes in managing those enter and exit states mentioned earlier. The state stack must ensure that the following happens during a state transition:

  • call enter once before calling run on a state that was previously inactive.
  • callexit on a state that becomes inactive.
  • enter and exitshould be called an equal number of times during a state’s life.
struct StateStack(T) {
private {
bool        _entered;
    SList!State _stack;
    T           _object;
  }

void push(State!T[] states ...) {
if (_entered) {
      _stack.top.exit(_object);
      _entered = false;
    }

// Note that we push the new states, but do _not_ call enter() yet
// If push is called again before run, we only want to enter the top state
foreach_reverse(state ; states) {
      _stack.insertFront(state);
    }
  }

void pop() {
// get ref to current state, top may change during exit
auto popped = _stack.top;
    _stack.removeFront;

if (_entered) {
// the state we are popping had been entered, so we need to exit it
      _entered = false;
      popped.exit(_object);
    }
  }

void run(T obj) {
// cache obj for calls to exit() that are triggered by pop().
    _object = obj;

// top.enter() could push/pop, so keep going until the top state is entered
while(!_entered) {
      _entered = true;
      top.enter(obj);
    }

// finally, our stack has stabilized
    top.run(obj);
  }
}

The implementation is mostly straightforward, but there are a few caveats.

It is valid (and useful) for a state to push() and pop() states during its enter. In the previous example, StartRound pushes a number of states during enter. Therefore, implementing StateStack.run like so would be incorrect:

if (!_entered) {
  _entered = true;
  top.enter(obj);
}
top.run(obj);

After pushing StartRound and calling StateStack.run, it could call StartRound.enter, which would push more states onto the stack. It would then call top.run(obj) on whatever state was last pushed, which hasn’t been entered yet!

For this reason, run uses the while (!_entered) loop to call enter until the stack ‘stabilizes’.

Similarly, a state may push or pop states during its exit call. To support this, we need to cache the object that get passed in to run so it can be used by pop.

Dissolving Complex Logic Flows

In Terra Arcana, the StateStack, a turn-based strategy game I developed, made the flow of combat manageable.

Here’s a quick description of the rules regarding attacks:

  • The attacker launches one or more strikes against the defender.
  • Each strike may hit (dealing damage or some effect) or miss.
  • If the defender’s health has dropped to 0, they are destroyed.
  • The defender may get a chance to counter-attack if:
  • They were not destroyed by the initial attack.
  • They have an attack that is in range of the attacker.
  • They have enough AP (action points) to use said attack.
  • The counter-attack, like the initial attack, may have multiple strikes.
  • The counter-attack may destroy the attacker.
  • You cannot counter-attack a counter-attack.

Now consider that an AOE attack may hit multiple defenders, each of which gets a chance to counter-attack!

Now, computing the result of this isn’t so bad — you can probably imagine a series of if/else statements that could do the job in a single pass.

The difficulty depicting the result to the player. We need to play animations for attacks and unit destruction, pop up text to indicate damage and status effects (or lack thereof), manipulate health/AP bars on the UI, and play sound effects at various points throughout the process.

This all happens over the course of multiple update cycles rather than a single function call, so managing it with a single function would involve a whole mess of state variables (attackCount, isAnimating, isDefenderDestroyed, isCounterAttackInProgress, ect.).

With a StateStack we can separate chunks of logic like applying damage or status effects, destroying a unit, and initiating a counter attack into its own independent state. When an attack begins, you push a whole bunch of these onto the stack at once, and then let everything play out.

Here’s an excerpt of code that initiates an attack:

battle.states.popState();

foreach(unit ; unitsAffected) {
    battle.states.push(newPerformCounter(unit, _actor));
}

foreach(unit ; unitsAffected) {
    battle.states.push(new CheckUnitDestruction(unit));

for(int i = 0 ; i < _action.hits ; i++) {
        battle.states.push(new ApplyEffect(_action, unit));
    }
}

Remember that we are dealing with a stack, so states pushed later end up at the top (ApplyEffect happens before CheckUnitDestruction).

This logic resides in the PerformAction state, so the first call removes this state from the stack before pushing the rest on.

To understand this a bit better, consider the following scenario: A unit launches an attack that hits twice. The target is not destroyed, and is capable of countering with an attack that hits three times. The states on the stack would progress like so (where the right side represents the top of the stack):

PerformAction
PerformCounter | CheckUnitDestruction | ApplyEffect | ApplyEffect
PerformCounter | CheckUnitDestruction | ApplyEffect
PerformCounter | CheckUnitDestruction
PerformCounter
CheckUnitDestruction | ApplyEffect | ApplyEffect | ApplyEffect
CheckUnitDestruction | ApplyEffect | ApplyEffect
CheckUnitDestruction | ApplyEffect
CheckUnitDestruction

Note that when PerformCounter becomes the active state, it replaces itself with three ApplyEffects and a CheckUnitDestruction. The states nicely encapsulate specific chunks of game logic, so we get to reuse the same states in PerformAction and PerformCounter.

Author:

Ryan Roden-Corrent is a software developer by trade and hobby. He is an active contributor in the free/open-source software community and has a passion for simple but effective tools. He started gaming at a young age and dabbles in all aspects of game development, from coding to art and music. He’s also an aspiring musician and yoga teacher. You can find his open source work here and Creative Commons art here.

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