State Machines

Manage AI behavior with Finite State Machines (FSM) and Hierarchical State Machines (HFSM) supporting nested states, transitions, guards, and history.

Overview#

Web Engine provides two types of state machines:

Simple FSM

Flat state structure with simple transitions, lifecycle hooks (enter/update/exit), generic key types, lightweight and fast.

Hierarchical FSM

Nested/hierarchical states with transition guards, event-driven transitions, history states (shallow and deep), and zero-GC optimization.

Simple State Machine#

Basic FSM#

basic-fsm.ts

typescript

import { StateMachine, State } from '@web-engine-dev/core/engine/ai';

// Define states
enum AIState {
  IDLE = 'IDLE',
  PATROL = 'PATROL',
  CHASE = 'CHASE',
  ATTACK = 'ATTACK',
}

// Create state machine
const fsm = new StateMachine<AIState>();

// Add states with lifecycle hooks
fsm.add(AIState.IDLE, {
  onEnter() {
    playIdleAnimation();
  },
  onUpdate(delta, time) {
    checkForEnemies();
  },
  onExit() {
    stopIdleAnimation();
  },
});

fsm.add(AIState.PATROL, {
  onEnter() {
    selectNextWaypoint();
  },
  onUpdate(delta, time) {
    moveToWaypoint(delta);
    if (atWaypoint()) {
      selectNextWaypoint();
    }
  },
});

fsm.add(AIState.CHASE, {
  onEnter() {
    playChaseAnimation();
  },
  onUpdate(delta, time) {
    chaseTarget(delta);
  },
});

fsm.add(AIState.ATTACK, {
  onEnter() {
    startAttack();
  },
  onUpdate(delta, time) {
    if (attackComplete()) {
      fsm.changeTo(AIState.CHASE);
    }
  },
});

// Start in IDLE state
fsm.changeTo(AIState.IDLE);

// In game loop
fsm.update(delta, time);

// Transition to new states
if (enemySpotted) {
  fsm.changeTo(AIState.CHASE);
}

if (inAttackRange) {
  fsm.changeTo(AIState.ATTACK);
}

FSM System Integration#

fsm-system.ts

typescript

import {
  registerFSM,
  assignFSM,
  changeFSMState,
  getCurrentFSMState,
} from '@web-engine-dev/core/engine/ai';

// Register an FSM template
registerFSM('GuardFSM', () => {
  const fsm = new StateMachine<AIState>();

  fsm.add(AIState.IDLE, {
    onUpdate(delta) {
      // Check for targets
    },
  });

  fsm.add(AIState.PATROL, {
    onUpdate(delta) {
      // Patrol logic
    },
  });

  return fsm;
});

// Assign to entity
assignFSM(eid, 'GuardFSM', AIState.IDLE);

// Change state externally
changeFSMState(eid, AIState.PATROL);

// Get current state
const currentState = getCurrentFSMState(eid);
console.log('Current state:', currentState);

Hierarchical State Machine#

HFSM Basics#

Hierarchical FSM supports nested states with parent-child relationships:

hfsm-basics.ts

typescript

import { HierarchicalStateMachine, HistoryType } from '@web-engine-dev/core/engine/ai';

interface AIContext {
  targetEid: number;
  health: number;
  alertLevel: number;
}

const fsm = new HierarchicalStateMachine<AIContext>({
  initial: 'Idle',
  states: {
    Idle: {
      onEnter: (ctx) => {
        console.log('Entering idle state');
        playIdleAnimation();
      },
      on: {
        ENEMY_SPOTTED: 'Combat',
        PATROL_START: 'Patrol',
      },
    },

    Patrol: {
      initial: 'MovingToWaypoint',
      states: {
        MovingToWaypoint: {
          onUpdate: (ctx, delta) => {
            moveToWaypoint(delta);
          },
          on: {
            WAYPOINT_REACHED: 'WaitingAtWaypoint',
          },
        },
        WaitingAtWaypoint: {
          onEnter: () => selectNextWaypoint(),
          on: {
            WAIT_COMPLETE: 'MovingToWaypoint',
          },
        },
      },
      on: {
        ENEMY_SPOTTED: 'Combat',
      },
    },

    Combat: {
      initial: 'Approach',
      history: HistoryType.Shallow,  // Remember last combat state
      states: {
        Approach: {
          onUpdate: (ctx, delta) => {
            moveToTarget(ctx.targetEid, delta);
          },
          on: {
            IN_RANGE: 'Attack',
          },
        },
        Attack: {
          onEnter: (ctx) => {
            startAttack(ctx.targetEid);
          },
          on: {
            OUT_OF_RANGE: 'Approach',
            TARGET_DEAD: 'Search',
          },
        },
        Search: {
          onUpdate: (ctx, delta) => {
            searchForTargets();
          },
          on: {
            ENEMY_SPOTTED: 'Approach',
            TIMEOUT: '#Idle',  // Absolute path to root state
          },
        },
      },
      on: {
        HEALTH_CRITICAL: '#Flee',
      },
    },

    Flee: {
      onUpdate: (ctx, delta) => {
        fleeFromThreat(delta);
      },
      on: {
        SAFE_DISTANCE: 'Idle',
      },
    },
  },

  // Global transitions (available from any state)
  on: {
    DEATH: 'Dead',
  },
});

// Start the state machine
const context: AIContext = {
  targetEid: 0,
  health: 100,
  alertLevel: 0,
};

fsm.start(context);

// Update in game loop
fsm.update(context, delta, time);

// Send events to trigger transitions
fsm.send('ENEMY_SPOTTED', context);
fsm.send('IN_RANGE', context);

// Check current state
console.log('Current state:', fsm.getCurrentState()); // "Combat.Attack"

Transition Guards#

Guards control whether a transition should occur:

transition-guards.ts

typescript

const fsm = new HierarchicalStateMachine<AIContext>({
  initial: 'Idle',
  states: {
    Idle: {
      on: {
        ENEMY_SPOTTED: {
          target: 'Combat',
          guard: (ctx) => ctx.health > 20,  // Only enter combat if healthy
        },
        ENEMY_SPOTTED_WEAK: {
          target: 'Flee',
          guard: (ctx) => ctx.health <= 20,  // Flee if weak
        },
      },
    },

    Combat: {
      states: {
        Attack: {
          on: {
            RELOAD: {
              target: 'Reloading',
              guard: (ctx) => ctx.ammo === 0,  // Only reload when empty
              action: (ctx) => {
                console.log('Transitioning to reload');
              },
            },
          },
        },
        Reloading: {
          onEnter: (ctx) => startReload(),
          on: {
            RELOAD_COMPLETE: 'Attack',
          },
        },
      },
    },

    Flee: {
      onUpdate: (ctx, delta) => {
        fleeFromThreat(delta);
      },
    },
  },
});

History States#

History states remember which substate was active:

history-states.ts

typescript

import { HistoryType } from '@web-engine-dev/core/engine/ai';

const fsm = new HierarchicalStateMachine<AIContext>({
  initial: 'Combat',
  states: {
    Combat: {
      initial: 'Approach',
      history: HistoryType.Shallow,  // Remember last combat substate
      states: {
        Approach: {
          on: { IN_RANGE: 'Attack' },
        },
        Attack: {
          on: { OUT_OF_RANGE: 'Approach' },
        },
      },
      on: {
        TAKE_COVER: '#Cover',
      },
    },

    Cover: {
      onUpdate: (ctx, delta) => {
        hideInCover();
      },
      on: {
        SAFE: 'Combat',  // Returns to last combat state (Attack or Approach)
      },
    },
  },
});

// First time: enters Combat.Approach (initial state)
fsm.send('TAKE_COVER', context);  // Go to Cover
// If was in Combat.Attack when we took cover...
fsm.send('SAFE', context);  // Returns to Combat.Attack (history)

History Types:

None — No history, always enter initial substate
Shallow — Remember only direct child state
Deep — Remember full nested state path (all descendants)

HFSM System Integration#

hfsm-system.ts

typescript

import {
  registerHFSM,
  assignHFSM,
  sendFSMEvent,
  getCurrentFSMState,
  isInFSMState,
} from '@web-engine-dev/core/engine/ai';

// Register HFSM template
registerHFSM('EnemyAI', () => {
  return new HierarchicalStateMachine<AIContext>({
    // ... config as above
  });
});

// Assign to entity
assignHFSM(eid, 'EnemyAI');

// Send events
sendFSMEvent(eid, 'ENEMY_SPOTTED');
sendFSMEvent(eid, 'IN_RANGE');

// Check current state
const state = getCurrentFSMState(eid);
console.log('State:', state);  // "Combat.Attack"

// Check if in a state (supports partial matching)
if (isInFSMState(eid, 'Combat')) {
  // True for "Combat", "Combat.Attack", "Combat.Approach", etc.
  console.log('In combat!');
}

State Machine Context#

FSM Context#

Simple FSM uses context passed to state methods:

fsm-context.ts

typescript

import { registerFSM } from '@web-engine-dev/core/engine/ai';

registerFSM('GuardFSM', (eid: number, world: IWorld) => {
  const fsm = new StateMachine<AIState>();

  fsm.add(AIState.PATROL, {
    onUpdate(delta, time) {
      // Access entity data
      const pos = Transform.position[eid];
      const vel = Velocity.linear[eid];

      // Use blackboard for shared data
      const blackboardId = AIAgent.blackboardId[eid];
      const waypoints = getBlackboardValue(blackboardId, 'waypoints');
    },
  });

  return fsm;
});

HFSM Context#

HFSM uses a typed context object:

hfsm-context.ts

typescript

interface EnemyContext {
  eid: number;
  targetEid: number;
  health: number;
  maxHealth: number;
  alertLevel: number;
  lastSeenPosition: THREE.Vector3 | null;
  ammo: number;
  maxAmmo: number;
}

const fsm = new HierarchicalStateMachine<EnemyContext>({
  initial: 'Idle',
  states: {
    Combat: {
      states: {
        Attack: {
          onUpdate: (ctx, delta) => {
            // Access context
            if (ctx.ammo > 0) {
              shootAtTarget(ctx.eid, ctx.targetEid);
              ctx.ammo--;
            } else {
              // Trigger reload
              fsm.send('RELOAD', ctx);
            }
          },
        },
      },
    },
  },
});

// Create context for entity
const context: EnemyContext = {
  eid,
  targetEid: 0,
  health: 100,
  maxHealth: 100,
  alertLevel: 0,
  lastSeenPosition: null,
  ammo: 30,
  maxAmmo: 30,
};

fsm.start(context);

Advanced HFSM Features#

Numeric State IDs#

For hot paths, use numeric state IDs for O(1) state checks:

numeric-ids.ts

typescript

// Get numeric ID once
const attackStateId = fsm.getStateId('Combat.Attack');

// In hot path (60 FPS loop)
if (fsm.isInStateById(attackStateId)) {
  // O(1) check - no string hashing
  performAttack();
}

// Or get current state ID
const currentId = fsm.getCurrentStateId();

// Use in switch statement
switch (currentId) {
  case attackStateId:
    performAttack();
    break;
  // ... other states
}

Event Data#

Pass data with events:

event-data.ts

typescript

// Define event with data
fsm.send('ENEMY_SPOTTED', context, {
  enemyEid: targetEid,
  distance: 10.5,
  position: new THREE.Vector3(5, 0, 5),
});

// Access in transition action
const fsm = new HierarchicalStateMachine<AIContext>({
  states: {
    Idle: {
      on: {
        ENEMY_SPOTTED: {
          target: 'Combat',
          action: (ctx, event, data: any) => {
            ctx.targetEid = data.enemyEid;
            ctx.lastSeenPosition = data.position;
            console.log(`Enemy at ${data.distance}m`);
          },
        },
      },
    },
  },
});

State Queries#

state-queries.ts

typescript

// Check if FSM is started
if (fsm.isStarted()) {
  // Update FSM
  fsm.update(context, delta, time);
}

// Get current state path
const state = fsm.getCurrentState();  // "Combat.Attack"

// Check if in state (partial matching)
if (fsm.isInState('Combat')) {
  // True for Combat, Combat.Attack, Combat.Approach, etc.
}

// Check exact state
if (fsm.getCurrentState() === 'Combat.Attack') {
  // Exact match only
}

Integration with Behavior Trees#

FSM within Behavior Tree#

fsm-in-bt.ts

typescript

import { BTAction, BTStatus } from '@web-engine-dev/core/engine/ai';

// Create action that runs FSM
const fsmAction = new BTAction('runFSM', (ctx) => {
  const fsmInstance = getFSMInstance(ctx.eid);

  if (!fsmInstance) {
    return BTStatus.FAILURE;
  }

  // Update FSM
  fsmInstance.update(ctx.delta, ctx.time);

  // Check FSM state to determine BT status
  const currentState = getCurrentFSMState(ctx.eid);

  if (currentState === 'Combat.Attack') {
    return BTStatus.RUNNING;
  } else if (currentState === 'Idle') {
    return BTStatus.SUCCESS;
  }

  return BTStatus.RUNNING;
});

// Use in behavior tree
const tree = new BTSequence([
  new BTCondition('hasTarget', (ctx) => ctx.blackboard.has('targetEid')),
  fsmAction,
]);

Performance Optimization#

Zero-GC HFSM#

HFSM is optimized for zero garbage collection:

Pre-computed path segments (no split() calls)
Pre-allocated buffers for state traversal
Numeric state IDs for O(1) comparisons
Cached current state string
Reused arrays for active states

Update Throttling#

throttling.ts

typescript

// Throttle FSM updates based on distance
const distanceToCamera = calculateDistance(eid, cameraPosition);

if (distanceToCamera < 20) {
  // Near: update every frame
  fsm.update(context, delta, time);
} else if (distanceToCamera < 50) {
  // Mid: update every 0.1s
  if (time - lastUpdateTime > 0.1) {
    fsm.update(context, delta, time);
    lastUpdateTime = time;
  }
} else {
  // Far: update every 0.5s
  if (time - lastUpdateTime > 0.5) {
    fsm.update(context, delta, time);
    lastUpdateTime = time;
  }
}

Debugging State Machines#

State Logging#

logging.ts

typescript

// Log all state transitions
const fsm = new HierarchicalStateMachine<AIContext>({
  states: {
    Idle: {
      onEnter: (ctx) => {
        console.log('[FSM] Entered Idle');
      },
      onExit: (ctx) => {
        console.log('[FSM] Exited Idle');
      },
    },
    Combat: {
      onEnter: (ctx) => {
        console.log('[FSM] Entered Combat');
      },
      states: {
        Attack: {
          onEnter: (ctx) => {
            console.log('[FSM] Entered Combat.Attack');
          },
        },
      },
    },
  },
});

State History#

history.ts

typescript

// Track state history for debugging
const stateHistory: string[] = [];

const fsm = new HierarchicalStateMachine<AIContext>({
  states: {
    Idle: {
      onEnter: () => {
        stateHistory.push('Idle');
        if (stateHistory.length > 10) stateHistory.shift();
      },
    },
    // ... other states
  },
});

// View state history
console.log('State history:', stateHistory.join(' -> '));

Example: Complete Enemy AI#

complete-enemy-ai.ts

typescript

import {
  HierarchicalStateMachine,
  HistoryType,
  registerHFSM,
  assignHFSM,
  sendFSMEvent,
} from '@web-engine-dev/core/engine/ai';

interface EnemyContext {
  eid: number;
  targetEid: number;
  health: number;
  alertLevel: number;
}

// Register enemy AI FSM
registerHFSM('EnemyAI', () => {
  return new HierarchicalStateMachine<EnemyContext>({
    initial: 'Patrol',
    states: {
      Patrol: {
        initial: 'Moving',
        states: {
          Moving: {
            onUpdate: (ctx, delta) => {
              moveToWaypoint(ctx.eid, delta);
            },
            on: {
              WAYPOINT_REACHED: 'Waiting',
            },
          },
          Waiting: {
            onEnter: () => selectNextWaypoint(),
            on: {
              WAIT_COMPLETE: 'Moving',
            },
          },
        },
        on: {
          ENEMY_SPOTTED: {
            target: 'Combat',
            guard: (ctx) => ctx.health > 20,
            action: (ctx, event, data: any) => {
              ctx.targetEid = data.enemyEid;
            },
          },
        },
      },

      Combat: {
        initial: 'Approach',
        history: HistoryType.Shallow,
        states: {
          Approach: {
            onUpdate: (ctx, delta) => {
              moveToTarget(ctx.eid, ctx.targetEid, delta);
            },
            on: {
              IN_RANGE: 'Attack',
            },
          },
          Attack: {
            onEnter: (ctx) => {
              startAttack(ctx.eid, ctx.targetEid);
            },
            on: {
              OUT_OF_RANGE: 'Approach',
              TARGET_LOST: 'Search',
            },
          },
          Search: {
            onUpdate: (ctx, delta) => {
              searchArea(ctx.eid, delta);
            },
            on: {
              ENEMY_SPOTTED: 'Approach',
              SEARCH_TIMEOUT: '#Patrol',
            },
          },
        },
        on: {
          HEALTH_LOW: {
            target: '#Flee',
            guard: (ctx) => ctx.health < 20,
          },
        },
      },

      Flee: {
        onUpdate: (ctx, delta) => {
          fleeFromTarget(ctx.eid, ctx.targetEid, delta);
        },
        on: {
          SAFE_DISTANCE: 'Patrol',
          HEALTH_RECOVERED: {
            target: 'Combat',
            guard: (ctx) => ctx.health > 50,
          },
        },
      },
    },

    on: {
      DEATH: 'Dead',
    },
  });
});

// Setup enemy
function createEnemy(eid: number) {
  // Add AI component
  addComponent(world, AIAgent, eid);
  initAIAgent(eid, AIControllerType.StateMachine, AIAffiliation.Hostile);

  // Assign HFSM
  assignHFSM(eid, 'EnemyAI');

  // Set initial context via blackboard
  const blackboardId = AIAgent.blackboardId[eid];
  setBlackboardValue(blackboardId, 'health', 100);
  setBlackboardValue(blackboardId, 'alertLevel', 0);
}

// Send events from game logic
function onEnemySpotted(eid: number, targetEid: number) {
  sendFSMEvent(eid, 'ENEMY_SPOTTED', { enemyEid: targetEid });
}

function onHealthChange(eid: number, health: number) {
  const blackboardId = AIAgent.blackboardId[eid];
  setBlackboardValue(blackboardId, 'health', health);

  if (health < 20) {
    sendFSMEvent(eid, 'HEALTH_LOW');
  } else if (health > 50) {
    sendFSMEvent(eid, 'HEALTH_RECOVERED');
  }
}

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Last updated: 1/15/2026

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