# Life Arcs

Life arcs are state machines that model how characters or other entities evolve over time. They define discrete states and the conditions under which an entity transitions between states. Life arcs capture character development, quest progress, relationship phases, and any other finite-state processes.

## What is a Life Arc?

A life arc is a finite state machine (FSM) composed of:

- **States**: Discrete phases or modes (e.g., "happy", "angry", "sleeping")
- **Transitions**: Conditional edges between states (e.g., "when health < 20, go to 'fleeing'")
- **On-enter actions**: Field updates that occur when entering a state

```
[State A] --condition--> [State B] --condition--> [State C]
    |                         |                       |
 on enter                  on enter                on enter
 (set fields)              (set fields)            (set fields)
```

Life arcs run continuously, evaluating transitions every tick and moving between states as conditions become true.

## Syntax

```bnf
<life-arc-decl> ::= "life_arc" <identifier> <body>

<body> ::= "{" <prose-blocks>* <state>+ "}"

<state> ::= "state" <identifier> "{" <state-body>* "}"

<state-body> ::= <on-enter-block>
               | <transition>
               | <prose-block>

<on-enter-block> ::= "on" "enter" "{" <field>+ "}"

<transition> ::= "on" <expression> "->" <identifier>

<prose-block> ::= "---" <identifier> <content> "---"
```

## States

A state represents a discrete phase in an entity's lifecycle.

### Basic State

```storybook
life_arc SimpleArc {
    state idle {
        ---narrative
        The character is doing nothing, waiting for something to happen.
        ---
    }

    state active {
        ---narrative
        The character is engaged in their primary activity.
        ---
    }
}
```

### State Names

State names are identifiers that must be unique within the life arc. Use descriptive names:

- **Good**: `idle`, `combat`, `sleeping`, `enlightened`
- **Avoid**: `state1`, `s`, `temp`

## On-Enter Actions

When entering a state, the `on enter` block updates entity fields.

### Syntax

```storybook
state state_name {
    on enter {
        EntityName.field_name: value
        EntityName.other_field: other_value
    }
}
```

### Examples

```storybook
life_arc CharacterMood {
    state happy {
        on enter {
            Martha.emotional_state: "happy"
            Martha.energy: 100
        }
    }

    state tired {
        on enter {
            Martha.emotional_state: "exhausted"
            Martha.energy: 20
        }
    }
}
```

### Field Update Semantics

- **Target**: `EntityName.field_name` must reference an existing character/entity
- **Value**: Any valid [value type](./18-value-types.md)
- **Effect**: Field is set to the specified value when state is entered
- **Timing**: Happens immediately upon transition to the state

### Multiple Field Updates

```storybook
state exhausted_baker {
    on enter {
        Martha.energy: 0.1
        Martha.mood: "stressed"
        Martha.quality_output: 0.7
        Martha.needs_break: true
    }

    ---narrative
    After a sixteen-hour shift during the harvest festival,
    Martha's hands are shaking. She knows her bread quality is
    suffering and reluctantly steps away from the oven.
    ---
}
```

## Transitions

Transitions define conditional edges between states. When a transition's condition becomes true, the state machine moves to the target state.

### Syntax

```storybook
state source_state {
    on condition_expression -> target_state
    on another_condition -> another_target
}
```

### Basic Transitions

```storybook
life_arc Combat {
    state fighting {
        on health < 20 -> fleeing
        on enemy_defeated -> victorious
    }

    state fleeing {
        on safe_distance_reached -> idle
    }

    state victorious {
        on celebration_complete -> idle
    }

    state idle {}
}
```

### Expression Conditions

Transitions use the full [expression language](./17-expressions.md):

**Comparisons:**
```storybook
on health < 20 -> low_health
on distance > 100 -> far_away
on count == 0 -> empty
```

**Boolean fields:**
```storybook
on is_hostile -> combat
on completed -> done
on not ready -> waiting
```

**Logical operators:**
```storybook
on health < 20 and not has_potion -> desperate
on is_day or is_lit -> visible
```

**Complex conditions:**
```storybook
on (health < 50 and enemy_count > 3) or surrounded -> retreat
on forall e in enemies: e.defeated -> victory
```

### Multiple Transitions

A state can have multiple outgoing transitions:

```storybook
state monitoring {
    on status is "active" -> active_state
    on status is "inactive" -> inactive_state
    on status is "error" -> error_state
    on shutdown_requested -> shutting_down
}
```

**Evaluation order:**
- Transitions are evaluated in declaration order (top to bottom)
- First transition with a true condition is taken
- If no conditions are true, state remains unchanged

### Self-Transitions

A state can transition to itself:

```storybook
state patrolling {
    on enemy_spotted -> combat
    on checkpoint_reached -> patrolling  // Reset patrol state
}
```

## Complete Examples

### Elena's Career Journey

```storybook
life_arc ElenaCareer {
    ---description
    Tracks Elena's progression from nervous apprentice to confident
    master baker. Each state represents a key phase of her career.
    ---

    state early_apprentice {
        on enter {
            Elena.skill_level: novice
            Elena.confidence: timid
        }

        on recipes_mastered > 5 -> growing_apprentice

        ---narrative
        Elena's hands shake as she measures flour. She checks the
        recipe three times before adding each ingredient. Martha
        patiently corrects her technique.
        ---
    }

    state growing_apprentice {
        on enter {
            Elena.skill_level: beginner
            Elena.confidence: uncertain
        }

        on recipes_mastered > 15 -> journeyman

        ---narrative
        The shaking stops. Elena can make basic breads without
        looking at the recipe. She still doubts herself but
        Martha's encouragement is taking root.
        ---
    }

    state journeyman {
        on enter {
            Elena.skill_level: intermediate
            Elena.confidence: growing
            Elena.can_work_independently: true
        }

        on recipes_mastered > 30 -> senior_journeyman

        ---narrative
        Elena runs the morning shift alone while Martha handles
        special orders. Customers start asking for "Elena's rolls."
        She begins experimenting with her own recipes.
        ---
    }

    state senior_journeyman {
        on enter {
            Elena.skill_level: advanced
            Elena.confidence: steady
        }

        on recipes_mastered > 50 -> master

        ---narrative
        Elena develops her signature recipe: rosemary olive bread
        that becomes the bakery's bestseller. She handles difficult
        customers with grace and trains new helpers.
        ---
    }

    state master {
        on enter {
            Elena.skill_level: master
            Elena.confidence: commanding
            Elena.can_teach: true
        }

        ---narrative
        Master Baker Elena. She has earned it. The guild acknowledges
        her mastery, and Martha beams with pride. Elena begins
        mentoring her own apprentice.
        ---
    }
}
```

### Quest Progress

```storybook
life_arc HeroQuest {
    state not_started {
        on talked_to_elder -> received_quest
    }

    state received_quest {
        on enter {
            Hero.quest_active: true
            Hero.quest_step: 0
        }

        on found_first_artifact -> collecting
    }

    state collecting {
        on enter {
            Hero.quest_step: 1
        }

        on artifact_count == 3 -> returning
        on failed_trial -> failed
    }

    state returning {
        on enter {
            Hero.quest_step: 2
        }

        on reached_elder -> completed
    }

    state completed {
        on enter {
            Hero.quest_active: false
            Hero.quest_completed: true
            Hero.reputation: 100
        }

        ---narrative
        The hero returns triumphant, artifacts in hand. The elder
        bestows great rewards and the village celebrates.
        ---
    }

    state failed {
        on enter {
            Hero.quest_active: false
            Hero.quest_failed: true
        }

        on retry_accepted -> received_quest
    }
}
```

### Name Checker (Equality Examples)

```storybook
life_arc NameCheck {
    state checking {
        on name is "Martha" -> found_martha
        on name is "Jane" -> found_jane
    }

    state found_martha {
        on ready -> checking
    }

    state found_jane {
        on ready -> checking
    }
}
```

### Status Monitor

```storybook
life_arc StatusMonitor {
    state monitoring {
        on status is active -> active_state
        on status is inactive -> inactive_state
        on status is error -> error_state
    }

    state active_state {
        on enter {
            System.led_color: "green"
        }

        on status is inactive -> inactive_state
        on status is error -> error_state
    }

    state inactive_state {
        on enter {
            System.led_color: "yellow"
        }

        on status is active -> active_state
        on status is error -> error_state
    }

    state error_state {
        on enter {
            System.led_color: "red"
            System.alarm: true
        }

        on error_cleared -> monitoring
    }
}
```

### Character Mood Swings

```storybook
life_arc MoodSwings {
    state neutral {
        on provoked -> angry
        on complimented -> happy
        on tired -> sleepy
    }

    state angry {
        on enter {
            Character.aggression: 0.9
            Character.willingness_to_talk: 0.1
        }

        on calmed_down -> neutral
        on escalated -> furious
    }

    state furious {
        on enter {
            Character.aggression: 1.0
            Character.will_attack: true
        }

        on timeout_elapsed -> angry
        on apologized_to -> neutral
    }

    state happy {
        on enter {
            Character.aggression: 0.0
            Character.willingness_to_talk: 1.0
            Character.gives_discounts: true
        }

        on insulted -> neutral
        on bored -> neutral
    }

    state sleepy {
        on enter {
            Character.responsiveness: 0.2
        }

        on woke_up -> neutral
        on fell_asleep -> sleeping
    }

    state sleeping {
        on enter {
            Character.responsiveness: 0.0
            Character.is_vulnerable: true
        }

        on woke_up -> neutral
    }
}
```

## Execution Semantics

### Tick-Based Evaluation

Life arcs evaluate transitions every "tick" (typically once per frame):

1. **Current state**: Start in the current state
2. **Evaluate transitions**: Check each outgoing transition's condition (in order)
3. **First true transition**: Take the first transition with a true condition
4. **Enter new state**: Execute the new state's `on enter` block
5. **Repeat next tick**

### Transition Priority

When multiple transitions could fire, the **first one in declaration order** is taken:

```storybook
state combat {
    on health < 10 -> desperate       // Checked first
    on health < 50 -> defensive       // Checked second
    on surrounded -> retreat          // Checked third
}
```

If health is 5, only `desperate` transition fires (even though `defensive` condition is also true).

### State Persistence

The current state persists across ticks until a transition fires.

```storybook
state waiting {
    on signal_received -> active
}
```

The entity remains in `waiting` state indefinitely until `signal_received` becomes true.

### On-Enter Execution

`on enter` blocks execute **once** when entering the state, not every tick:

```storybook
state combat {
    on enter {
        Character.weapon_drawn: true  // Runs once when entering combat
    }
}
```

## Validation Rules

1. **At least one state**: Life arc must contain at least one state
2. **Unique state names**: State names must be unique within the life arc
3. **Valid transitions**: Transition targets must reference defined states
4. **No orphan states**: All states should be reachable (warning, not error)
5. **Expression validity**: Transition conditions must be well-formed expressions
6. **Field targets**: On-enter field updates must reference valid entities/fields
7. **No cyclic immediate transitions**: Avoid transitions that fire immediately in a loop

## Design Patterns

### 1. Hub-and-Spoke

A central "hub" state with transitions to specialized states:

```storybook
life_arc HubPattern {
    state idle {
        on combat_triggered -> combat
        on quest_accepted -> questing
        on entered_shop -> shopping
    }

    state combat {
        on combat_ended -> idle
    }

    state questing {
        on quest_completed -> idle
    }

    state shopping {
        on left_shop -> idle
    }
}
```

### 2. Linear Progression

States form a linear sequence (quests, tutorials):

```storybook
life_arc Tutorial {
    state intro {
        on clicked_start -> movement
    }

    state movement {
        on moved_forward -> combat
    }

    state combat {
        on defeated_enemy -> inventory
    }

    state inventory {
        on opened_inventory -> complete
    }

    state complete {}
}
```

### 3. Cyclic States

States form a cycle (day/night, seasons):

```storybook
life_arc DayNightCycle {
    state dawn {
        on hour >= 8 -> day
    }

    state day {
        on hour >= 18 -> dusk
    }

    state dusk {
        on hour >= 20 -> night
    }

    state night {
        on hour >= 6 -> dawn
    }
}
```

### 4. Hierarchical (Simulated)

Use multiple life arcs for hierarchical state:

```storybook
// Top-level life arc
life_arc CharacterState {
    state alive {
        on health <= 0 -> dead
    }

    state dead {}
}

// Nested life arc (only active when alive)
life_arc CombatState {
    state idle {
        on enemy_nearby -> combat
    }

    state combat {
        on enemy_defeated -> idle
    }
}
```

## Best Practices

### 1. Use Descriptive State Names

**Avoid:**
```storybook
state s1 { ... }
state s2 { ... }
```

**Prefer:**
```storybook
state waiting_for_player { ... }
state engaged_in_combat { ... }
```

### 2. Add Narrative Prose Blocks

```storybook
state master_baker {
    ---narrative
    Master Baker Elena. She has earned it. The guild acknowledges
    her mastery, and Martha beams with pride. Elena begins
    mentoring her own apprentice.
    ---
}
```

### 3. Order Transitions by Priority

```storybook
state health_check {
    on health <= 0 -> dead          // Most urgent
    on health < 20 -> critical      // Very urgent
    on health < 50 -> wounded       // Moderate
}
```

### 4. Avoid Orphan States

Ensure all states are reachable from the initial state.

**Avoid:**
```storybook
life_arc Broken {
    state start {
        on ready -> middle
    }

    state middle {
        on done -> end
    }

    state unreachable {}  // No transition leads here!

    state end {}
}
```

### 5. Use On-Enter for State Initialization

```storybook
state combat {
    on enter {
        Character.weapon_drawn: true
        Character.combat_stance: "aggressive"
        Character.target: nearest_enemy
    }
}
```

## Cross-References

- [Characters](./10-characters.md) - Characters can have associated life arcs
- [Expression Language](./17-expressions.md) - Transition condition syntax
- [Value Types](./18-value-types.md) - On-enter field value types
- [Validation Rules](./19-validation.md) - Life arc validation constraints

## Related Concepts

- **Finite State Machines (FSM)**: Life arcs are FSMs
- **Character development**: Track character growth over time
- **Quest states**: Model quest progression
- **Mood systems**: Model emotional states
- **Lifecycle modeling**: Birth, growth, aging, death
