docs: add NPC social interactions design

Covers awareness zone, interaction state machine, cooldowns, brain/movement guards, broadcast format, and client emoji overlay. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2026-03-07 12:52:31 +00:00
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+# NPC Social Interactions Design
+
+## Overview
+
+Add NPC-to-NPC social interactions triggered by proximity. NPCs within a 5-tile awareness zone can initiate interactions that play out as a face-each-other → pause → emoji sequence. Outcomes are currently random but the system is designed to support future personality, relationships, and seek-out behavior.
+
+## New ECS Component: SocialState
+
+Every NPC receives a `SocialState` component at spawn time.
+
+```typescript
+type InteractionPhase = 'none' | 'facing' | 'pausing' | 'emoting';
+
+interface SocialState {
+  phase: InteractionPhase;
+  partnerId: string | null;
+  phaseTimer: number;           // ticks remaining in current phase
+  outcome: 'positive' | 'negative' | null;
+  globalCooldown: number;       // ticks until this NPC can interact with anyone
+  pairCooldowns: Map<string, number>; // entityId -> ticks remaining
+}
+```
+
+- `globalCooldown`: 50 ticks (5s)
+- `pairCooldowns` entries: 300 ticks (30s)
+- Cooldowns decrement each tick regardless of interaction state
+
+## Social System
+
+New `socialSystem` runs after `npcBrainSystem`, before `movementSystem`.
+
+System order: needsDecay → npcBrain → **social** → movement.
+
+### Phase 1: Update active interactions
+
+For NPCs with `phase !== 'none'`, decrement `phaseTimer`. When timer hits 0, advance:
+
+| Transition | Duration | Action |
+|---|---|---|
+| `facing` → `pausing` | 10 ticks (1s) | NPCs face each other, movement set to idle |
+| `pausing` → `emoting` | 15 ticks (1.5s) | Roll outcome independently per NPC (50/50 positive/negative) |
+| `emoting` → `none` | 20 ticks (2s) | Emoji visible; on exit set global + pair cooldowns |
+
+### Phase 2: Detect new interactions
+
+For NPCs with `phase === 'none'` and `globalCooldown === 0`:
+
+1. Find all other NPCs within 5-tile Manhattan distance
+2. Filter out those with active pair cooldowns or already interacting
+3. Pick closest eligible NPC as partner
+4. **Target rejection**: If target is not wandering AND their relevant need is below threshold (hunger < 30 or energy < 20), skip — they're too busy
+5. If valid partner: set both NPCs to `facing` phase, update directions to face each other
+
+Process in entity ID order to resolve conflicts — first initiator to claim an NPC wins.
+
+### Cooldown decay
+
+Decrement `globalCooldown` and all `pairCooldowns` each tick. Remove pair entries that hit 0.
+
+## Integration: Brain & Movement Guards
+
+- **npcBrainSystem**: If entity has `socialState.phase !== 'none'`, skip entirely
+- **movementSystem**: Same check — skip entities in active social phase
+- Path preservation: movement component's `target` and `path` are left untouched during interaction. When phase returns to `none`, movement resumes naturally (brain skips idle check if path exists, movement continues walking).
+
+## Edge Cases
+
+- **NPC despawn during interaction**: Reset remaining partner's phase to `none`
+- **Simultaneous targeting**: Process in entity ID order; first claim wins
+- **Player entities**: Filter out entities with `PlayerControlled` component
+
+## Broadcast & Client Rendering
+
+### Server: add to EntityState
+
+```typescript
+socialState?: {
+  phase: InteractionPhase;
+  partnerId: string | null;
+  outcome: 'positive' | 'negative' | null;
+}
+```
+
+Only phase, partnerId, and outcome are serialized — no cooldowns or timers.
+
+### Client: floating emoji overlay
+
+- When `socialState.phase === 'emoting'` with non-null outcome, render HTML element above sprite
+- Positive: happy emoji, Negative: angry emoji
+- Float-up animation: translate Y upward ~20px over 2s, fade out in last 0.5s
+- Clean up element when emoting phase ends or entity leaves view
+- Track active elements to avoid duplicates
+- Styling is placeholder (plain emoji text) — will be reworked to match game aesthetic later
+
+## Future Extensions (not in scope now)
+
+- NPC personality/likes/dislikes influencing outcome
+- Persistent relationship tracking between NPC pairs
+- NPCs seeking out liked/disliked NPCs for interactions
+- Interaction outcomes affecting needs or mood