# Design: Tactical AI and Goalie (M4)

## Goal
Implement a "smart" goalie that covers angles and a zone-based tactical system for players to make the game feel more professional and less like a simple "chase the puck" simulation.

## Architecture
The system extends the existing steering-based movement. Instead of a simple target, the target is now determined by a combination of the player's role and the current game zone.

### 1. Goalie AI (The Interceptor)
The goalie's primary objective is to block the path between the puck and the goal center.
- **Anchor Point**: The center of the goalie's own goal.
- **Positioning Logic**:
    - Calculate the vector from the goal center to the puck.
    - The goalie attempts to stay on this vector, effectively "cutting off the angle".
    - **Constraint**: The goalie is restricted to a small semi-circle (radius ~100-150px) around the goal center to prevent them from wandering into the midfield.
    - **Idle State**: When the puck is far away, the goalie returns to the center of the goal.
- **Movement**: Uses the existing steering physics for smooth transitions.

### 2. Zone-Based Tactics
The field is divided into three zones for each team: **Defensive**, **Middle**, and **Offensive**.

| Role | Defensive Zone | Middle Zone | Offensive Zone |
| :--- | :--- | :--- | :--- |
| **Striker** | Return to Middle/Offensive | Support attack | Aggressively pursue puck |
| **Defender** | Aggressively pursue puck | Maintain position/Support | Stay back (Insurance) |
| **Goalie** | Intercept puck | Stay in goal area | Stay in goal area |

### 3. Technical Details
- **Constants**: Define `WorldZoneWidth` (1/3 of field width) and `GoalieRadius` to avoid magic numbers.
- **Role Update**: Add `RoleGoalie` to `PlayerRole` enum.
- **Logic Flow**: `Puck Position` $\rightarrow$ `Zone Detection` $\rightarrow$ `Role-based Target Calculation` $\rightarrow$ `Steering` $\rightarrow$ `Movement`.

## File Changes
- `internal/entities/player.go`: Add `RoleGoalie`.
- `internal/game/world.go`: 
    - Add zone and goal constants.
    - Implement `updateGoalieAI`.
    - Refactor `updatePlayerAI` to use zone-based logic.
- `internal/game/world_test.go`: Add tests for goalie positioning and zone transitions.

## Edge Cases
- **Puck behind goal**: Goalie resets to center.
- **Crowding**: Existing anti-clumping logic prevents players from stacking on top of the goalie.
- **Jitter**: Implement a small distance threshold before updating the target to prevent micro-oscillations.