Tech Tree: Mutually Exclusive Branch Design

Overview

The tech tree system implements mutually exclusive branch choices that create meaningful decisions and encourage multiple playthroughs. Players must choose between competing specializations, permanently locking out alternate paths.

Design Philosophy

Core Principles

Meaningful Choice: Each branch offers distinct playstyles
Permanent Consequences: Choices matter and can't be undone mid-run
Replayability: Different branches encourage new playthroughs
Strategic Depth: Players must commit to a strategy early

Balance Goals

No branch is objectively better
Each branch counters different enemy types
Branches complement different playstyles (aggressive vs defensive)
Late-game content accessible regardless of early choices

Implementation Approaches

Option A: Permanent Exclusive Choices ⭐ RECOMMENDED

Description: At decision points, choosing one branch permanently locks others.

Pros:

Highest replayability
Most meaningful choices
Clear strategic commitment
Encourages experimentation across runs

Cons:

Players might feel "locked out" of content
Requires careful balance
Need clear communication of consequences

Example Flow:

Level 1-2: Basic turret + wall (everyone gets these)
Level 3: CHOICE POINT
  → Rapid Fire Specialization (unlock fast turrets, lock Heavy branch)
  → Heavy Damage Specialization (unlock powerful turrets, lock Rapid branch)
Level 5: Continue down chosen path
Level 8: Advanced weapons (available to both paths)

Option B: Sequential Unlocking

Description: Complete one branch to unlock the next.

Pros:

All content accessible in one run
Guided progression
Less confusion

Cons:

Lower replayability
Less strategic choice
Linear progression

Option C: Hybrid

Description: Combine both systems - early exclusive choices, late sequential unlocks.

Implementation:

Early Game (Levels 1-5): Mutually exclusive branches
Mid Game (Levels 6-10): Branch-specific upgrades
Late Game (Levels 10+): Universal upgrades after branch completion

CHOSEN APPROACH: Option A - Permanent Exclusive Choices ✅

Rationale:

Maximizes replayability (like Factorio)
Each playthrough feels distinct and meaningful
Encourages experimentation across multiple saves
Clear strategic commitment creates engaging gameplay
Players can restart with different builds to explore all content

Save System Integration:

Support multiple save slots (like Factorio)
Each save maintains its own tech tree choices
In-game achievements reset per save (per-run progression)
Steam achievements unlock globally (account-wide, permanent)
Players can have parallel runs with different tech choices

Technical Implementation

TechNodeResource Fields

class_name TechNodeResource
extends Resource

# Standard fields
@export var id: String
@export var name: String
@export var description: String
@export var icon: Texture2D
@export var branch_name: String

# Unlock conditions
@export var unlock_level_requirement: int
@export var unlock_achievement_ids: Array[String]
@export var prerequisite_tech_ids: Array[String]

# Exclusive branch system (NEW)
@export var mutually_exclusive_with: Array[String] = []
## List of tech_ids that become permanently locked when this is unlocked

@export var requires_branch_completion: Array[String] = []
## List of branch_names that must be fully completed first

@export var unlocked_obstacle_ids: Array[String]

TechTreeManager Logic

var unlocked_tech_ids: Array[String] = []
var locked_tech_ids: Array[String] = []  # Permanently locked

func unlock_tech(tech_id: String) -> bool:
  var tech_node = _get_tech_node_by_id(tech_id)
  
  # Unlock this tech
  unlocked_tech_ids.append(tech_id)
  
  # Lock mutually exclusive techs and their entire branches
  for exclusive_id in tech_node.mutually_exclusive_with:
    locked_tech_ids.append(exclusive_id)
    _lock_branch_recursively(exclusive_id)
    emit_signal("tech_locked", exclusive_id)
  
  return true

func _lock_branch_recursively(tech_id: String):
  # Find all techs that depend on this locked tech
  for node in all_tech_nodes:
    if tech_id in node.prerequisite_tech_ids:
      if node.id not in locked_tech_ids:
        locked_tech_ids.append(node.id)
        _lock_branch_recursively(node.id)

Example Tech Tree Structure

Phase 1: Basic Defense (Levels 1-2)

All players get these:

Basic Turret (level 1)
Basic Wall (level 1)

Phase 2: Offensive Specialization (Level 3) - EXCLUSIVE CHOICE

Option A: Rapid Fire Path

Rapid Fire Turret (level 3)
Multi-Target Turret (level 5)
Gatling Turret (level 7)
Locks: Heavy Damage Path

Option B: Heavy Damage Path

Heavy Turret (level 3)
Sniper Turret (level 5)
Cannon Turret (level 7)
Locks: Rapid Fire Path

Phase 3: Defensive Specialization (Level 6) - EXCLUSIVE CHOICE

Option A: Fortress Strategy

Reinforced Walls (level 6)
Barrier Towers (level 8)
Regenerating Walls (level 10)
Locks: Mobile Defense Path

Option B: Mobile Defense

Support Towers (level 6)
Slow Fields (level 8)
Buff Amplifiers (level 10)
Locks: Fortress Strategy Path

Phase 4: Advanced Weapons (Level 12)

Requires completing one offensive AND one defensive branch:

Laser Turret
Missile Launcher
Tesla Coil

UI/UX Considerations

Visual Indicators

Unlocked: ✅ Green checkmark, full color
Available: 💡 Yellow glow, clickable
Locked (prerequisites): 🔒 Gray, shows requirements tooltip
Permanently Locked: ❌ Red X, strikethrough, "Locked due to [choice]"
Exclusive Choice: ⚠️ Yellow warning border

Warning Dialog

When hovering over an exclusive choice:

┌─────────────────────────────────────────┐
│  ⚠️  IMPORTANT CHOICE                    │
├─────────────────────────────────────────┤
│  Unlocking "Rapid Fire Specialization"  │
│  will PERMANENTLY LOCK:                 │
│                                         │
│  ❌ Heavy Damage Specialization         │
│  ❌ Sniper Turret                       │
│  ❌ Cannon Turret                       │
│                                         │
│  This choice cannot be undone!          │
│                                         │
│  [Cancel]              [Confirm Unlock] │
└─────────────────────────────────────────┘

Branch Completion Display

Offensive Branch: ████████░░ 80% (4/5 nodes)
Defensive Branch: Not Started
Advanced Branch: 🔒 Requires Offensive completion

Balancing Considerations

Rapid Fire vs Heavy Damage

Attribute	Rapid Fire	Heavy Damage
DPS (vs many enemies)	⭐⭐⭐⭐⭐	⭐⭐
DPS (vs single target)	⭐⭐⭐	⭐⭐⭐⭐⭐
Scrap efficiency	⭐⭐⭐	⭐⭐⭐⭐
Best vs	Swarms	Tanks/Bosses

Fortress vs Mobile Defense

Attribute	Fortress	Mobile Defense
Enemy delay	⭐⭐⭐⭐⭐	⭐⭐
Turret buffing	⭐⭐	⭐⭐⭐⭐⭐
Flexibility	⭐⭐	⭐⭐⭐⭐
Best vs	Linear attacks	Multi-path attacks

Player Communication

First-Time Experience

Tutorial mentions choices: "You'll face strategic decisions..."
First exclusive node: Show detailed explanation
Confirmation dialog: Always require explicit confirmation
Post-choice feedback: "You chose Rapid Fire! Heavy Damage path is now locked."

Tech Tree Screen

Legend/Key: Explain all icons and states
Filter options: "Show available only" vs "Show all"
Branch overview: Birds-eye view of all branches
Reset warning: "Starting new game resets all tech choices"

Testing Checklist

Future Expansion

Meta-Progression (Optional)

Could add persistent unlocks across runs:

Unlock ability to respec mid-game (one-time use)
Unlock "hybrid" nodes that work with both branches
Unlock third branch options

Challenge Runs

"Pure Rapid Fire" - complete game without walls
"Fortress Only" - no offensive turrets allowed
These would have achievement rewards

Save Game System Requirements

Multiple Save Slots

Like Factorio, support multiple parallel saves:

Minimum 3 save slots (expandable)
Each slot is independent
Display: Save name, playtime, level, last played date
Quick save / Quick load (F5 / F9)
Auto-save every level completion

Achievement System Split

In-Game Achievements (per save slot):

Reset when starting new game in a slot
Track per-run progress ("Defeated 50 zombies")
Used for tech tree unlock conditions
Stored in save file

Steam Achievements (global, permanent):

Unlock once, stay unlocked forever
Track across all save slots
Milestones: "Reached level 10", "Completed Rapid Fire path"
Never reset

Save File Structure

user://saves/
├── save_slot_1.save
│   ├── player_progression (level, XP, scrap)
│   ├── tech_tree_state (unlocked, locked)
│   ├── in_game_achievements (progress, unlocked)
│   ├── level_completion_status
│   └── stats (per-run stats)
├── save_slot_2.save
└── save_slot_3.save

user://global.save
├── steam_achievements (permanent)
├── global_stats (all-time totals)
└── settings (shared across saves)

Implementation Priority

✅ Issue #5: Design tech tree structure (includes exclusive branches) - OPTION A CHOSEN
✅ Issue #6: Implement TechTreeManager with locking logic (Option A approach)
✅ Issue #8.5: Create example exclusive branch configs
Issue #4.5: Player progression persistence (UPDATE: multiple save slots)
Issue #38: Save game system (UPDATE: multiple slots + achievement split)
Issue #8: Build UI with exclusive choice warnings
Issue #7: Connect to ObstacleRegistry
NEW: Main menu save slot selection UI

Notes

DECISION LOCKED: Option A (Permanent Exclusive Choices)
Start simple: 2 exclusive choices in early tech tree
Gather player feedback on choice difficulty
Balance can be adjusted via .tres files (no code changes)
Each save slot can explore different tech paths
Steam achievements provide account-wide progression sense
In-game achievements drive tech unlocks per run

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