# Skill Mechanics - Detailed Design

## Core Design Principles

1. **Walking is NEVER punished** - No HP drain from travel. Hiking should be rewarded, not penalized.
2. **Combat is optional and asynchronous** - Monsters queue up during your hike, fight them when convenient.
3. **Turn-based battles** - Classic RPG style combat when you choose to engage.
4. **Skills alternate** - Offensive skills on odd levels, Utility/Defensive on even levels.

---

## Combat System Overview

### Monster Encounters

While hiking, you may randomly encounter monsters based on:
- Location (different biomes spawn different monsters)
- Time of day (nocturnal creatures at night)
- Trail difficulty
- Player level (scaled encounters)

### The Monster Entourage

When you encounter a monster and don't want to fight, **it follows you around on the map**, yelling insults and taunts at you until you deal with it.

**Maximum followers:** 2 monsters per player level
- Level 1 player: Max 2 monsters following
- Level 5 player: Max 10 monsters following
- Level 10 player: Max 20 monsters following

If you're at max capacity, new monsters can't spawn until you fight or dismiss some.

**Map Display:**
```
        🐺 "FIGHT ME COWARD!"
    🦇 "I'LL GET YOU!"          🧭 You
        🌿 "COME BACK HERE!"     ↓
                               [===trail===]
```

**Monster Taunts (examples):**
- 🐺 Trail Wolf: "I've been chasing you for 2 MILES!"
- 🦇 Cave Bat: "You can't ignore me forever!"
- 🌿 Angry Shrub: "I'm a BUSH and I'm faster than you!"
- 🐗 Wild Boar: "My legs are getting tired but my RAGE isn't!"
- 🦎 Trail Lizard: "This is embarrassing for both of us!"

**Interacting with followers:**
- Tap a monster icon to start combat with just that one
- "Fight All" button for sequential battles
- "Dismiss" to shoo them away (no XP, no penalty)
- Monsters stay between sessions (persistent)

**Comedy escalation:**
- Monsters following for 10+ min: Taunts get more desperate
- Monsters following for 30+ min: Start complimenting you instead
- Monsters following for 1+ hour: Existential crisis dialogue

### Turn-Based Combat

When you choose to fight:

```
┌─ BATTLE ──────────────────────┐
│                               │
│   🐺 Trail Wolf               │
│   HP: ████████░░ 45/60        │
│                               │
│   ─────────────────────────   │
│                               │
│   🏃 You (Trail Runner Lv 3)  │
│   HP: ██████████ 100/100      │
│   MP: ████████░░ 40/50        │
│                               │
├───────────────────────────────┤
│ [⚔️ Attack]  [🔥 Double Strike] │
│ [🎒 Items ]  [🏃 Flee        ] │
└───────────────────────────────┘
```

**Combat Flow:**
1. Player turn: Choose action
2. Action resolves
3. Enemy turn: Enemy attacks
4. Repeat until someone hits 0 HP

**Victory:** Gain XP, possible loot drops
**Defeat:** Lose some gold? Lose queued monsters? (TBD - should be minor)

---

## HP System (Revised)

### HP is for COMBAT only

- HP does NOT drain from walking
- HP only changes during battle, skills, or items
- **HP persists between battles** - adds strategy to combat decisions

### HP Recovery (Out of Combat)

| Method | Recovery | Notes |
|--------|----------|-------|
| Time (passive) | 1 HP per minute | Slow but free |
| Rest at geocache | 5 HP per minute | Must be stationary at location |
| Food items | Varies | Snacks, trail mix, etc. |
| Healing skills | Varies | Class abilities |
| Level up | Full restore | Reward for leveling |

### What Happens at 0 HP?

**In Battle:**
- You lose the fight
- Monster escapes (gets dismissed, no XP)
- You're left at 1 HP (can't die, just knocked out of that fight)
- Any remaining monsters in your entourage keep following you

**Strategic Implications:**
- Can't just chain-fight 20 monsters - need to manage HP
- Healing skills become valuable between fights
- Might need to dismiss some monsters if HP is low
- Adds tension: "Can I take one more fight before resting?"

---

## Skill Leveling System

Skills have their own level that improves with use! This is separate from your character level.

### How Skills Level Up

**XP per skill:** Each time you use a skill, it gains Skill XP
- Using a skill in combat: +10 Skill XP
- Skill contributes to victory: +5 bonus Skill XP
- Using skill effectively (e.g., heal when low HP): +5 bonus Skill XP

**Skill XP thresholds:**
| Skill Level | Total XP Needed |
|-------------|-----------------|
| 1 | 0 (starting) |
| 2 | 50 |
| 3 | 150 |
| 4 | 300 |
| 5 | 500 |
| 6 | 800 |
| 7 | 1200 |
| 8+ | +500 per level |

### What Skill Levels Do

Each skill defines its own leveling bonuses. Common patterns:
- **Cooldown reduction** - Use more often
- **Damage/healing increase** - More powerful
- **MP cost reduction** - More efficient
- **Additional effects** - New mechanics unlock

---

## Trail Runner Skills (Revised)

**Skill Pattern:** Odd levels = Offensive, Even levels = Utility/Defensive

**Class Identity:** Gear snob, endurance athlete, trail obsessed, owns $200 shoes, has opinions about hydration vests

| Level | Skill | Type | Flavor |
|-------|-------|------|--------|
| 1 | Brand New Hokas | ⚔️ Offensive | Lightning fast double-strike in premium footwear |
| 2 | Runner's High | 💚 Utility | Endorphins are nature's healing |
| 3 | Shin Kick | ⚔️ Offensive | Targets the most painful spot |
| 4 | Gel Pack | 💚 Utility | Disgusting but effective energy boost |
| 5 | Pole Vault | ⚔️ Offensive | Trekking pole to the FACE |
| 6 | Foam Roll | 💚 Utility | Painful recovery that actually works |
| 7 | Downhill Bomb | ⚔️ Offensive | Reckless descending attack |
| 8 | Aid Station | 💚 Utility | Full rest stop recovery |
| 9 | Final Kick | ⚔️ Offensive | That sprint finish energy |
| 10 | Hundred Miler | 🌟 Ultimate | You simply refuse to stop |

---

### Reserved for Gym Bro (Barbarian) class:
- Rep It Out
- PR Attempt
- Beast Mode
- Protein Shake
- Max Effort
- Drop Set
- Pre-Workout
- Spot Me Bro

---

## Level 1: Brand New Hokas ⚔️

### The Skill
```
Name: Brand New Hokas
Type: Offensive
MP Cost: 12 (reduces with skill level)
Cooldown: None (usable every turn)
Effect: Attack twice in a single turn
```

*"These cost $180 and they're worth EVERY PENNY."*

The Trail Runner's premium cushioned footwear allows for lightning-fast strikes. Two rapid kicks delivered with the confidence that only expensive gear can provide.

### Skill Leveling Progression

Brand New Hokas improves its damage and efficiency as you level it:

| Skill Level | Damage per Hit | MP Cost | Total vs Normal Attack |
|-------------|---------------|---------|------------------------|
| 1 | 80% | 12 | 160% damage |
| 2 | 82% | 12 | 164% damage |
| 3 | 85% | 11 | 170% damage |
| 4 | 87% | 11 | 174% damage |
| 5 | 90% | 10 | 180% damage |
| 6 | 92% | 10 | 184% damage |
| 7 | 95% | 9 | 190% damage |
| 8 | 97% | 9 | 194% damage |
| 9 | 100% | 8 | 200% damage (two full attacks!) |
| 10+ | 100% + crit bonus | 8 | 200%+ with +5% crit chance per level |

**Milestone unlocks:**
- **Level 5:** "Swift Strikes" - Animation speeds up (QoL)
- **Level 9:** "True Double" - Each hit deals full damage
- **Level 10:** "Relentless" - +5% critical hit chance per level beyond 10

### Combat Mechanics

**Base Attack Damage Formula:**
```
Damage = (STR × 2) + WeaponDamage + RandomVariance(0 to STR/2)
```

For a Level 1 Trail Runner with 12 STR and no weapon:
- Base Attack: (12 × 2) + 0 + (0-6) = 24-30 damage

**Double Strike at Skill Level 1:**
- Hit 1: 80% of base = 19-24 damage
- Hit 2: 80% of base = 19-24 damage
- Total: 38-48 damage (vs 24-30 from normal attack)

**Double Strike at Skill Level 9:**
- Hit 1: 100% of base = 24-30 damage
- Hit 2: 100% of base = 24-30 damage
- Total: 48-60 damage for only 8 MP!

### How It Works In-Game

**Scenario:** Battle against a Trail Wolf (45 HP)

```
┌─ BATTLE ──────────────────────┐
│   🐺 Trail Wolf               │
│   HP: ████████░░ 45/45        │
│                               │
│   🏃 You                      │
│   HP: ██████████ 100/100      │
│   MP: ██████████ 50/50        │
├───────────────────────────────┤
│ [⚔️ Attack]  [⚔️⚔️ Double Strike] │
│ [🎒 Items ]  [🏃 Flee         ] │
└───────────────────────────────┘
```

**Player selects Double Strike:**

```
┌─ BATTLE ──────────────────────┐
│                               │
│   ⚔️ Double Strike!           │
│                               │
│   💥 Hit 1: 21 damage!        │
│   💥 Hit 2: 19 damage!        │
│                               │
│   🐺 Trail Wolf               │
│   HP: █░░░░░░░░░ 5/45         │
│                               │
└───────────────────────────────┘
```

**Enemy turn follows, then back to player...**

### Why This Skill is Good for Level 1

1. **Simple to understand** - "Attack twice" is intuitive
2. **Immediately useful** - Works in first battle
3. **Teaches MP management** - Can't spam it forever
4. **Fighter identity** - Establishes Trail Runner as damage dealer

### Edge Cases

- **Not enough MP?** Skill greyed out, tooltip shows "Need 12 MP"
- **Can both hits crit?** Yes, each hit rolls independently
- **Can enemy die on first hit?** Yes, second hit is wasted (no overkill bonus)

---

## Level 2: Runner's High 💚

### The Skill
```
Name: Runner's High
Type: Utility (Healing)
MP Cost: 15
Cooldown: Battle-based (improves with Skill Level)
Effect: Restore 30% of maximum HP
```

*"Around mile 8, something magical happens... the pain becomes pleasure."*

The Trail Runner taps into that euphoric state where endorphins flood the system and everything feels possible. Pain melts away, replaced by a serene confidence.

### Skill Leveling Progression

Runner's High cooldown is based on **number of battles**, and improves as you level the skill:

| Skill Level | Cooldown | In Practice |
|-------------|----------|-------------|
| 1 | Once per 5 battles | Very limited, save for emergencies |
| 2 | Once per 4 battles | Slightly more available |
| 3 | Once per 3 battles | Every third fight |
| 4 | Once per 2 battles | Every other fight |
| 5 | Once per battle | Reliable, use every fight |
| 6 | Twice per battle | Now we're cooking |
| 7 | Three times per battle | Significant sustain |
| 8+ | +1 use per level | Approaching immortality |

**Cooldown Tracking:**
```
Skill Level 1: [✓ Used] [░ 4 battles left] [░] [░] [░]
Skill Level 5: [✓ Used] [Ready next battle!]
Skill Level 7: [✓ Used] [✓ Used] [░ Available]
```

### Combat Mechanics

For a player with 100 max HP:
- Runner's High restores 30 HP (scales slightly with skill level)
- Healing increases with skill level:

| Skill Level | HP Restored |
|-------------|-------------|
| 1-3 | 30% max HP |
| 4-5 | 35% max HP |
| 6-7 | 40% max HP |
| 8+ | 45% max HP |

### How It Works In-Game

**Scenario:** Mid-battle, you've taken some hits. Runner's High is available (cooldown ready).

```
┌─ BATTLE ──────────────────────┐
│   🐺 Trail Wolf               │
│   HP: ██░░░░░░░░ 12/45        │
│                               │
│   🏃 You                      │
│   HP: ████░░░░░░ 42/100       │  ← Hurting!
│   MP: ██████░░░░ 30/50        │
├───────────────────────────────┤
│ [⚔️ Attack     ] [👟 New Hokas    ] │
│ [🏃‍♂️ Runner's High] [🚪 Flee       ] │
│     Lv.3 (1/1)                     │
└───────────────────────────────┘
```

**Player selects Runner's High:**

```
┌─ BATTLE ──────────────────────┐
│                               │
│   🏃‍♂️ RUNNER'S HIGH!           │
│                               │
│   *Breathes rhythmically*     │
│   *Zones out completely*      │
│   *Achieves flow state*       │
│                               │
│   ✨ Recovered 30 HP!         │
│                               │
│   🏃 You                      │
│   HP: ███████░░░ 72/100       │  ← The endorphins are REAL
│   MP: ████░░░░░░ 15/50        │
│                               │
│   Runner's High: 0/1 (recharges in 3 battles)
└───────────────────────────────┘
```

### Strategic Implications

**At Skill Level 1 (once per 5 battles):**
- VERY precious resource
- Only use when absolutely necessary
- Might lose fights you could have won if you waste it

**At Skill Level 5 (once per battle):**
- Reliable tool in every fight
- Can be more aggressive knowing you have a heal
- Still need to time it right

**At Skill Level 7+ (multiple per battle):**
- Major sustain advantage
- Can take on tougher monsters
- Enables riskier strategies

### Edge Cases

- **Cooldown not ready?** Skill greyed out, shows "2 battles remaining"
- **Multiple uses available?** Shows "(2/3)" to indicate uses left this battle
- **At full HP?** Greyed out with "HP Full" tooltip
- **Would overheal?** Caps at max HP (no waste, shows actual heal amount)

---

## Level 3: Shin Kick ⚔️

### The Skill
```
Name: Shin Kick
Type: Offensive
MP Cost: 20
Cooldown: Once per battle
Effect: A devastating strike dealing 250% damage
```

*"Right in the shin. You KNOW how much that hurts."*

Every trail runner knows the agony of a shin strike - whether from a hidden root, a rock, or an aggressive trail marker. Now weaponize that pain against your enemies.

### Skill Leveling Progression

| Skill Level | Damage | MP Cost | Bonus Effect |
|-------------|--------|---------|--------------|
| 1 | 250% | 20 | - |
| 2 | 265% | 20 | - |
| 3 | 280% | 19 | - |
| 4 | 295% | 19 | - |
| 5 | 310% | 18 | "New PR!" - 10% chance to reset cooldown |
| 6 | 325% | 18 | 15% reset chance |
| 7 | 340% | 17 | 20% reset chance |
| 8 | 355% | 17 | 25% reset chance |
| 9 | 370% | 16 | 30% reset chance |
| 10+ | 400% | 15 | "BONE BRUISE" - 35% reset + stuns enemy 1 turn |

### Combat Mechanics

**Comparison to other attacks:**
- Normal Attack: 100% damage, 0 MP
- Brand New Hokas (Lv1): 160% damage, 12 MP
- Shin Kick (Lv1): 250% damage, 20 MP

**When to use:**
- Opening move to chunk a tough enemy
- Finishing blow on a low-HP monster
- When you can afford the MP investment

### How It Works In-Game

```
┌─ BATTLE ──────────────────────┐
│   🐗 Wild Boar                │
│   HP: ██████████ 80/80        │
│                               │
│   🏃 You                      │
│   HP: ██████████ 100/100      │
│   MP: ██████████ 50/50        │
├───────────────────────────────┤
│ [⚔️ Attack   ] [👟 New Hokas   ] │
│ [🦵 Shin Kick] [🏃‍♂️ Runner's High] │
└───────────────────────────────┘
```

**Player selects Shin Kick:**

```
┌─ BATTLE ──────────────────────┐
│                               │
│   🦵 SHIN KICK!               │
│                               │
│   *Winds up*                  │
│   *Aims for the shin*         │
│   *CRACK*                     │
│                               │
│   💥💥💥 62 DAMAGE! 💥💥💥     │
│                               │
│   🐗 Wild Boar                │
│   HP: ██░░░░░░░░ 18/80        │  ← Oof, right in the shin
│                               │
└───────────────────────────────┘
```

### Edge Cases

- **Already used this battle?** Greyed out, shows "Shin Sore" (already gave 'em one)
- **Not enough MP?** Greyed out, shows "Need 20 MP"
- **Kills the enemy?** Displays "CRITICAL SHIN!" celebration text
- **Cooldown resets (Lv5+)?** Shows "OTHER SHIN!" and skill becomes available again

---

## Level 4: Gel Pack 💚

### The Skill
```
Name: Gel Pack
Type: Utility (Buff)
MP Cost: 18
Cooldown: Once per 3 battles
Effect: Boost attack damage by 25% for 4 turns
```

*"It tastes like chemical birthday cake but it WORKS."*

The Trail Runner squeezes down an energy gel - that weird gooey substance that costs $3 per packet and has the consistency of toothpaste. Disgusting? Yes. Effective? Absolutely.

### Skill Leveling Progression

| Skill Level | Damage Boost | Duration | Cooldown |
|-------------|-------------|----------|----------|
| 1 | +25% | 4 turns | 1 per 3 battles |
| 2 | +27% | 4 turns | 1 per 3 battles |
| 3 | +30% | 4 turns | 1 per 3 battles |
| 4 | +32% | 5 turns | 1 per 2 battles |
| 5 | +35% | 5 turns | 1 per 2 battles |
| 6 | +37% | 5 turns | 1 per battle |
| 7 | +40% | 6 turns | 1 per battle |
| 8 | +42% | 6 turns | 2 per battle |
| 9 | +45% | 6 turns | 2 per battle |
| 10+ | +50% | 8 turns | 3 per battle, also +10% crit |

**Bonus at Level 10:** "GU Guru" - Also adds +10% critical hit chance

### Combat Mechanics

**How the buff stacks with attacks:**
```
Normal Attack: 25 damage
With Gel Pack (+25%): 25 × 1.25 = 31 damage

Brand New Hokas: 40 damage (two hits)
With Gel Pack: 40 × 1.25 = 50 damage

Shin Kick: 62 damage
With Gel Pack: 62 × 1.25 = 77 damage
```

**Optimal timing:**
- Use Gel Pack turn 1, then unload damage skills
- 4 turns is enough for 3-4 buffed attacks
- Pairs especially well with Brand New Hokas (double benefit)

### How It Works In-Game

```
┌─ BATTLE ──────────────────────┐
│   🦎 Trail Lizard             │
│   HP: ██████████ 50/50        │
│                               │
│   🏃 You                      │
│   HP: ██████████ 100/100      │
│   MP: ██████████ 50/50        │
├───────────────────────────────┤
│ [⚔️ Attack   ] [👟 New Hokas   ] │
│ [🦵 Shin Kick] [🧴 Gel Pack    ] │
└───────────────────────────────┘
```

**Player selects Gel Pack:**

```
┌─ BATTLE ──────────────────────┐
│                               │
│   🧴 GEL PACK!                │
│                               │
│   *Tears open packet*         │
│   *Squeezes directly into     │
│    mouth*                     │
│   *Tries not to gag*          │
│                               │
│   ⚡ ATK +25% for 4 turns!    │
│                               │
│   🏃 You [🧴 GELLED]          │
│   HP: ██████████ 100/100      │
│   MP: ████████░░ 32/50        │
│                               │
└───────────────────────────────┘
```

**Buff indicator persists:**
```
│   🏃 You [🧴 3 turns]         │  ← Shows remaining duration
```

### Edge Cases

- **Already have the buff?** Can't stack, refreshes duration instead
- **Buff expires mid-battle?** Shows "Sugar crash..." message
- **Multiple uses (Lv8+)?** Can refresh before expiring for continuous uptime
- **Does it affect healing?** No, damage only (it's SUGAR not PROTEIN)

### Synergy Notes

**Gel Pack + Brand New Hokas combo:**
- Gel Pack Turn 1 (+25% damage)
- Brand New Hokas Turn 2, 3, 4 (all buffed)
- Each Hokas attack now deals ~50 damage instead of 40
- Total extra damage over 4 turns: ~30-40 bonus damage

**Gel Pack + Shin Kick combo:**
- Gel Pack Turn 1
- Shin Kick Turn 2 for ~77 damage instead of 62
- Finish with buffed normal attacks

---

## Level 5: Pole Vault ⚔️

### The Skill
```
Name: Pole Vault
Type: Offensive
MP Cost: 16
Cooldown: None (usable every turn)
Effect: 150% damage attack with 40% chance to stun enemy for 1 turn
```

*"These carbon fiber trekking poles cost $250. Time to get my money's worth."*

The Trail Runner plants their premium trekking pole and launches themselves foot-first into the enemy's face. It's not what the manufacturer intended, but it's devastatingly effective.

### Skill Leveling Progression

| Skill Level | Damage | Stun Chance | MP Cost |
|-------------|--------|-------------|---------|
| 1 | 150% | 40% | 16 |
| 2 | 155% | 42% | 16 |
| 3 | 160% | 45% | 15 |
| 4 | 165% | 47% | 15 |
| 5 | 170% | 50% | 14 |
| 6 | 175% | 52% | 14 |
| 7 | 180% | 55% | 13 |
| 8 | 185% | 57% | 13 |
| 9 | 190% | 60% | 12 |
| 10+ | 200% | 65% | 12, "Pole Position" - stun lasts 2 turns |

**Milestone at Level 10:** "Pole Position" - Stuns now last 2 turns instead of 1

### Combat Mechanics

**Comparison to other offensive skills:**
| Skill | Damage | Special | MP Cost |
|-------|--------|---------|---------|
| Brand New Hokas | 160% (2×80%) | Two hits | 12 |
| Shin Kick | 250% | Once per battle | 20 |
| Pole Vault | 150% | 40% stun | 16 |

**When to use Pole Vault:**
- Enemy is about to use a big attack (stun prevents it)
- You need breathing room to heal next turn
- Setting up for a Shin Kick (stunned enemies can't dodge)

**Stun mechanics:**
- Stunned enemies skip their next turn
- Stunned enemies take 10% bonus damage from all sources
- Stun doesn't stack (refreshes duration if re-applied)

### How It Works In-Game

```
┌─ BATTLE ──────────────────────┐
│   🐗 Wild Boar                │
│   HP: ██████░░░░ 48/80        │
│   [🔄 Charging up...]         │  ← About to do something bad
│                               │
│   🏃 You                      │
│   HP: ██████░░░░ 55/100       │
│   MP: ████████░░ 40/50        │
├───────────────────────────────┤
│ [⚔️ Attack   ] [👟 New Hokas   ] │
│ [🦵 Shin Kick] [🥢 Pole Vault  ] │
└───────────────────────────────┘
```

**Player selects Pole Vault:**

```
┌─ BATTLE ──────────────────────┐
│                               │
│   🥢 POLE VAULT!              │
│                               │
│   *Plants trekking pole*      │
│   *YEETS self at enemy*       │
│   *Both feet connect*         │
│                               │
│   💥 38 DAMAGE!               │
│   ⭐ STUNNED!                 │
│                               │
│   🐗 Wild Boar [💫 STUNNED]   │
│   HP: ███░░░░░░░ 10/80        │
│                               │
└───────────────────────────────┘
```

**If stun fails (60% chance at Lv1):**
```
│   💥 38 DAMAGE!               │
│   (Boar shakes it off)        │
```

### Strategic Considerations

**Pole Vault vs Brand New Hokas:**
- Hokas: More reliable damage (160% vs 150%)
- Pole Vault: Chance to prevent enemy turn entirely
- Use Hokas for pure DPS, Pole Vault for control

**Pole Vault + Shin Kick combo:**
1. Pole Vault (hope for stun)
2. If stunned: Free Shin Kick next turn (250% damage + 10% stun bonus = 275%)
3. Enemy wasted a turn, you dealt massive damage

**Pole Vault + Runner's High combo:**
1. Pole Vault to stun
2. Runner's High to heal safely while enemy is stunned
3. Resume attacking at higher HP

### Edge Cases

- **Not enough MP?** Greyed out, shows "Need 16 MP"
- **Enemy already stunned?** Can still use, refreshes stun duration
- **Enemy immune to stun?** Damage still applies, shows "Immune to stun!"
- **Critical hit?** Stun chance increases to 60% on crits

---

## Implementation Considerations

### Data We Need to Track

```javascript
player = {
  hp: 80,
  maxHp: 100,
  mp: 35,
  maxMp: 50,

  skills: {
    secondWind: {
      unlocked: true,
      cooldownEnds: 1704567890000, // Unix timestamp
    },
    steadyPace: {
      unlocked: true,
      // No cooldown for passives
    }
  },

  // For passive tracking
  stats: {
    hpSavedBySteadyPace: 47,
    distanceTraveledToday: 8500, // meters
    elevationGainedToday: 320,  // meters
  }
}
```

### GPS Integration

The drain system needs to integrate with GPS tracking:

```javascript
function onPositionUpdate(newPosition) {
  const distance = calculateDistance(lastPosition, newPosition);
  const elevation = calculateElevationGain(lastPosition, newPosition);

  let drain = 0;

  // Base distance drain
  drain += distance / 100; // 1 HP per 100m

  // Elevation drain
  drain += (elevation / 10) * 2; // 2 HP per 10m climbed

  // Apply terrain modifier (from trail data)
  drain *= getTerrainModifier(currentTrail);

  // Apply passive skills
  if (player.skills.steadyPace.unlocked) {
    drain *= 0.85; // -15%
  }

  // Apply racial bonuses
  drain *= player.race.movementDrainModifier;

  // Apply the drain
  player.hp = Math.max(0, player.hp - drain);

  updateUI();
}
```

---

## Next Steps

- [ ] Define how MP regenerates
- [ ] Define what Difficult Terrain actually means (GPS data? Manual marking?)
- [ ] Design the skills UI
- [ ] Define Level 3: Trail Legs mechanics