2nd/10_Wiki/Topics/Architecture/가차(Gacha).md

---
id: wiki-2026-0508-가차-gacha
title: 가차(Gacha)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Gacha, ガチャ, 뽑기, Lootbox]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [game-design, monetization, randomness, mobile-game]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript
  framework: Game Server (Node.js)
---

# 가차(Gacha)

## 매 한 줄
> **"매 weighted random pull + pity system — 매 mobile game monetization 의 dominant 메커니즘."**. 1965 일본 캡슐 토이 (gachapon) 어원. 2010s Puzzle & Dragons / FGO / Genshin Impact 매 globalization. 매 2026 regulatory scrutiny (Belgium ban, China 概率 disclosure, Japan 上限) 매 mature.

## 매 핵심

### 매 components
- **Pool / Banner**: 매 pullable item set — rate-up, character-specific, standard.
- **Rate**: 매 rarity 별 확률 (e.g. SSR 0.6%, SR 5.4%, R 94%).
- **Pity**: 매 N pull guarantee — 매 player frustration cap (Genshin 90 pull, FGO 330).
- **Soft pity**: 매 pity 근접 시 rate ramp (Genshin 75+ → SSR rate 폭증).
- **Currency**: 매 premium (paid) vs free — 매 dual track.
- **50/50**: 매 rate-up 미달 시 standard pool 으로 떨어짐.

### 매 design 축
- **Convergent vs divergent**: 매 dupe 가 useful (constellation) vs useless (shard).
- **Hard pity**: 매 deterministic ceiling — 매 player trust.
- **Spark / mileage**: 매 pull 누적 → 매 직접 교환 — 매 ultimate pity.

### 매 응용
1. Mobile RPG (Genshin Impact, Honkai Star Rail).
2. Card collection (Hearthstone pack 매 gacha 변형).
3. Battle pass 매 deterministic 변형.
4. Cosmetic-only gacha (매 regulatory safer).

## 💻 패턴

### Weighted random pull
```typescript
type Item = { id: string; rarity: 'SSR'|'SR'|'R'; rateUp?: boolean };

function pull(rates: Record<string, number>, items: Item[]): Item {
  const r = Math.random();
  let acc = 0;
  let chosenRarity: keyof typeof rates;
  for (const [rarity, p] of Object.entries(rates)) {
    acc += p;
    if (r < acc) { chosenRarity = rarity as any; break; }
  }
  const pool = items.filter(i => i.rarity === chosenRarity!);
  return pool[Math.floor(Math.random() * pool.length)];
}

const rates = { SSR: 0.006, SR: 0.054, R: 0.94 };
```

### Soft + hard pity
```typescript
class PitySystem {
  pulls = 0;
  softStart = 75;
  hardCap = 90;

  ssrRate(): number {
    if (this.pulls >= this.hardCap) return 1.0;
    if (this.pulls < this.softStart) return 0.006;
    // 매 linear ramp 75 → 89
    const t = (this.pulls - this.softStart) / (this.hardCap - this.softStart);
    return 0.006 + t * (1.0 - 0.006);
  }

  pull(): { rarity: string; pulls: number } {
    this.pulls++;
    if (Math.random() < this.ssrRate()) {
      const out = { rarity: 'SSR', pulls: this.pulls };
      this.pulls = 0;
      return out;
    }
    return { rarity: 'SR/R', pulls: this.pulls };
  }
}
```

### 50/50 rate-up
```typescript
class RateUpBanner {
  guaranteed = false; // 매 직전 SSR 이 off-banner 이면 다음 SSR rate-up 확정

  rollSSR(featured: Item, standardSSRs: Item[]): Item {
    if (this.guaranteed) {
      this.guaranteed = false;
      return featured;
    }
    if (Math.random() < 0.5) {
      this.guaranteed = false;
      return featured;
    }
    this.guaranteed = true;
    return standardSSRs[Math.floor(Math.random() * standardSSRs.length)];
  }
}
```

### Spark / mileage shop
```typescript
type Wallet = { stones: number; mileage: number };

function spend(wallet: Wallet, cost: number) {
  if (wallet.stones < cost) throw new Error('insufficient');
  wallet.stones -= cost;
  wallet.mileage += 1; // 매 pull 당 1 mileage
}

function exchangeMileage(wallet: Wallet, item: Item, price = 300) {
  if (wallet.mileage < price) throw new Error('mileage 부족');
  wallet.mileage -= price;
  return item;
}
```

### 매 rate disclosure (compliance)
```typescript
// 매 일본 JOGA / 중국 网信办 매 mandatory disclosure.
function bannerDisclosure(banner: Banner) {
  return {
    rates: banner.rates,            // SSR 0.6% etc
    pity: { soft: 75, hard: 90 },
    rateUpItems: banner.featured.map(i => i.id),
    expectedCostJPY: estimateExpectedCost(banner),
  };
}
```

### Server-side roll (anti-cheat)
```typescript
import { randomBytes } from 'node:crypto';

// 매 client 의 RNG 신뢰 X — 매 server 결정.
async function serverPull(userId: string, bannerId: string) {
  const seed = randomBytes(16).toString('hex');
  const result = pull(bannerRates[bannerId], items);
  await db.gachaLog.insert({ userId, bannerId, seed, result, ts: Date.now() });
  return result;
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 매 player 신뢰 | hard pity + spark |
| 매 monetization 극대화 | soft pity 후반 ramp |
| 매 collection 게임 | convergent (constellation 유용) |
| 매 PVP 밸런스 | 매 cosmetic-only 우선 (regulatory safe) |
| 매 신규 banner | 50/50 with rate-up |

**기본값**: 매 soft pity 75 / hard 90 / 50-50 / spark 300 — 매 Genshin model 의 industry baseline.

## 🔗 Graph
- 부모: [[Game_Monetization]]
- 변형: [[Lootbox]]
- 응용: [[Genshin_Impact]]

## 🤖 LLM 활용
**언제**: 매 mobile RPG monetization 설계, gacha sim, drop rate calculation.
**언제 X**: 매 Belgium / Netherlands market (매 lootbox 금지). 매 PEGI 18 관련 region risk.

## ❌ 안티패턴
- **Hidden rate**: 매 disclosure X — 매 regulatory ban / store removal 위험.
- **No pity**: 매 player frustration → churn 폭증.
- **Client-side RNG**: 매 trivially exploit — 매 packet replay attack.
- **Stacking dupes 무용**: 매 SSR pull 후 dupe = 매 무가치 → 매 trust 손실. 매 conversion 시스템 mandatory.
- **PvP P2W gacha**: 매 정체된 player base — 매 cosmetic 분리.

## 🧪 검증 / 중복
- Verified (Genshin Wiki rate data, JOGA guidelines, Belgium Gaming Commission report).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — gacha mechanics + pity / regulatory |