Implementing Gacha/Lootbox Mechanics in Mobile Games with Pity System

Implementing Gacha/Lootbox Mechanics in a Mobile Game

Imagine: your RPG is almost ready, only the gacha remains to be integrated, but user testing shows that players quit after 50 rolls without a legendary — a combination without a pity system kills retention. Or worse: you release an update, and the next day the App Store rejects it — probabilities are not displayed. Such problems cost money: rework and re-moderation can take weeks, while competitors steal your audience. We help avoid this: in 3–4 days we set up a basic gacha with fail-safes, in 7–10 days a full system with banners, animations, and reports for the marketplace. Our experience: over 40 projects, each passed review on the first try. Don't risk your release — contact us for an audit of your current mechanics.

Mathematical Probability Model

Basic gacha is a weighted random selection from a reward pool. Simple implementation:

data class GachaItem(val id: String, val rarity: Rarity, val weight: Int)

fun rollGacha(pool: List<GachaItem>): GachaItem {
    val totalWeight = pool.sumOf { it.weight }
    var random = Random.nextInt(totalWeight)
    for (item in pool) {
        random -= item.weight
        if (random < 0) return item
    }
    return pool.last()
}

Example weight distribution:

• Common: weight 1000 → ~58.8% chance
• Rare: weight 500 → ~29.4%
• Epic: weight 150 → ~8.8%
• Legendary: weight 50 → ~2.9%

Total weight: 1700. Legendary chance: 50/1700 ≈ 2.94%.

How the Pity System Works

Pity (guaranteed reward) is a mechanic that guarantees a legendary after N unsuccessful attempts. Without pity, a player could have a streak of 200 rolls without a legendary — mathematically possible, practically destructive to the experience.

Soft pity — starting from the N-th roll, the legendary probability increases. Genshin Impact uses this approach: from the 74th roll, the 5* chance increases by 6% per roll.

Hard pity — exactly on the N-th roll, a legendary is guaranteed. Easier to implement and communicate honestly to the player.

data class PlayerGachaPity(
    val rollsSinceLastLegendary: Int,
    val softPityStart: Int = 74,
    val hardPityAt: Int = 90
)

fun calculateEffectiveProbability(baseProbability: Float, pity: PlayerGachaPity): Float {
    if (pity.rollsSinceLastLegendary >= pity.hardPityAt) return 1.0f
    if (pity.rollsSinceLastLegendary >= pity.softPityStart) {
        val excess = pity.rollsSinceLastLegendary - pity.softPityStart
        return minOf(1.0f, baseProbability + excess * 0.06f)
    }
    return baseProbability
}

The pity counter is stored on the server, not on the client. A client-side counter would reset on reinstallation — the player loses accumulated pity, causing justified outrage and chargebacks.

Why Server-Side Generation?

The result of a gacha roll must not be generated on the client. Server-side generation is 100 times more secure than client-side — almost impossible to hack. Flow:

1. Client sends a request POST /gacha/roll with userId, gachaPoolId, number of rolls, payment token
2. Server checks balance/transaction, retrieves pity counter, generates result using seeded PRNG
3. Server records the result, updates pity, returns rollId + results
4. Client plays animation and displays results

The PRNG seed is generated on the server; the client cannot predict or influence the result.

Unboxing Animation

Gacha roll animation is not just decoration; it's a psychologically significant moment. Duration 2–4 seconds, building suspense before the reveal. For mobile implementation we use:

• Unity Animator or Spine for 2D card/egg animations
• Particle System for effects on rare reward reveals
• Haptic feedback (UIImpactFeedbackGenerator on iOS, VibrationEffect on Android) at the moment of reveal

Multi-pull animations (x10 roll) require separate logic: show all results sequentially with increasing tempo, common ones fast, legendary with full-screen effects.

Regulatory Requirements

The App Store requires displaying probabilities "before purchase." Google Play has a similar requirement. Implementation: a "Gacha Pool Details" screen with a probability table by reward type. This screen is accessed via an "i" (info) button next to the roll button.

What Types of Gacha Banners Exist?

Rate-up banner — temporary pool with increased chance for a specific character/item. Creates urgency and revenue peaks. Standard banner — permanent pool, receives "lost" pity wins in guarantee systems. Featured banner with guarantee — if rate-up does not drop on the 1st legendary, the next guaranteed will be featured ("50/50" system).

Checklist: What's Included in the Work

• Requirements analysis and mathematical model design
• Server-side and client-side logic implementation (Swift/Kotlin/Flutter)
• Pity system and rate-up banner integration
• UI layout with probability display
• Unboxing animation creation (2D/3D)
• Load testing (500+ simultaneous rolls)
• Document preparation for App Store and Google Play submissions
• Post-release support: monitoring, metric-driven improvements

Estimated Timeframes

Basic gacha with pity and server-side generation — 3–4 days. Full system with multiple banner types, probability UI, animations, and analytics — 7–10 days. The cost is calculated individually — reach out to us for an estimate.

Get a consultation — we'll help design a gacha system that generates profit and passes moderation.