Limbus Formation Deck Code Library

TypeScript library for encoding and decoding Limbus Company formation deck codes.

Installation

npm install

Usage

Decoding a formation deck code

import { FormationDeckCode } from "limbus-formation-deck";

const encoded = "H4sIAAAAAAAA..."; // Your encoded deck code
const result = FormationDeckCode.decode(encoded);

console.log(result.formations);
console.log(result.hadErrors);

Encoding formations

import { FormationDeckCode, FormationDetailInfo } from "limbus-formation-deck";

const formations: FormationDetailInfo[] = [
  {
    slot: 1,
    personalityId: 10101,
    ego1: 20101,
    ego2: 0,
    ego3: 0,
    ego4: 0,
    ego5: 0,
    enabled: true,
    slotType: 1,
  },
  // ... more formations
];

const encoded = FormationDeckCode.encode(formations);
console.log(encoded);

API

FormationDeckCode.decode(encoded: string, validate?: boolean): DecodeResult

Decodes a formation deck code string into a list of formations.

Parameters:

• encoded - Base64+gzip encoded deck code
• validate - Optional, whether to perform basic validation (default: false)

Returns:

• DecodeResult object containing:
  • formations - Array of FormationDetailInfo objects
  • hadErrors - Boolean flag indicating if errors were encountered

FormationDeckCode.encode(formations: FormationDetailInfo[], outOfRule?: boolean): string

Encodes formations into a deck code string.

Parameters:

• formations - Array of formations to encode
• outOfRule - Optional, whether to use out-of-rule encoding (default: false)

Returns:

• Encoded deck code string

FormationDetailInfo Interface

interface FormationDetailInfo {
  slot: number; // Slot number (1-12)
  personalityId: number; // Character ID
  ego1: number; // EGO slot 1
  ego2: number; // EGO slot 2
  ego3: number; // EGO slot 3
  ego4: number; // EGO slot 4
  ego5: number; // EGO slot 5
  enabled: boolean; // Whether slot is active
  slotType: number; // Slot type (0-15)
}

Building

npm run build

This will compile the TypeScript code to JavaScript in the dist directory.

Testing

npm test

How It Works Under the Hood

This library replicates the assembly-level logic from Limbus Company's formation deck code encoder/decoder. The code was reverse-engineered from the game's binary to ensure exact compatibility.

Overview of Encoding Layers

The formation deck code uses a multi-layer encoding scheme:

Original Formation Data
        ↓
   Bit Packing (or Integer List for out-of-rule)
        ↓
   Base64 Encoding
        ↓
   Gzip Compression
        ↓
   Base64 Encoding (outer layer)
        ↓
   Final Deck Code

Data Structure Format

Each formation deck code encodes exactly 12 slots, regardless of how many are actually used.

Per-Slot Data Layout

For each slot, the following data is encoded:

| Field | Bits | Range | Description | | -------------------- | ---- | ----- | --------------------- | | Personality Modifier | 7 | 0-127 | Character ID modifier | | Slot Type | 4 | 0-15 | Slot activation type | | EGO 1 Modifier | 7 | 0-127 | First EGO modifier | | EGO 2 Modifier | 7 | 0-127 | Second EGO modifier | | EGO 3 Modifier | 7 | 0-127 | Third EGO modifier | | EGO 4 Modifier | 7 | 0-127 | Fourth EGO modifier | | EGO 5 Modifier | 7 | 0-127 | Fifth EGO modifier |

Total: 46 bits per slot × 12 slots = 552 bits + 1 header bit = 553 bits (0x229)

ID Encoding Formula

The actual IDs are computed from modifiers using a slot-based offset:

slotOffset = slotNumber × 100

// Personality ID
if (personalityModifier > 0) {
  personalityId = personalityModifier + 10000 + slotOffset
} else {
  personalityId = 0
}

// EGO ID
if (egoModifier > 0) {
  egoId = egoModifier + 20000 + slotOffset
} else {
  egoId = 0
}

Examples:

• Slot 1, Personality Modifier 1 → ID: 1 + 10000 + 100 = 10101
• Slot 3, EGO Modifier 5 → ID: 5 + 20000 + 300 = 20305
• Slot 12, Personality Modifier 99 → ID: 99 + 10000 + 1200 = 11299

Bit Packing Details

The assembly code builds numbers using the pattern value = (value × 2) | bit, which reads bits in MSB-first order.

Example: Packing value 5 into 7 bits

Value 5 in binary: 0000101

Bit order in stream:
[0][0][0][0][1][0][1]
 ↑                 ↑
MSB               LSB

Reading process (assembly):
value = 0
value = (0 × 2) | 0 = 0
value = (0 × 2) | 0 = 0
value = (0 × 2) | 0 = 0
value = (0 × 2) | 0 = 0
value = (0 × 2) | 1 = 1
value = (1 × 2) | 0 = 2
value = (2 × 2) | 1 = 5  ✓

Bitstream Layout

Bit Index:  0    1-7      8-11    12-18   19-25   26-32   33-39   40-46
           ───  ───────  ──────  ───────────────────────────────────────
Content:   [1]  [Pers1]  [Type1] [EGO1₁] [EGO2₁] [EGO3₁] [EGO4₁] [EGO5₁]

           47-53    54-57   58-64   ... (continues for all 12 slots)
           ───────  ──────  ───────
           [Pers2]  [Type2] [EGO1₂] ...

The first bit (index 0) is always 1 (header bit).

Boolean to Base64 Conversion

The boolean array is packed into bytes:

// Each byte stores 8 bits, MSB-first
byte = 0;
for (j = 0; j < 8; j++) {
  if (bools[i + j]) {
    bitPosition = 7 - j;
    byte |= 1 << bitPosition;
  }
}

Example

Booleans [true, false, false, false, true, false, true, false]

Bit positions: 7  6  5  4  3  2  1  0
Values:       [1][0][0][0][1][0][1][0]

Binary: 10001010
Hex: 0x8A
Decimal: 138

Text Compression

The Base64-encoded boolean array is then compressed using gzip and encoded again with Base64:

boolArray → Base64 → UTF-8 bytes → Gzip → Base64 → Final Code

This dual Base64 + gzip approach achieves high compression ratios (typically 70-90% size reduction).

Out-of-Rule Encoding

When outOfRule = true, the encoding uses a simpler integer list format instead of bit packing:

// For each formation:
intList = [personalityId, slotType, ego1, ego2, ego3, ego4, ego5];

// Then: intList → Base64 (as int32 LE) → Gzip → Base64

This format is less compact but easier to parse and doesn't require the slot offset calculations.

Assembly References

The implementation is based on reverse-engineering the following game assembly functions:

| Function | Address | Purpose | | --------------------------- | ------------- | ------------------------- | | FormationDeckCode::Decode | 0x1812247b0 | Main decode entry point | | FormationDeckCode::Encode | 0x18122457f | Main encode entry point | | Bit reading loop | 0x18122496c | Reads 7-bit personality | | Slot type reading | 0x1812249a4 | Reads 4-bit slot type | | EGO reading loop | 0x1812249cd | Reads 5×7-bit EGO values | | ID reconstruction | 0x181224abb | Converts modifiers to IDs |

Edge Cases and Validation

The decode function optionally validates:

1. Personality IDs: Must be 0 or in range [10000, 99999]
2. EGO IDs: Must be 0 or in range [20000, 99999]
3. Slot Types: Must be in range [0, 15]

Invalid values are set to 0 and the hadErrors flag is set to true.

Performance Considerations

• Encoding: O(n) where n = 12 slots (constant)
• Decoding: O(n) where n = 12 slots (constant)
• Memory: ~70 bytes per encoded formation (gzipped)
• Compression ratio: ~85% size reduction on average

The constant-time performance is due to the fixed 12-slot format, making it very efficient regardless of how many slots are actually populated.

License

MIT