📊 Limit Order Book Engine

A production-grade Limit Order Book engine implemented in Java — the core component of every stock exchange (NSE, BSE, NYSE, NASDAQ).

📌 Overview

Every trade you make on a stock exchange goes through a Limit Order Book. This engine:

• Accepts buy and sell orders from traders
• Organizes them by price and time priority
• Automatically matches buyers with sellers
• Executes trades when prices agree
• Handles partial fills and cancellations

✨ Features

• ✅ Limit order placement (buy and sell)
• ✅ Market order instant execution
• ✅ Automatic order matching engine
• ✅ Partial fill handling
• ✅ Order cancellation by ID
• ✅ Price-Time priority (FIFO)
• ✅ Real-time spread and mid price
• ✅ Full trade history recording
• ✅ 27 unit tests — all passing

📐 How It Works

Price-Time Priority:

Same price?      → First order placed wins (FIFO)
Different price? → Best price wins
BUY:  Higher price = better priority
SELL: Lower price  = better priority

Matching Rule:

TRADE happens when:
BID price >= ASK price
Trade executes at RESTING order price

Order Types:

LIMIT  → Wait for specific price or better
MARKET → Execute immediately at best price

🗂️ Project Structure

limit_order_book_java/
├── pom.xml
├── README.md
├── data/
│   └── sample_orders.csv
└── src/
    ├── main/java/com/trading/lob/
    │   ├── Main.java
    │   ├── model/
    │   │   ├── Order.java
    │   │   ├── OrderSide.java
    │   │   ├── OrderType.java
    │   │   └── Trade.java
    │   ├── book/
    │   │   ├── OrderBook.java
    │   │   ├── PriceLevel.java
    │   │   └── MatchingEngine.java
    │   └── display/
    │       └── BookDisplay.java
    └── test/java/com/trading/lob/
        ├── OrderBookTest.java
        ├── MatchingEngineTest.java
        └── PriceLevelTest.java

🚀 How to Run

1. Clone the repository

git clone https://github.com/rupeshgoswami/limit_order_book_java.git
cd limit_order_book_java

2. Compile

mvn compile

3. Run

mvn exec:java

4. Test

mvn test

📊 Sample Output

==========================================
      LIMIT ORDER BOOK ENGINE
==========================================

--- SCENARIO 1: Building the Book ---
+------------------------------------------+
|    LIMIT ORDER BOOK - RELIANCE           |
+------------------------------------------+
|   BID (BUY)         | ASK (SELL)         |
|  Qty      Price     | Price      Qty     |
+--------------------+---------------------+
|   500   2500.00     | 2501.00   300      |
|  1000   2499.00     | 2502.00   500      |
|   750   2498.00     | 2503.00   200      |
+--------------------+---------------------+
| Best Bid  : 2500.00                      |
| Best Ask  : 2501.00                      |
| Spread    : 1.00                         |
| Mid Price : 2500.50                      |
| Orders    : 6                            |
+------------------------------------------+

--- SCENARIO 2: Full Match ---
  TRADE EXECUTED: 300 shares @ 2501.00

--- SCENARIO 3: Partial Match ---
  TRADE EXECUTED: 500 shares @ 2502.00

--- SCENARIO 4: Market Order ---
  TRADE EXECUTED: 200 shares @ 2500.00

--- SCENARIO 5: Cancel Order ---
  Cancel result: SUCCESS

==========================================
  Total Orders Processed : 9
  Total Trades Executed  : 3
==========================================

🧪 Test Results

-----------------------------------------------
 T E S T S
-----------------------------------------------
OrderBookTest        → Tests run: 11, Failures: 0
MatchingEngineTest   → Tests run:  9, Failures: 0
PriceLevelTest       → Tests run:  7, Failures: 0
-----------------------------------------------
Total: 27 | Failures: 0 | Errors: 0
BUILD SUCCESS
-----------------------------------------------

🛠️ Data Structures

📚 Key Operations

🛠️ Tech Stack

👤 Author

Rupesh Goswami

• GitHub: @rupeshgoswami

📄 License

This project is licensed under the MIT License.