Deterministic Combat Simulation: A Simple Guide

Hey guys! Today, we're diving into how to implement a deterministic combat simulation endpoint using POST /combat/simulate. This is super useful when you need consistent and predictable results for testing, balancing, or even replaying combat scenarios. Let's break it down!

Understanding Deterministic Simulations

First off, what does "deterministic" even mean in this context? Simply put, it means that given the same initial conditions, the simulation will always produce the same outcome. This is achieved by controlling the random number generation (RNG) process. In a typical combat simulation, you might use RNG for things like hit chances, damage rolls, critical hits, and special abilities. To make it deterministic, we need to seed the RNG.

Seeding the RNG involves providing an initial value (the seed) to the random number generator. The RNG then uses this seed to produce a sequence of pseudo-random numbers. The beauty of it is that if you use the same seed, you'll get the same sequence of numbers every single time. This is the key to determinism.

Now, why is this important? Imagine you're testing a new character ability. You want to make sure it's balanced and doesn't lead to unfair advantages. If your combat simulation is random, it's hard to get consistent results. One time the ability might seem overpowered because of lucky critical hits, and another time it might seem weak because of missed attacks. With a deterministic simulation, you can run the same scenario multiple times and get the same results, allowing you to accurately assess the ability's impact. Moreover, determinism greatly aids in debugging and identifying the root cause of issues. If a bug occurs during a simulation, being able to reproduce the exact conditions that led to the bug is invaluable. By using the same seed, developers can step through the simulation and pinpoint the problem.

Deterministic simulations also open the door to combat replays. Players can share the seed of a particularly exciting or controversial battle, allowing others to recreate and analyze the events. This is particularly useful in competitive games or esports, where understanding the nuances of combat is crucial.

Implementing POST /combat/simulate

Okay, let's get into the nitty-gritty of implementing the POST /combat/simulate endpoint. The goal is to create an endpoint that accepts a seed parameter (optional), runs a deterministic combat simulation, and returns the results along with the seed_used.

Endpoint Structure

We'll use a POST request because we're potentially sending data to the server to define the combat scenario (e.g., the characters involved, their stats, equipment, etc.). The endpoint URL will be /combat/simulate.

Input Parameters

The main input parameter is the seed. This can be passed in the request body (e.g., as a JSON payload) or as a query parameter in the URL. If the seed parameter is provided, we'll use it to seed the RNG. If it's not provided, we'll generate a random seed on the server. This allows users to either control the simulation or let the server handle it.

Server-Side Logic

Here's a step-by-step breakdown of the server-side logic:

1. Receive the Request: The server receives the POST request to /combat/simulate.
2. Extract the Seed: The server checks if the seed parameter is present in the request.
   • If the seed is present, use it as the seed for the RNG.
   • If the seed is absent, generate a random seed (e.g., using a secure random number generator).
3. Seed the RNG: Use the extracted or generated seed to initialize the random number generator. Most programming languages have built-in RNG functions that allow you to set the seed. For example, in Python, you can use random.seed(seed_value). In Java, you can use new Random(seed_value). It’s crucial to use a good pseudo-random number generator (PRNG) algorithm suitable for simulations, such as Mersenne Twister, to ensure high-quality and consistent random numbers.
4. Run the Simulation: Execute the combat simulation, using the seeded RNG for all random events (hit chances, damage rolls, etc.).
5. Prepare the Response: Create a response payload containing the results of the simulation and the seed_used. The seed_used field is important because it tells the user which seed was actually used for the simulation, whether it was the one they provided or one generated by the server.
6. Send the Response: Send the response back to the client. The response should be in a structured format like JSON.

Example Code (Python)

import random
from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/combat/simulate', methods=['POST'])
def simulate_combat():
    data = request.get_json()
    seed = data.get('seed')

    if seed is None:
        seed = random.randint(0, 1000000)  # Generate a random seed
        
    random.seed(seed)

    # Simulate combat (replace with your actual combat logic)
    attacker_hit = random.random() > 0.3  # 70% hit chance
    damage = random.randint(10, 20)

    result = {
        'attacker_hit': attacker_hit,
        'damage': damage,
    }

    response = {
        'seed_used': seed,
        'result': result
    }

    return jsonify(response)

if __name__ == '__main__':
    app.run(debug=True)

In this example, we're using Flask to create a simple web server. The simulate_combat function handles the POST request to /combat/simulate. It extracts the seed from the JSON payload, generates a random seed if one is not provided, seeds the RNG using random.seed(seed), runs a very basic combat simulation (with a 70% hit chance and damage between 10 and 20), and returns the results along with the seed_used.

Response Payload

The response payload should be a JSON object containing the simulation results and the seed_used. Here's an example:

{
  "seed_used": 12345,
  "result": {
    "attacker_hit": true,
    "damage": 15
  }
}

The seed_used field tells the client which seed was used for the simulation. The result field contains the simulation results, which can include things like whether the attacker hit, the amount of damage dealt, and any other relevant information.

Unit Tests for Determinism

Testing is crucial to ensure that your deterministic simulation is actually deterministic. You need to write unit tests that verify that repeated calls with the same seed produce identical results. Here's how you can do it:

1. Choose a Seed: Select a specific seed value for your test.
2. Run the Simulation Multiple Times: Call the /combat/simulate endpoint multiple times with the same seed.
3. Compare the Results: Compare the results of each simulation run. They should be identical. If they're not, then there's something wrong with your RNG seeding or your simulation logic.

Example Unit Test (Python)

import unittest
import json
from your_app import app  # Replace with your Flask app

class TestCombatSimulation(unittest.TestCase):

    def setUp(self):
        self.app = app.test_client()
        self.app.testing = True

    def test_deterministic_simulation(self):
        seed = 12345
        num_runs = 5
        results = []

        for _ in range(num_runs):
            response = self.app.post('/combat/simulate', json={'seed': seed})
            self.assertEqual(response.status_code, 200)
            data = json.loads(response.get_data(as_text=True))
            results.append(data['result'])

        # Verify that all results are the same
        for i in range(1, num_runs):
            self.assertEqual(results[0], results[i])

if __name__ == '__main__':
    unittest.main()

In this example, we're using the unittest module to create a unit test for our combat simulation endpoint. The test_deterministic_simulation function runs the simulation five times with the same seed (12345) and then verifies that all the results are the same. If any of the results are different, the test will fail, indicating that our simulation is not deterministic.

Acceptance Criteria

To ensure that your implementation meets the requirements, here are the acceptance criteria:

• Repeated calls with the same seed produce identical results: This is the core of determinism. You need to verify this with unit tests.
• Response includes seed_used: The response payload must include the seed_used field, so the client knows which seed was actually used for the simulation.

Wrapping Up

Implementing a deterministic combat simulation endpoint using POST /combat/simulate can be a powerful tool for testing, balancing, and debugging your game or application. By controlling the RNG with a seed, you can ensure that your simulations are consistent and predictable. Remember to write unit tests to verify determinism and to include the seed_used in the response payload. Have fun building!