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OnePhysics for .NET

OnePhysics 0.4.0-preview.1README.md at fa8eed5 Markdown

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Deterministic 2D rigid-body physics for .NET, written from first principles: the same world state, bit for bit, on every platform, CPU, vector width and thread count. Package RecursiveAI.OnePhysics, built on onemath for its deterministic math.

Status: preview. The 2D engine is the supported surface: circles, capsules, rounded polygons, segments and chains; revolute, prismatic, weld, wheel, distance, motor and mouse joints; bullets and continuous collision; a character mover; layers, filters, sensors, events and queries; byte-exact snapshot and restore; per-step world hashes; debug geometry. The 3D engine (RecursiveAI.OnePhysics.Physics3D) ships as a preview marked [Experimental("OPHYS0002")]: it works and is deterministic, but its API and its results may change between previews. The raw tier (RecursiveAI.OnePhysics.LowLevel) is [Experimental("OPHYS0001")]. Previews are not bit-compatible with each other: a snapshot or replay from one preview does not carry to the next.

Install

dotnet add package RecursiveAI.OnePhysics --prerelease

Targets net10.0; works under NativeAOT, trimming and browser WebAssembly. Its only dependency is RecursiveAI.OneMath.

2D in a minute

C#
using System;
using RecursiveAI.OneMath;               // Vector2F, Vector2D
using RecursiveAI.OnePhysics;            // BodyType, events, exceptions
using RecursiveAI.OnePhysics.Physics2D;  // PhysicsWorld2D, Body, Character
 
var world = new PhysicsWorld2D();
world.AddBox(new Vector2F(20, 1), at: new Vector2D(0, -1), type: BodyType.Static);
var ball = world.AddCircle(0.5f, at: new Vector2D(0, 5));
ball.OnCollisionEnter += c => Console.WriteLine($"hit {c.Other} along {c.Normal}");
 
var player = world.AddCharacter(at: new Vector2D(3, 1));
for (int i = 0; i < 600; i++)
{
    player.Move(new Vector2F(0.05f, -0.1f), 1f / 60);
    world.Step(1f / 60);                        // fixed dt; events fire after the step, in a deterministic order
}
if (world.Raycast(new Vector2D(0, 10), new Vector2F(0, -1), out var hit)) Console.WriteLine(hit.Point);
 
byte[] saved = world.Snapshot();                // byte-exact; Restore(saved) continues bit for bit
ulong hash = world.ComputeHash();               // the same on every platform

3D, a preview

The 3D world has the same members with 3D math types. Using it raises the experimental diagnostic OPHYS0002 until you acknowledge it:

C#
#pragma warning disable OPHYS0002        // the 3D engine is a preview
using System;
using RecursiveAI.OneMath;               // Vector3F, Vector3D
using RecursiveAI.OnePhysics;            // BodyType, events, exceptions
using RecursiveAI.OnePhysics.Physics3D;  // PhysicsWorld3D, Body, Character
 
var world = new PhysicsWorld3D();
world.AddBox(new Vector3F(20, 1, 20), at: new Vector3D(0, -1, 0), type: BodyType.Static);
var ball = world.AddSphere(0.5f, at: new Vector3D(0, 5, 0));
ball.OnCollisionEnter += c => Console.WriteLine($"hit {c.Other} along {c.Normal}");
 
for (int i = 0; i < 300; i++) world.Step(1f / 60);
if (world.Raycast(new Vector3D(0, 10, 0), new Vector3F(0, -1, 0), out var hit)) Console.WriteLine(hit.Point);
 
byte[] saved = world.Snapshot();
ulong hash = world.ComputeHash();

What you get

  • Deterministic. Only IEEE-exact + - * / and sqrt and our own transcendental functions; fixed-order reductions and constraint coloring. The world hash after N steps is the same on x64, arm64 and WebAssembly, scalar or SIMD, one thread or all cores.
  • Three tiers, one engine. One-liners (world.AddBox(...), body.OnCollisionEnter), definitions and polled events, and a raw tier, each built on the one below.
  • Rollback-ready. Byte-exact Snapshot/TrySnapshot and Restore, ComputeHash per step, SuppressCallbacks for resimulation, allocation-free steady state after Warmup.
  • Handles you can hold. Body, Shape and Joint are small structs; a destroyed one throws a StaleHandleException naming it.
  • Hostile input is refused, with typed errors that name the value and the limit.

Guide

Licence

MIT OR Apache-2.0, at your option. Copyright (c) Recursive AI, LLC. See LICENSE-MIT and LICENSE-APACHE.