Fast local TRS matrix C# MIT

Helpers.cs

using System.Numerics;
using System.Runtime.CompilerServices;

public static class Helpers
{
    private static Matrix4x4 LocalMatrix(Vector3 translation, Quaternion rotation, Vector3 scale)
    {
        // every element in matrix except of translation
        // ends up being multiplied by 2, so we multiply whole vector by sqrt2
        // to skip multiplication at the end
        const float sqrt2 = 1.4142135623730951f;
        Vector4 rot = Unsafe.As<Quaternion, Vector4>(ref rotation) * sqrt2;

        // xx is actually equals to rotation.x * rotation.x * 2
        float xx = rot.X * rot.X;
        float yy = rot.Y * rot.Y;
        float zz = rot.Z * rot.Z;

        float xy = rot.X * rot.Y;
        float xz = rot.X * rot.Z;
        float xw = rot.X * rot.W;
        float yz = rot.Y * rot.Z;
        float yw = rot.Y * rot.W;
        float zw = rot.Z * rot.W;

        Matrix4x4 mat4 = default;
        mat4.M12 = scale.Y * (xy + zw);
        mat4.M13 = scale.Z * (xz - yw);
        mat4.M21 = scale.X * (xy - zw);
        mat4.M23 = scale.Z * (yz + xw);
        mat4.M31 = scale.X * (xz + yw);
        mat4.M32 = scale.Y * (yz - xw);
        mat4.M11 = scale.X * (1f - (yy + zz));
        mat4.M22 = scale.Y * (1f - (xx + zz));
        mat4.M33 = scale.Z * (1f - (xx + yy));
        mat4.M41 = translation.X;
        mat4.M42 = translation.Y;
        mat4.M43 = translation.Z;
        mat4.M44 = 1;

        return mat4;
    }
}