NanoBrain-unitypackage/Runtime/Scripts/Core/Synapse.cs

using System;
using UnityEngine;
using Unity.Mathematics;

namespace NanoBrain {

    /// <summary>
    /// A Synapse connects the ouput of a Neuron to another Neuron
    /// </summary>
    [Serializable]
    public class Synapse {
        /// <summary>
        /// The neuron from which input is received
        /// </summary>
        [SerializeReference]
        [HideInInspector]
        public Neuron neuron;

        /// <summary>
        /// The weight value to apply to the Neuron input
        /// </summary>
        public float weight;

        /// <summary>
        /// Indicator whether the weight can be trained
        /// </summary>
        public bool trainable = false;

        /// <summary>
        /// Create a new Synapse
        /// </summary>
        /// <param name="nucleus">The neuron from which input is received</param>
        /// <param name="weight">The weight value to apply to the Neuron input</param>
        public Synapse(Neuron nucleus, float weight = 1.0f) {
            this.neuron = nucleus;
            this.weight = weight;
        }

        public static bool EqualStructure(Synapse synapse1, Synapse synapse2) {
            return synapse1.neuron.name == synapse2.neuron.name;    
        }

        public virtual void BackPropagation(Neuron receiver, float derivative, float learningRate) {
            switch (receiver.activator) {
                case Neuron.ActivationType.Linear:
                    derivative *= 1;
                    break;
                case Neuron.ActivationType.Power:
                    // untested
                    derivative *= 2 * math.length(this.neuron.combination);
                    break;
                case Neuron.ActivationType.Reciprocal:
                    // untested
                    derivative *= -1 / Mathf.Pow(math.length(this.neuron.combination), 2);
                    break;
                default:
                    Debug.Log("other activator");
                    break;
            }

            this.neuron.BackPropagation1D(derivative * this.weight, learningRate);

            derivative *= math.length(this.neuron.activation);

            if (this.trainable) {
                float deltaWeight = learningRate * derivative;
                this.weight += deltaWeight;
            }
        }

        public virtual void BackPropagation3D(Neuron receiver, Vector3 derivative, float learningRate) {
            // As the weight cannot change the direction of the derivative
            // we can use the simpler, 1D backpropagation here
            // But we still need to determine the sign of the derivative

            if (Synapse.AreOpposed(derivative, this.neuron.activation))
                BackPropagation(receiver, -derivative.magnitude, learningRate);
            else
                BackPropagation(receiver, derivative.magnitude, learningRate);

            // switch (receiver.activator) {
            //     case Neuron.ActivationType.Linear:
            //         derivative *= 1;
            //         break;
            //     case Neuron.ActivationType.Power:
            //         // untested
            //         derivative *= 2 * math.length(this.neuron.combination);
            //         break;
            //     case Neuron.ActivationType.Reciprocal:
            //         // untested
            //         derivative *= -1 / Mathf.Pow(math.length(this.neuron.combination), 2);
            //         break;
            //     default:
            //         Debug.Log("other activator");
            //         break;
            // }

            // this.neuron.BackPropagation3D(derivative * this.weight, learningRate);

            // derivative *= math.length(this.neuron.activation);

            // if (this.trainable) {
            //     float deltaWeight = learningRate * derivative.magnitude;
            //     // Compared to the 1D solution, this does not decrease the weight because magnitude is always positive
            //     // derivative.direction and derivative.sign are different....
            //     if (AreOpposed(derivative, this.neuron.activation))
            //         this.weight -= deltaWeight;
            //     else
            //         this.weight += deltaWeight;
            // }
        }

        public static bool AreOpposed(Vector3 a, Vector3 b) {
            // Check if the angle between the vectors is > 90 degrees
            return Vector3.Dot(a, b) < 0f;
        }

        public void GaussianAdditiveMutation(float sigma) {
            if (this.trainable == false)
                return;

            float deltaWeight = NormalDistribution.Sample(sigma);
            this.weight += deltaWeight;
        }
    }

    public static class NormalDistribution {
        private static readonly System.Random rng = new();

        // Returns a single sample from N(0, sigma^2)
        public static float Sample(float sigma) {
            // u1 must be > 0 to avoid log(0)
            float u1 = 1.0f - (float)rng.NextDouble(); // in (0,1]
            float u2 = (float) rng.NextDouble();       // in [0,1)

            float stdNormal =
                Mathf.Sqrt(-2.0f * Mathf.Log(u1)) * Mathf.Cos(2.0f * Mathf.PI * u2);

            return sigma * stdNormal;
        }
    }

    [Serializable]
    public class SynapseData {
        public string clusterName;
        public string neuronName;
        public float weight;
        public bool trainable;

        public SynapseData(Synapse synapse) {
            // if (synapse.neuron.parent.prefab != null)
            //     this.clusterName = synapse.neuron.parent.prefab.name;
            // else
            //     this.clusterName = synapse.neuron.parent.name;
            this.clusterName = synapse.neuron.parent.baseName;
            this.neuronName = synapse.neuron.name;
            this.weight = synapse.weight;
            this.trainable = synapse.trainable;
        }
    }
}