Revolver

Synchronous evolutionary solver for Rhino Compute written in C# for Grasshopper. Uses examples from Microsoft and wraps everything in native RhinoCommon types, then modifies runtime order to prevent blocking and avoid cyclical errors.

What Is Evolutionary Optimization?

An evolutionary optimization algorithm is an implementation of a meta-heuristic modeled on the behavior of biological evolution. These algorithms can be used to find approximate solutions to difficult or impossible numerical minimization problems.

Selected Code

public class Individual : IComparable<Individual> { public double[] Chromosome; // Represents a solution. public double Error; // Smaller values are better for minimization. private int NumGenes; // Problem dimension. private double MutateRate; // Used during reproduction by mutate. private double MutateChange; // Used during reproduction. static Random rnd = new(0); // Used by ctor. public Individual(List<IGH_Param> genomes, double mutateRate, double mutateChange, double error) { this.NumGenes = genomes.Count; this.MutateRate = mutateRate; this.MutateChange = mutateChange; this.Chromosome = new double[genomes.Count]; for (int i = 0; i < this.Chromosome.Length; ++i) this.Chromosome[i] = (maxGene - minGene) * rnd.NextDouble() + minGene; this.Error = error; } public int CompareTo(Individual other) // From smallest error (better) to largest. { if (this.Error < other.Error) return -1; else if (this.Error > other.Error) return 1; else return 0; } }

Pseudocode

Choosing A Mutation Value

The mutation rate value controls how many genes in the chromosome will be modified. One heuristic for the value of the mutateRate field is to use 1.0 / numGenes so that on average one gene in the chromosome will be mutated every time Mutate is called.

Fitness Values vs. Cost Functions

With arbitrary minimization problems, the target function to be minimized is usually called a cost function. In the context of evolutionary and genetic algorithms, however, the function is usually called a fitness function. Notice the terminology is a bit awkward because lower values of fitness are better than higher values. In this example, the fitness function is completely self-contained. In many optimization problems, the fitness function requires additional input parameters, such as a matrix of data or a reference to an external data file.

Evolve

The Evolve method begins by initializing the best fitness and best chromosomes to the first ones in the population. The method iterates exactly maxGenerations times, using gen (generation) as a loop counter. One of several alternatives is to stop when no improvement has occurred after some number of iterations. The Select method returns two good, but not necessarily best, individuals from the population.

These two parents are passed to Reproduce, which creates and returns two children. The Accept method places the two children into the population, replacing two existing individuals. The Immigrate method generates a new random individual and places it into the population. The new population is then scanned to see if any of the three new individuals in the population is a new best solution.

Select

The Select method uses a technique called tournament selection. A subset of random candidate individuals is generated and the best n of them are returned. The number of candidates is computed into variable tournSize, which is some fraction, tau, of the total population size. Larger values of tau increase the probability that the best two individuals will be selected.

References

Some of the original demonstration code and description text in this repo was taken from

here