selection.js

// define a function to select individuals for the next generation
const selection = (population) => {
  // calculate the fitness of each individual
  let fitnesses = population.map(calculateFitness);
  // normalize the fitness values
  let totalFitness = fitnesses.reduce((a, b) => a + b, 0);
  let probabilities = fitnesses.map((fitness) => fitness / totalFitness);
  // select individuals for the next generation
  let nextGeneration = [];
  for (let i = 0; i < POPULATION_SIZE; i++) {
    // choose two individuals based on their probabilities
    let individual1 = Math.random();
    let individual2 = Math.random();
    let prob1 = 0;
    let prob2 = 0;
    let index1 = 0;
    let index2 = 0;

    for (let j = 0; j < POPULATION_SIZE; j++) { //find the first suitable individual based on probability
      prob1 += probabilities[j];
      if (individual1 < prob1) {
        index1 = j;
        break;
      }
    }
    //find the second one as long as it's not the one we picked before
    for (let j = 0; j < POPULATION_SIZE; j++) {
      if (j === index1) continue;
      prob2 += probabilities[j];
      if (individual2 < prob2) {
          index2 = j;
          break;
        }
    }
    // add the fittest individual to the next generation
    nextGeneration.push(calculateFitness(population[index1]) < calculateFitness(population[index2]) ? population[index1] : population[index2]);
  }
  return nextGeneration
}