/**
     * Calculate rankings for all chromosomes. High ranking numbers denote very fit chromosomes.
     */
    private function computeFitnessRankings() {
        // recalc the fitness of each chromosome
        for ($i = 0; $i < $this->chromosomeDim; ++$i) {
            //$chromosome instanceof Chromosome;
            $this->chromosomes[$i]->fitness = $this->getFitness($i);
        }
        for ($i = 0; $i < $this->chromosomeDim; ++$i) {
            $this->chromosomes[$i]->fitnessRank = $this->getFitnessRank($this->chromosomes[$i]->fitness);
        }

        $rBestFitnessVal;
        $rWorstFitnessVal;
        for ($i = 0; $i < $this->chromosomeDim; ++$i) {
            if ($this->chromosomes[$i]->fitnessRank == $this->populationDim - 1) {
                $rBestFitnessVal = $this->chromosomes[$i]->fitness;
                $this->bestFitnessChromIndex = $i;
            } else
            if ($this->chromosomes[$i]->fitnessRank == 0) {
                $rWorstFitnessVal = $this->chromosomes[$i]->fitness;
                $this->worstFitnessChromIndex = $i;
            }
        }
    }