Contents

########################
# stochastic_grammar.rb
# unweighted rules accepted
# with default weight = 1
# complex stochastic rule
########################
class StochasticGrammar
  PROB = 1
  attr_accessor :axiom, :srules

  def initialize(axiom)
    @axiom = axiom
  end

  ######################################################
  # randomly selects a rule (with a weighted probability)
  #####################################################

  def stochastic_rule(rules)
    total = rules.inject(0) do |sum, rule_and_weight|
      sum + rule_and_weight[PROB]
    end
    srand
    chance = rand * total
    rules.each do |item, weight|
      return item unless chance > weight
      chance -= weight
    end
    return rule
  end

  def rule?(pre)
    @srules.key?(pre)
  end

  def add_rule(pre, rule, weight = 1.0)    # default weighting 1 (can handle non-stochastic rules)
    @srules ||= Hash.new { |h, k| h[k] = 1 }
    if rule?(pre)                      # add to existing hash
      srules[pre][rule] = weight
    else
      srules[pre] = { rule => weight }     # store new hash with pre key
    end
  end

  def new_production(prod)  # note the use of gsub!, we are changing prod as we go
    prod.gsub!(/./) do |ch|
      rule?(ch) ? stochastic_rule(srules[ch]) : ch
    end
  end

  def generate(repeat = 0)
    prod = axiom
    repeat.times { prod = new_production(prod) }
    return prod
  end
end

Version data entries

5 entries across 5 versions & 1 rubygems

Version	Path
ruby-processing-2.6.2	samples/processing_app/topics/lsystems/library/stochastic_grammar/stochastic_grammar.rb
ruby-processing-2.6.1	samples/processing_app/topics/lsystems/library/stochastic_grammar/stochastic_grammar.rb
ruby-processing-2.6.0	samples/processing_app/topics/lsystems/library/stochastic_grammar/stochastic_grammar.rb
ruby-processing-2.5.1	samples/processing_app/topics/lsystems/library/stochastic_grammar/stochastic_grammar.rb
ruby-processing-2.5.0	samples/processing_app/topics/lsystems/library/stochastic_grammar/stochastic_grammar.rb