Class: Operator::CrossoverLHS

Inherits:
Object
  • Object
show all
Defined in:
../lib/crossover_lhs.rb

Overview

Two-points LHS (Left Hand Side) structure-preserving GE crossover operator. It uses track_support information collected during genotype-phenotype mapping.

See Harper, R. Blair, A.: A structure preserving crossover in grammatical evolution ieeexplore.ieee.org/search/freesrchabstract.jsp?tp=&arnumber=1555012

Instance Attribute Summary (collapse)

Instance Method Summary (collapse)

Constructor Details

- (CrossoverLHS) initialize

Create a new crossover facility with the default attribute values. 



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# File '../lib/crossover_lhs.rb', line 17

def initialize
  @random = Kernel
end

Instance Attribute Details

- (Object) random

The source of randomness, used for calling “random.rand( limit )”, defaulting to ‘Kernel’ class. 



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# File '../lib/crossover_lhs.rb', line 22

def random
  @random
end

Instance Method Details

- (Object) crossover(parent1, parent2, track1, track2)

Take parent1, parent2 genotypes and produce [offspring1, offspring2], utilising track1 and track2 information. parent1 and parent2 are regular chromosomes ie. arrays (enumerables) of numbers. track1 and track2 are hints (all allowed splitting points) obtained from Mapper::Base#track_support. 



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# File '../lib/crossover_lhs.rb', line 28

def crossover( parent1, parent2, track1, track2 )
  raise "CrossoverLHS: no track_support. Check Mapper#track_support_on" unless track1.kind_of?( Enumerable ) and track2.kind_of?( Enumerable )
  offspring1 = parent1.clone
  offspring2 = parent2.clone

  # handle possible wrapping of the chromozome (track node info indices bigger than the actual length)
  offspring1.concat parent1 while offspring1.size < track1.last.to+1
  offspring2.concat parent2 while offspring2.size < track2.last.to+1

  # organize track nodes by symbols
  hash1 = {}
  track1.each do |node| 
    choices = hash1.fetch( node.symbol, [] )
    choices.push node
    hash1[node.symbol] = choices 
  end
  hash2 = {}
  track2.each do |node| 
    choices = hash2.fetch( node.symbol, [] )
    choices.push node
    hash2[node.symbol] = choices 
  end

  # randomly select the symbol from all _common_ symbols
  symbols = Set.new( hash1.keys ).intersection( hash2.keys ) # extract only symbols present in both tracks
  return offspring1, offspring2 if symbols.empty? # no common symbols -> cloning fallback
  sym = symbols.to_a.sort[ @random.rand( symbols.size ) ] # select a random one

  # randomly select trace nodes by the (compatible) symbol
  choices = hash1[ sym ]
  choice1 = choices[ @random.rand( choices.size ) ]
  choices = hash2[ sym ]
  choice2 = choices[ @random.rand( choices.size ) ]

  # swap parts
  part1 = offspring1[ choice1.from .. choice1.to ]
  part2 = offspring2[ choice2.from .. choice2.to ]
  offspring1[ choice1.from .. choice1.to ] = part2
  offspring2[ choice2.from .. choice2.to ] = part1
  return offspring1, offspring2
end