Biased shuffle

gistfile1.rb

def swap(array,indexa,indexb)
   value_to_swap = array[indexa]
   array[indexa] = array[indexb]
   array[indexb] = value_to_swap
end

def biased_shuffle(array)
  array = array.dup
  array.size.times do |i|
    swap(array,rand(array.size),i)
  end
  array
end

def fair_shuffle(array)
  array = array.dup
  array.length.downto(1) do |i|
    swap(array,rand(i),i-1)
  end
  array
end

def montecarlo(source,samples=60000)
  results = Hash.new(0)
  samples.times do
     sample = yield source
     key = sample.join("")
     results[key] = results[key] + 1
  end
  results
end

biased = montecarlo(["a","b","c"]) do |y|
  biased_shuffle y
end

fair = montecarlo(["a","b","c"]) do |y|
  fair_shuffle y
end

puts "BIASED SHUFFLE RESULTS"
puts biased.inspect
puts
puts "FAIR SHUFFLE RESULTS"
puts fair.inspect
puts

# BIASED SHUFFLE RESULTS
# {"acb"=>11104, "bca"=>11225, "cba"=>8947, "abc"=>8874, "cab"=>8832, "bac"=>11018}

# FAIR SHUFFLE RESULTS
# {"acb"=>9829, "bca"=>10206, "cba"=>9888, "abc"=>10065, "cab"=>9970, "bac"=>10042}