optimise_fill.rb

# Matches an Array (of trues and falses) to a given Array. Fills itself
# with preference to fill where true first, then any spare items are put 
# in place of false, then nils are used for remaining falses.
#
#   input = ["a", "b", "c"]
#   match = [true, false, false, true]
#
#   match.optimise_fill(input)
#   #=> ["a", "b", nil, "c"]
# 
class Array
  def optimise_fill(input)
    match = self
    diff = input.size - match.reject{|i| i == false}.size
    
    result = []
    match.each_index do |i|
      curr_item = match[i]
      if curr_item == true
        result << input.shift
      else
        if diff > 0
          result << input.shift
          diff -= 1
        else
          result << nil
        end
      end
    end
    result
  end
end