A solution to including duplicates of some classes but not others

special_prune.rb

#!/usr/bin/env ruby

# A solution to the problem where one wants to create a new array
# that includes all instances of A and B but only the first instances
# of C & D.

require 'set'

class A; end
class B; end
class C; end
class D; end


def class_duplicates_ok?(klass)
  [A, B].include?(klass)
end


def prune_array(array)
  classes_found = Set.new
  array.each_with_object([]) do |element, pruned_array|
    klass = element.class
    if class_duplicates_ok?(klass) || (! classes_found.include?(klass))
      pruned_array << element
    end
    classes_found << klass
  end
end


def main
  options = [A.new, B.new, A.new, B.new, C.new, C.new, D.new]
  pruned = prune_array(options)
  puts "\nOriginal: "; puts options
  puts "\nPruned: "; puts pruned
end


main

=begin

Sample Output:

Original:
#<A:0x00007f8c81915130>
#<B:0x00007f8c81915108>
#<A:0x00007f8c819150e0>
#<B:0x00007f8c819150b8>
#<C:0x00007f8c81915018>
#<C:0x00007f8c81914ff0>
#<D:0x00007f8c81914f28>

Pruned:
#<A:0x00007f8c81915130>
#<B:0x00007f8c81915108>
#<A:0x00007f8c819150e0>
#<B:0x00007f8c819150b8>
#<C:0x00007f8c81915018>
#<D:0x00007f8c81914f28>

=end