stackoverflow.com/q/31194315

require 'benchmark'

def get_array number, recurring
    (1..number-recurring).map {|i| [i, i]} + (number-recurring..number).map {|i| [i, i-recurring]}
end

def simons array
    hash = array.reduce({}) {|hash, pair| (hash[pair.last] ||= []).push pair.first; hash}
    hash.keys.combination(16).map do |array|
      array.map {|parent| hash[parent]}
    end.map {|array| array.first.product *array.drop(1)}.flatten(1).size
end

def original array
    array.combination(16).select {|c| c.map(&:last).uniq.size == c.size }.size
end

puts "Original: #{Benchmark.measure { puts original get_array 33, 1}}"
puts "Simons:   #{Benchmark.measure { puts simons   get_array 33, 1}}"