readme.md

Self-referential number (10 digit)

Which ten-digit number has the n-th digit representing how many occurrences of the digit (n−1) there are in the number, for all n≤10 ?

brute_force_solution.rb

class Fixnum
  def autoref?
    a = to_s.rjust(10, "0").split("").map(&:to_i) # a = [2,3,1,1,3,4,5,5,1,9]
    h = Hash[(0..9).map{|e| [e, 0]}]
    a.each{|e| h[e]+=1}
    a.each_with_index do |n, i|
      return false if n!=h[i]
    end
    true
  end
end


0.upto(10**10-1) do |n|
  puts n if n%20_000==0
  if n.autoref?
    puts ">>>>#{n}<<<<<" 
  end
end