Ruby script to compute modular-exponentiation iteratively.

modular_exponentiation_1.rb

#!/usr/local/bin/ruby
#*************************************
# Modular Exponentiation (iterative). 
#*************************************
class ModularExponentiation
  def comp_mod_exp(b, e, m)
    ans = 1
    while e > 0
      ans = (ans * b) % m if (e & 1) == 1
      e >>= 1
      b = (b * b) % m
    end
    return ans
  rescue => e
    raise
  end
end

exit unless __FILE__ == $0

begin
  # me = b^e mod m
  b, e, m = 12345, 6789, 4567
  obj = ModularExponentiation.new
  me = obj.comp_mod_exp(b, e, m)
  puts "#{b}^#{e} mod #{m} = #{me}"
rescue => e
  puts "[#{e.class}] #{e.message}"
  e.backtrace.each{ |bt| puts "\t#{bt}" }
  exit 1
end