Ruby script to compute modular-exponentiation recursively.

modular_exponentiation_2.rb

#!/usr/local/bin/ruby
#*************************************
# Modular Exponentiation (recursive). 
#*************************************
class ModularExponentiation
  def comp_mod_exp(b, e, m)
    return 1 if e == 0
    ans = comp_mod_exp(b, e / 2, m)
    ans = (ans * ans) % m
    ans = (ans * b) % m if e % 2 == 1
    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