coin_change_algorithm.rb

#! /usr/bin/env ruby

def banknotes_calculator(banknotes, requested_sum)
  sorted_values = banknotes.keys.sort.reverse

  total_sum = banknotes.inject(0) {|total_sum, banknote| total_sum += banknote[0] * banknote[1]}
  result = Hash[banknotes.keys.map {|value| [value, 0]}]

  return if requested_sum > total_sum

  while requested_sum > 0
    current_value = sorted_values.first

    break unless current_value

    if banknotes[current_value] > 0 && current_value <= requested_sum
      value_diff = current_value - requested_sum

      if value_diff <= 0
        requested_sum -= current_value

        banknotes[current_value] -= 1
        result[current_value] += 1
      else
        sorted_values.shift
      end
    else
      sorted_values.shift
    end
  end

  result
end

p change_calculator({100 => 3, 500 => 5}, 230)