Polynomial Manipulation in Ruby

polynomial.rb

class Polynomial
  # maps e.g. {x: 1, y: 2} => 3 which corresponds to term 3 * x ** 1 * y ** 2
  attr_reader :coefficients

  def initialize(coefficients = {})
    puts @coefficients.inspect
    @coefficients = coefficients.reject do |_index, coefficient|
      coefficient == 0
    end
  end

  def +(summand)
    Polynomial.new(hash_sum(coefficients, summand.coefficients))
  end

  def -(subtrahend)
    self + (-subtrahend)
  end

  def -@
    Polynomial.new(Hash[@coefficients.map { |key, value| [key, -value] }])
  end

  def *(factor)
    case factor.coefficients.size
      when 0
        0
      when 1
        factor_multi_index = factor.coefficients.keys.first
        factor_coefficient = factor.coefficients.values.first
        Polynomial.new(
          Hash[
            @coefficients.map do |multi_index, coefficient|
              [hash_sum(multi_index, factor_multi_index),
               coefficient * factor_coefficient]
            end
          ]
        )
      else
        product = self
        factor.coefficients.each do |multi_index, coefficient|
          product *= Polynomial.new(multi_index => coefficient)
        end
        product
    end
  end

  private

  def hash_sum(*hashes)
    sum = {}
    hashes.each do |hash|
      hash.each do |key, value|
        sum[key] ||= 0
        sum[key] += value
        sum.delete(key) if sum[key] == 0
      end
    end
    sum
  end
end