polynomial.rb

# Submission for http://rubylearning.com/blog/2009/11/26/rpcfn-rubyfun-4/
# Author:       Rohit Arondekar
# Author-URL:   http://rohitarondekar.com
# Author-email: hello@rohitarondekar.com

# Notes:
# 1. Storing whether a term is the 1st non zero term as bool in the term
# 2. Added a unit test in polynomial_test.rb

class Polynomial
  def initialize(coeffs = [])
    raise ArgumentError, 'Need at least 2 coefficients' if coeffs.size < 2
    @terms = []
    # The following is done to identify the first non-zero term so that inputs like [0,0,3,2,1] work
    ft_not_set = true
    coeffs.each_with_index do |c, i|
      if(c != 0 && ft_not_set)
        @terms << PolynomialTerm.new(coeffs[i], coeffs.size - i - 1, true)
        ft_not_set = false
      else
        @terms << PolynomialTerm.new(coeffs[i], coeffs.size - i - 1)
      end
    end
  end
  
  def to_s
    output = ''
    @terms.each { |term| output = output + term.to_s }
    (output == '' && '0') || output
  end
end


class PolynomialTerm
  def initialize(coeff, expo, ft = nil)
    @coeff = coeff     # Coefficient
    @expo = expo       # Exponent
    @ft = ft || false  # Boolean indicating if this is the first term
  end
  
  def to_s()  
    return '' if @coeff == 0
    sign_of_coeff + format_coeff + format_expo
  end
  
  private  
  # Returns the sign of the coefficient based on whether this is the first term
  def sign_of_coeff()
    if @ft
      @coeff < 0 ? '-' : ''
    else
      @coeff < 0 ? '-' : '+'      
    end     
  end
   
  def format_expo
    case @expo
      when 0 then ''
      when 1 then 'x'
      else 'x^' + @expo.to_s
    end
  end
  
  def format_coeff
    if @expo == 0 || @coeff.abs > 1
      @coeff.abs.to_s
    else
      ''
    end
  end
end

polynomial_test.rb

# assuming that your solution is polynomial.rb
require 'test/unit'
require 'test/unit/ui/console/testrunner'
require 'polynomial'

class Polynomial_Test < Test::Unit::TestCase
  def setup
    @p1 = Polynomial.new([-3,-4,1,0,6])
    @p2 = Polynomial.new([1,0,2])
    @p3 = Polynomial.new([-1,-2,3,0])
    @p4 = Polynomial.new([0,0,0])
    @p5 = Polynomial.new([0,0,1,2,-1,2])
  end

  def test_first_negative
    assert_equal("-3x^4-4x^3+x^2+6", @p1.to_s) 
  end

  def test_simple
    assert_equal("x^2+2", @p2.to_s) 
  end

  def test_first_minus_one
    assert_equal("-x^3-2x^2+3x", @p3.to_s)
  end

  def test_all_zero
    assert_equal("0", @p4.to_s)
  end

  def test_error
    e = assert_raise(ArgumentError) { Polynomial.new([1]) }
    assert_match(/Need at least 2 coefficients/, e.message)
  end
  
  # Added the following test to check for cases like [0,0,12,-1,3,4]
  def test_non_zero_leading_coeffients_in_input_array
    assert_equal("x^3+2x^2-x+2", @p5.to_s)
  end
end

Test::Unit::UI::Console::TestRunner.run(Polynomial_Test)