polynomial.rb

class Polynomial
  def initialize(coefficients)
    @coefficients = coefficients
    @highest_exponent = coefficients.size-1
    raise ArgumentError, 'Need at least 2 coefficients' if @highest_exponent < 1
    @sprintf = ['%ix^%i'] + ['%+ix^%i']*(coefficients.size-2) << '%+i'
  end
  def to_s
    @coefficients.to_enum(:each_with_index).inject([]) {|builder, (coefficient, index)|
      builder.push coefficient == 0 ?
                     builder.compact.size == 0 && index == @highest_exponent && '0' || nil : # Push zero at the end, if empty
                     (@sprintf[index] % [coefficient, @highest_exponent-index])               # Use the format specified in @sprintf
    }.join.
    gsub(/(^|\+|-)1{1}x/, '\1x').  # Clean out 1x
    gsub(/x\^1{1}($|\+|-)/, 'x\1') # Clean out x^1
  end
end

polynomial_test.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([6,5,4,3,2,1])
    @p6 = Polynomial.new([0,5,4,3,2,1])
    @p7 = Polynomial.new([1,0])
    @p8 = Polynomial.new([1,0,0])
  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_last_is_one
    assert_equal("6x^5+5x^4+4x^3+3x^2+2x+1", @p5.to_s)
  end
  
  def zero_up_front
    assert_equal("5x^4+4x^3+3x^2+2x+1", @p6.to_s)
  end
  
  def testing_short
    assert_equal("x", @p7.to_s)
  end

  def test_all_zero
    assert_equal("0", @p4.to_s)
  end
  
  def test_last_two_zero
    assert_equal("x^2", @p8.to_s)
  end

  def test_error
    e = assert_raise(ArgumentError) { Polynomial.new([1]) }
    assert_match(/Need at least 2 coefficients/, e.message)
  end
end

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