michaellang.rb

# 
# Design Goal: strike a balance between terseness and readability
# 
# I always hear Ruby developers saying Ruby code doesn't need commenting.  But for 
# me, no matter how well you might name those routines routines or or write 
# your code, if you pair a comment clearly stating intent of the routine, 
# it is always easier to quickly wrap one's head around what a block of code is 
# *supposed* to be doing and you can answer the question, "does this code do what 
# the author thinks its doing?"  I'm still learning Ruby and I don't translate Ruby
# into a clear mental model instantly, but I generally do interpret comments quite quickly!
#
# Another readability enhancement is the use of parentheses to enhance readablity even when 
# they're not strictly required, and the use of whitespace, such as blank line between
# guard statements that exit a routine early and the rest of the routine.  Unlike other languages
# that might always require parentheses for conditionals or blocks, I've come to love 
# the fact that Ruby can omit them because now when they're present, they have a semantic 
# meaning to me, much like punctuation does in a sentence!
#
class Polynomial

  ##
  # pair each given coefficient with its respective exponential as [coeff, exp], removing zeroes
  def initialize(coefficients)
    raise ArgumentError.new "Need at least 2 coefficients" if coefficients.size < 2
    
    @coefficients = coefficients.reverse
    @coefficients.each_with_index{ |c, i| @coefficients[i] = (c == 0 ? nil : [c, i]) }
    @coefficients.reverse!.compact!
  end
  
  ##
  # make 1 and -1 implicit
  def multiplier_to_s(coefficient)
    coefficient[0] == 1 ? "" : coefficient[0] == -1 ? "-" : coefficient[0].to_s
  end
  
  ##
  # only raise to power if exp is > 0
  def exp_to_s(coefficient)
    coefficient[1] == 0 ? "" : coefficient[1] == 1 ? "x" : "x^#{coefficient[1]}"
  end
    
  ##
  # return "+" only when there is no prefix and the coefficient is positive
  def sign_to_s(prefix, coefficient)
    (prefix == '' || coefficient[0] < 0) ? '' : "+" 
  end
  
  ##
  # output all coefficients for the polynominal in a human readable equation.
  # Example: "-3x^4-4x^3+x^2+6"
  def to_s
    return "0" if @coefficients.empty?
    
    @coefficients.inject("") do |str, c| 
      str << sign_to_s(str, c) << multiplier_to_s(c) << exp_to_s(c)
    end
  end
end