module Silicium


  module Algebra
    ##
    # TODO: class docs
    class PolynomialDivision

      DELTA = 0.01

      # This function returns an array of coefficients obtained by parsing input string in format: "<coeff>*x**<degree>+..."
      # Even if in your expression don't exist x with some degree, you should to write it with 0 coefficient
      # Also free term you should to write with 0 degree
      # Example: "2*x**5-3*x**4+0*x**3+0*x**2-5*x**1-6*x**0"
      def polynom_parser(str)
        copy_str = str.clone
        sgn_array = []                    # Array of signs
        if copy_str[0] != '-'
          sgn_array.push('+')
        else
          sgn_array.push('-')
          copy_str[0] = ''
        end
        token = copy_str.split(/[-+]/)
        (0..copy_str.size-1).each do |i|
          sgn_array.push(copy_str[i]) if copy_str[i] == '-' || copy_str[i] == '+'
        end
        size = token.size - 1
        coeff = []                 # Array of coefficients
        (0..size).each do |i|
          degree = token[i].split('*')    # Split by '*' to get coefficient and degree
          degree[0] == 'x' ? coeff[i] = 1.0 : coeff[i] = degree[0].to_f
          coeff[i] *= -1 if sgn_array[i] == '-'
        end
        coeff
      end

# String implementation of result
      def str_res_impl(coeff_res, sgn_array)
        res_size = coeff_res.size
        res_exp = ""
        (0..res_size-1).each do |i|
          res_exp += ((coeff_res[i].ceil(3)).to_s+"*x**"+(res_size - i - 1).to_s)
          res_exp += sgn_array[i+1] if sgn_array[i+1] != '-'
        end
        res_exp
      end

# String implementation of remained part
      def str_rem_impl(coeff_1)
        c = coeff_1.size
        rem_exp = ""
        (0..c-1).each do |i|
          rem_exp += '+' if coeff_1[i] >= 0.0
          rem_exp += ((coeff_1[i].ceil(3)).to_s+"*x**"+(c - i - 1).to_s)
        end
        rem_exp[0] = '' if rem_exp[0] == '+'
        rem_exp
      end

# This function returns array of 2 strings: first is the result of division polynom poly_1 on polynom poly_2
# Second - remainder
      def polynom_division(poly_1, poly_2)
        coeff_1 = polynom_parser(poly_1)
        coeff_2 = polynom_parser(poly_2)
        res_size = coeff_1.size - coeff_2.size + 1
        coeff_result = Array.new(res_size)
        sgn_array = Array.new(res_size + 1,'')
        (0..res_size-1).each do |i|
          cur_coeff = coeff_1[i] / coeff_2[0]
          coeff_result[i] = cur_coeff
          coeff_result[i] < 0 ? sgn_array[i] = '-' : sgn_array[i] = '+'
          (0..coeff_2.size-1).each do |j|
            coeff_1[i+j] -= coeff_2[j]*cur_coeff
          end
        end
        res_exp = str_res_impl(coeff_result, sgn_array)
        rem_exp = str_rem_impl(coeff_1[coeff_result.size..coeff_1.size-1])
        [res_exp, rem_exp]
      end

      def compare_polynoms(poly1, poly2)
        polynom_parser(poly1).size - polynom_parser(poly2).size
      end

      def zero_coeffs?(polynom, delta = DELTA)
        polynom_parser(polynom).all?{ |item| item.abs < delta }
      end

      def round_coeffs(coefficients, delta = DELTA)
        coefficients.map do |element|
          (element.round - element).abs < delta ? element.round.to_f : element
        end
      end

      def build_polynom_from_coeffs(coefficients)
        "#{coefficients[0]}*x**#{coefficients.size - 1}" +
          coefficients[1..-1].each_with_index.inject('') do |acc, (coefficient, index)|
            leading_sign = coefficient >= 0 ? '+' : ''
            acc + "#{leading_sign}#{coefficient}*x**#{ coefficients.size - index - 2 }"
          end
      end
      
      # This function returns a string: greatest common integer divisor of two polynoms
      def polynom_gcd(poly1, poly2, delta = DELTA)
        divisor, remainder = order_gcd_operands(poly1, poly2)
        until zero_coeffs?(remainder) do
          division = polynom_division(divisor, remainder)
          divisor = remainder
          remainder = division[1]
        end
        normalizer = polynom_parser(divisor)[0]
        temp_result = polynom_division(divisor, normalizer.to_s+'*x**0')[0]
        coefficients = round_coeffs(polynom_parser(temp_result), delta)
        build_polynom_from_coeffs(coefficients)
      end

      private

      def order_gcd_operands(poly1, poly2)
        if compare_polynoms(poly1, poly2) >= 0
          divisor = poly2
          remainder = polynom_division(poly1, divisor)[1]
        else
          divisor = poly1
          remainder = polynom_division(poly2, divisor)[1]
        end
        [divisor, remainder]
      end
    end
  end
end