Lagrange interpolation w/ Barycentric formula

barycentric.rb

class Barycentric
  attr_reader :x, :y

  def initialize(samples)
    @x = samples.map(&:first).map(&:to_r)
    @y = samples.map(&:last).map(&:to_r)
    @w = @x.each_index.map { |j| w(j) }
  end

  def L(x)
    return @y[@x.index(x)] if @x.include? x
    l(x) * @x.each_index.map { |j| @w[j] * @y[j] / (x - @x[j]) }.reduce(:+)
  end

  def q
    @w
  end

  private

  def l(x)
    @x.each_index.map { |i| x - @x[i] }.reduce(:*)
  end

  def w(j)
    @x.each_index.map { |i| 1 / (@x[j] - @x[i]) unless i == j }.compact.reduce(:*)
  end
end