module MyChart::Rainbow

  SCALAR = (0..255)
  SCALAR_DESC = SCALAR.to_a.reverse.slice 1,254

  PALETTE = [SCALAR.map{|c| "rgba(255,#{c},0,1)"},
             SCALAR_DESC.map{|c| "rgba(#{c},255,0,1)"},
             SCALAR.map{|c| "rgba(0,255,#{c},1)"},
             SCALAR_DESC.map{|c| "rgba(0,#{c},255,1)"},
             SCALAR.map{|c| "rgba(#{c},0,255,1)"},
             SCALAR_DESC.map{|c| "rgba(255,0,#{c},1)"},].flatten

  def self.[] n
    picked = rand PALETTE.size
    steps = PALETTE.size / n
    n.times.map do |t|
      picked = picked + steps
      picked = picked - PALETTE.size if picked >= PALETTE.size
      picked
    end.map do |picked|
      RGBA.new PALETTE[picked]
    end
  end

  class RGBA
    def initialize rgba_str
      @rgba = rgba_str
    end

    def to_s
      @rgba
    end

    def alpha f
      self.class.new @rgba.sub(/1\)/, [f, ')'].join)
    end
  end

end