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module Xterm256Color
  # the 6 value iterations in the xterm color cube
  VALUERANGE = [
    0x00, 0x5F, 0x87, 0xAF, 0xD7, 0xFF
  ]

  # 16 basic colors
  BASIC16 = [
    [ 0x00, 0x00, 0x00 ], # 0
    [ 0xCD, 0x00, 0x00 ], # 1
    [ 0x00, 0xCD, 0x00 ], # 2
    [ 0xCD, 0xCD, 0x00 ], # 3
    [ 0x00, 0x00, 0xEE ], # 4
    [ 0xCD, 0x00, 0xCD ], # 5
    [ 0x00, 0xCD, 0xCD ], # 6
    [ 0xE5, 0xE5, 0xE5 ], # 7
    [ 0x7F, 0x7F, 0x7F ], # 8
    [ 0xFF, 0x00, 0x00 ], # 9
    [ 0x00, 0xFF, 0x00 ], # 10
    [ 0xFF, 0xFF, 0x00 ], # 11
    [ 0x5C, 0x5C, 0xFF ], # 12
    [ 0xFF, 0x00, 0xFF ], # 13
    [ 0x00, 0xFF, 0xFF ], # 14
    [ 0xFF, 0xFF, 0xFF ]  # 15
  ]

  # convert an xterm color value (0-253) to three elements array [r, g, b]
  def self.xterm2rgb(color)
    case color
    when 0..15
      return BASIC16[color].dup

    when 16..232
      color -= 16
      return [VALUERANGE[(color / 36) % 6], VALUERANGE[(color / 6) % 6], VALUERANGE[color % 6]]

    when 233..253
      return [8 + (color - 232) * 0x0a] * 3

    else
      raise ArgumentError, "expected color value in range 0..253 but was #{color}"
    end
  end

  # fill the colortable for use with rgb2xterm
  def self.make_table
    @colortable ||=
      begin
        colortable = []
        0.upto(253) do |color|
          rgb = xterm2rgb(color)
          colortable << rgb
        end
        colortable
      end
  end

  # selects the nearest xterm color for a given [r, g, b] color table
  def self.rgb2xterm(rgb)
    @rgb2xterm_cache ||= {}
    return @rgb2xterm_cache[rgb] ||=
      begin
        self.make_table
        smallest_distance = 1_000_000_000_000
        best_match = 0

        0.upto(253) do |color|
          d = self.euclidean_distance(@colortable[color], rgb)
          if d < smallest_distance
            smallest_distance = d
            best_match = color
          end
        end
        best_match
      end
  end

  # Return euclidean distance between two given vectors v1 and v2.
  # Example:
  #   Xterm256Color.euclidean_distance([0, 0], [1, 1]) # => 1.4142...
  def self.euclidean_distance(v1, v2)
    raise ArgumentError, "Expected two arrays" unless v1.is_a?(Array) && v2.is_a?(Array)
    raise ArgumentError, "Expected two array with the same size (#{v1.size} <=> #{v2.size})" if v1.size != v2.size

    #sum_of_squares = v1.zip(v2).inject(0) {|acc, (a, b)| acc + (a - b) ** 2 }
    sum_of_squares = 0
    v1.size.times do |i|
      sum_of_squares += (v1[i] - v2[i]) * (v1[i] - v2[i])
    end

    return Math.sqrt(sum_of_squares)
  end
end