gistfile1.txt

require 'rmagick'
require 'pp'
include Magick

image = ImageList.new("source.png")

rgb = image
        .color_histogram
        .keys
        .map {|color| [color.red, color.green, color.blue]}

PP.pp rgb

# outputs
# [[0, 0, 0],
# [7710, 3598, 2056],
# [6425, 6425, 6425],
# [11565, 11565, 11565],
# [7710, 35466, 30583],
# [13107, 60395, 52171],
# [65535, 65535, 65535]]

CHUNKS = (2 ** 16).times.each_slice(2 ** 8)
BIG_TO_SMALL = CHUNKS.with_index.reduce({}) do |acc, (numbers, index)|
  acc.merge numbers.to_h { |number| [number, index] }
end

outputs = [[0, 0, 0],
 [7710, 3598, 2056],
 [6425, 6425, 6425],
 [11565, 11565, 11565],
 [7710, 35466, 30583],
 [13107, 60395, 52171],
 [65535, 65535, 65535]]

outputs.map { |rgb| rgb.map &BIG_TO_SMALL }
[[0, 0, 0],
 [30, 14, 8],
 [25, 25, 25],
 [45, 45, 45],
 [30, 138, 119],
 [51, 235, 203],
 [255, 255, 255]]