gistfile1.txt

/original/1X/#{sha}#{extension}
/original/2X/#{sha[0]}/#{sha}#{extension}
/original/3X/#{sha[0]}/#{sha[1]}/#{sha}#{extension}
/original/4X/#{sha[0]}/#{sha[1]}/#{sha[2]}/#{sha}#{extension}

/optimized/1X/#{sha}_#{version}_#{width}x#{height}#{extension}
/optimized/2X/#{sha[0]}/#{sha}_#{version}_#{width}x#{height}#{extension}
/optimized/3X/#{sha[0]}/#{sha[1]}/#{sha}_#{version}_#{width}x#{height}#{extension}
/optimized/4X/#{sha[0]}/#{sha[1]}/#{sha[2]}/#{sha}_#{version}_#{width}x#{height}#{extension}


# of optimized images = avatar_sizes (6) * device_pixel_ratios (3)

avatar sizes:
- tiny
- small
- medium
- large
- extra_large
- huge

device pixel ratios
- 1 (normal displays)
- 2 (retina displays)
- 3 (iPhone 6+)

cf. http://bjango.com/articles/min-device-pixel-ratio/


BASED ON THE ID OF THE UPLOAD?

1X =       1 -     1 000
2X =   1 001 -    16 000
3X =  16 001 -   256 000
4X = 256 001 - 4 096 000

(0..4).map { |i| (16**i) * 1_000 }