Pride flag/gradient generator in julia

pride_flag.jl

#!/usr/bin/env julia

module PrideFlag
using Cairo

function make_flag(n, file; line_width=1)
  @assert n <= 256
  i  = 0
  f  = filtered_colors(n)
  w  = length(f) * line_width
  c  = CairoRGBSurface(round(Int, 16//9 * w), w)
  cr = CairoContext(c)
  for (r,g,b) in sort(f, by=hue(n))
    move_to(cr, 0, i*line_width+(line_width÷2))
    line_to(cr, c.width, i*line_width+(line_width÷2))
    set_line_width(cr, 1.0*line_width)
    set_source_rgb(cr, r/n, g/n, b/n)
    stroke(cr)
    i += 1
  end
  write_to_png(c, file)
end

hue(n) = c -> hue(c,n)

function hue(c,n)
  (r,g,b) = c
  cmax = max(r,g,b)
  cmin = min(r,g,b)

  h = if cmax == cmin
    0
  elseif cmax == r
    60 * (0 + (g - b)/(cmax - cmin))
  elseif cmax == g
    60 * (2 + (b - r)/(cmax - cmin))
  else
    60 * (4 + (r - g)/(cmax - cmin))
  end

  return h < 0 ? 360 + h : h
end

function color255(c, n)
  (x->round(Int, x)).(255//n .* c)
end

function colors_from_index(i, n)
  r = i ÷ (n * n)
  g = (i ÷ n) % n
  b = i % n
  return r,g,b
end

function colors(n)
  [colors_from_index(i,n) for i in 0:n^3]
end

function filter_shades(shade, colors)
  filter(x->maximum(x)==shade, colors)
end

function filter_contrast(cont, colors)
  filter(x->maximum(x) - minimum(x) == cont, colors)
end

function filtered_colors(n)
  filter_shades(n - 1, filter_contrast(n - 1, colors(n)))
end

module UI
import ..PrideFlag

function ask(prompt)
  print(prompt)
  return readline()
end

function main()
  main(ask("possible rgb values for each color: "))
end

function main(n)
  main(convert_bits(n), ask("output file name: "))
end

function main(n, file)
  main(convert_bits(n), file, ask("line width: "))
end

function main(ns, file, line_width_s)
  n = convert_bits(ns)
  line_width = convert_to_number(line_width_s)

  PrideFlag.make_flag(n, file, line_width=line_width)
end

function convert_bits(s)
  n = convert_to_number(s)

  if n < 2 || n > 256
    error("value must be in the [2,256] range")
  end

  return n
end

function convert_to_number(n::Int)
  return n
end

function convert_to_number(s::String)
  if any(!isnumeric, s)
    error("value must be a positive number")
  end
  if length(s) >= log2(typemax(Int))
    error("value is too large")
  end
  return parse(Int, s)
end

end

end

if !isinteractive()
  PrideFlag.UI.main(ARGS...)
end