A Sinatra-based webapp that retrieves the content of a gist and generates the PlantUML image URL from it. Apparently, I'm a Ruby beginner. Please feel free to fork and make it better.

gist2uml.rb

#!/usr/bin/env ruby

require 'rubygems'

require 'open-uri'
require 'open3'
require 'json'
require 'sinatra'
require 'zlib'

$encode6bit = [
  '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E',
  'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
  'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
  'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
  'y', 'z', '-', '_'
] 

configure do
  mime_type :png, 'image/png'
end

get %r{/uml/gist/([0-9]+)} do |gist_id|
  begin
    cache_filename = "/var/cache/gist2uml/#{gist_id}.uml"
    if File::exist?(cache_filename) && File::mtime(cache_filename) > Time::now - 300
      file = open(cache_filename)
      umldata = file.read
    end
  rescue
  ensure
    if file
      file.close
    end
  end

  if umldata == nil
    umldata = ''
    begin
      file = open("https://api.github.com/gists/#{gist_id}")
      gist = JSON.parse(file.read)
      if gist['files']
        for idx, f in gist['files']
          umldata = f['content']
          if umldata && umldata.length > 0
            break
          else
            umldata = ''
          end
        end
      end
      File.open("/var/cache/gist2uml/#{gist_id}.uml", 'w') { |f| f.write(umldata) }
    ensure
      if file
        file.close
      end
    end
  end

  if !umldata.empty?
    umldata = Zlib::Deflate.deflate(umldata, Zlib::BEST_COMPRESSION).each_byte.to_a
    umldata_encoded = ''
    i = 0
    while i < umldata.size
      b1 = umldata[i]
      i = i + 1
      b2 = if i < umldata.size then umldata[i] else 0 end
      i = i + 1
      b3 = if i < umldata.size then umldata[i] else 0 end
      i = i + 1

      c1 = b1 >> 2
      c2 = ((b1 & 0x3) << 4) | (b2 >> 4)
      c3 = ((b2 & 0xf) << 2) | (b3 >> 6)
      c4 = b3 & 0x3f

      umldata_encoded << $encode6bit[c1 & 0x3f]
      umldata_encoded << $encode6bit[c2 & 0x3f]
      umldata_encoded << $encode6bit[c3 & 0x3f]
      umldata_encoded << $encode6bit[c4 & 0x3f]
    end
    headers "Location" => "http://www.plantuml.com/plantuml/img/#{umldata_encoded}"
    302
  else
    404
  end
end