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task 'assets:precompile:before' do

  unless %w{profile production}.include? Rails.env
    raise "rake assets:precompile should only be run in RAILS_ENV=production, you are risking unminified assets"
  end

  # in the past we applied a patch that removed asset postfixes, but it is terrible practice
  # leaving very complicated build issues
  # https://github.com/rails/sprockets-rails/issues/49

  # let's make precompile faster using redis magic
  require 'sprockets'
  require 'digest/sha1'

  module ::Sprockets
    class UglifierCompressor
      def evaluate(context, locals, &block)
        expire_after = 1.week

        digest = Digest::SHA1.hexdigest(data)
        key = "SPROCKETS_#{digest}"

        if compiled = $redis.get(key)
          $redis.expire(key, expire_after)
        else
          compiled = Uglifier.new(:comments => :none).compile(data)
          $redis.setex(key, expire_after, compiled)
        end
        
        compiled
      end
    end
  end

end

task 'assets:precompile' => 'assets:precompile:before'

BEFORE: 

sam@ubuntu discourse % rm -fr tmp/cache                               
sam@ubuntu discourse % rm -fr public/assets 
sam@ubuntu discourse % time RAILS_ENV=production bin/rake assets:precompile

58.55s user 1.79s system 100% cpu 1:00.02 total


AFTER:

sam@ubuntu discourse % rm -fr tmp/cache                               
sam@ubuntu discourse % rm -fr public/assets 
sam@ubuntu discourse % time RAILS_ENV=production bin/rake assets:precompile

7.29s user 0.73s system 100% cpu 8.005 total