asciinema.org/app/decorators/asciicast_decorator.rb

class AsciicastDecorator < ApplicationDecorator
  decorates :asciicast

  THUMBNAIL_WIDTH = 20
  THUMBNAIL_HEIGHT = 10

  def os
    if uname =~ /Linux/
      'Linux'
    elsif uname =~ /Darwin/
      'OSX'
    else
      uname.split(' ', 2)[0]
    end
  end

  def shell_name
    File.basename(shell.to_s)
  end

  def smart_title
    if title.present?
      title
    elsif command.present?
      "$ #{command}"
    else
      "##{id}"
    end
  end

  def thumbnail
    if @thumbnail.nil?
      lines = model.snapshot.to_s.split("\n")

      top_lines = lines[0...THUMBNAIL_HEIGHT]
      top_text = prepare_lines(top_lines).join("\n")

      bottom_lines = lines.reverse[0...THUMBNAIL_HEIGHT].reverse
      bottom_text = prepare_lines(bottom_lines).join("\n")

      if top_text.gsub(/\s+/, '').size > bottom_text.gsub(/\s+/, '').size
        @thumbnail = top_text
      else
        @thumbnail = bottom_text
      end
    end

    @thumbnail
  end

  private

  def prepare_lines(lines)
    (THUMBNAIL_HEIGHT - lines.size).times { lines << '' }

    lines.map do |line|
      line = line[0...THUMBNAIL_WIDTH]
      line << ' ' * (THUMBNAIL_WIDTH - line.size)
      line
    end
  end
end