Nanoc::Filter.define(:sidenotes) do |content, params|
  @footnotes = {}

  # First pass: collect all footnote definitions
  content.scan(/\[\^([^\]]+)\]:\s*(.+?)($|\n\n)/) do |ref, text, _|
    @footnotes[ref] = text.strip
  end

  # Second pass: replace footnote references with sidenote markup
  result = content.gsub(/\[\^([^\]]+)\](?!:)/) do |match|
    ref = $1
    next match unless @footnotes[ref]  # Skip if no matching footnote definition
    
    <<~HTML
      <label for="sn-#{ref}" 
             class="margin-toggle sidenote-number">
      </label>
      <input type="checkbox"
             id="sn-#{ref}"
             class="margin-toggle"/>
      <span class="sidenote">
        #{@footnotes[ref]}
      </span>
    HTML
  end
  
  # Finally, remove the original footnote definitions
  result.gsub(/\[\^([^\]]+)\]:\s*(.+?)($|\n\n)/, '')
end