require 'tempfile'
require 'fileutils'

# Finish generation before reporting readiness; downloads never hold the full file in RAM.
class CountsExportDownload
  EXPIRY = 10.minutes

  def self.directory = Rails.root.join('tmp', 'counts_exports')
  def self.verifier = Rails.application.message_verifier('counts_export_download')

  def self.prepare(exporter, owner)
    FileUtils.mkdir_p(directory, mode: 0o700)
    Dir.glob(directory.join('counts-*.export')).each do |path|
      File.unlink(path) if File.mtime(path) < Time.current - EXPIRY
    rescue Errno::ENOENT
      # Another request may have removed an expired download.
    end
    file = Tempfile.create(['counts-', '.part'], directory)
    complete = false
    path = file.path
    begin
      file.binmode
      exporter.body.each { file.write(_1) }
      file.close
      finished_path = path.sub(/\.part\z/, '.export')
      File.rename(path, finished_path)
      path = finished_path
      token = verifier.generate({ 'file' => File.basename(path),
                          'resource' => exporter.resource, 'format' => exporter.format,
                          'owner' => owner }, expires_in: EXPIRY, purpose: 'counts_export')
      complete = true
      token
    ensure
      file.close unless file.closed?
      File.unlink(path) if !complete && File.exist?(path)
    end
  end

  def self.find(token, owner)
    metadata = verifier.verified(token, purpose: 'counts_export')
    return unless metadata.is_a?(Hash) && metadata['owner'] == owner
    return unless metadata['file']&.match?(/\Acounts-[a-zA-Z0-9_-]+\.export\z/)
    return unless CountsResourceDefinition::DEFINITIONS.key?(metadata['resource'])
    return unless ['json', 'csv'].include?(metadata['format'])

    path = directory.join(metadata['file'])
    return unless path.file?

    metadata.merge('path' => path.to_s)
  end
end