#!/usr/bin/env ruby
##
# pandoc2yaml.rb extracts the metadata from a pandoc markdown file and prints
# that metadata out again as a pandoc markdown file with nothing in it but that
# metadata
#
# Usage:
#
#  pandoc2yaml.rb input_file
#
##
module Pandoc2Yaml
  require 'json'
  require 'paru/pandoc'

  # Paru converters:
  # Note. When converting metadata back to the pandoc markdown format, you have
  # to use the option "standalone", otherwise the metadata is skipped
  PANDOC_2_JSON = Paru::Pandoc.new do
    from 'markdown'
    to 'json'
  end
  JSON_2_PANDOC = Paru::Pandoc.new do
    from 'json'
    to 'markdown'
    standalone
  end

  # When converting a pandoc document to JSON, or vice versa, the JSON object
  # has the following three properties:
  VERSION = 'pandoc-api-version'
  META = 'meta'
  BLOCKS = 'blocks'

  def extract_metadata(input_document)
    json = JSON.parse(PANDOC_2_JSON << File.read(input_document))
    yaml = ''

    version, metadata = json.values_at(VERSION, META)

    unless metadata.empty?
      metadata_document = {
        VERSION => version,
        META => metadata,
        BLOCKS => []
      }

      yaml = JSON_2_PANDOC << JSON.generate(metadata_document)
    end

    yaml
  end
end

if __FILE__ == $0
  include Pandoc2Yaml

  if ARGV.size != 1
    warn 'Expecting exactly one argument: the pandoc file to strip for metadata'
    exit
  end

  input_document = ARGV.first
  output_metadata = Pandoc2Yaml.extract_metadata input_document
  puts output_metadata
end