_plugins/create_data_docs.rb

require 'json'
require 'tempfile'

# This plugin generates the JSON data files _data/docs_*.json which are used
# in liquid tags across the site to generate the docs nav menu, forward/back
# links and breadcrumbs.
Jekyll::Hooks.register :site, :pre_render do |site|
    # The Polyglot plugin launches a Jekyll build per language, optionally in
    # parallel, meaning that this hook will be called once for each language.
     
	out_path = '_data/docs_' + site.active_lang + '.json'
    puts 'INFO: data_docs plugin begins to create ' + out_path
    
	docs_by_title = {}
	top_level_docs = []
	prev_doc_md = nil
	
    for doc in site.collections['docs'].docs
	    doc_md = {
	    	url: doc.url,
	    	title: doc.data['title'],
	    	parent: doc.data['parent'],
	    	relative_path: doc.relative_path,
	    	children: [], # populated as each child is encountered,
	    	previous_title: nil, # populated lower down if prev_doc_md != nil,
	    	previous_url: nil,   # populated lower down if prev_doc_md != nil,
	    	next_title: nil, # populated in the next loop iteration if there is one
	    	next_url: nil,   # populated in the next loop iteration if there is one
	    	breadcrumbs: []  # populated lower down if this doc has ancestors
	    }

		unless prev_doc_md.nil?
			doc_md[:previous_title] = prev_doc_md[:title]
			doc_md[:previous_url] = prev_doc_md[:url]
		    prev_doc_md[:next_title] = doc_md[:title]
		    prev_doc_md[:next_url] = doc_md[:url]
		end
		
	    docs_by_title[doc_md[:title]] = doc_md
	    prev_doc_md = doc_md

		# Note the assumption that parents always precede their children
		# in site.collections['docs'] in the code that follows.  For this to hold,
		# authors must use a file number+name convention that puts parents first
		# lexicographically.
	    if doc_md[:parent].nil?
		    top_level_docs << doc_md
		else
			# Append to the children of the immediate parent
		    anc_doc_md = docs_by_title[doc_md[:parent]]
		    if anc_doc_md.nil?
			    puts 'WARNING: the parent of "' + doc_md[:title] + '" is "' + \
				    doc_md[:parent] + '" but could not be found.'
		    else
				anc_doc_md[:children] << doc_md 
			end
			
		    while !anc_doc_md.nil?
				# Continue up the ancestor chain to build breadcrumbs
				doc_md[:breadcrumbs] << {
					title: anc_doc_md[:title],
					url: anc_doc_md[:url]
			    }

			    anc_doc_md = docs_by_title[anc_doc_md[:parent]]
		    end
	    end
    end

    docs_md = {
    	by_title: docs_by_title,
    	hierarchy: top_level_docs
    }

    File.write(out_path, JSON.pretty_generate(docs_md))
    puts 'INFO: data_docs plugin has finished creating ' + out_path

	# Nasty hack: ask Jekyll to read all of the data files for the site again
	# so that it will incorporate the new data_*.json files we generated here.
	puts 'INFO: data_docs plugin asks Jekyll to reload site data files'
	site.data = Jekyll::DataReader.new(site).read(site.config["data_dir"])
end