Tail Recursion is My Current Golden Hammer

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Picking up from last night, I am still keen on exploring mapping directory structures into JSON data structures. The ultimate goal is to create a CouchDB design document (which are stored in JSON format).

For example, I would like to take a document in couch/_design/lucene/transform.js to result in a JSON document along the lines of:

{
  "lucene": {
    "transform": <<contents of transform.js>>
  }
}

I devised a poor man's solution last night, but would like something that will work with directories that are deeper than 1 level. I also favor tail recursion because I have been learning Erlang. It may be something of a golden hammer for me, but it's a fun golden hammer. Besides, I am prototyping to learn.

First up, for each .js file, I build an array of the directories, basename of the file and the contents the file. For couch/_design/lucene/transform.js, I end up with something like this:

["lucene", "transform", "<<contents of transform.js>>"]

I can get that with:

  Dir["#{DESIGN_DOC_DIR}/**/*.js"].each do |filename|
    path = File.dirname(filename).
      split(/\//)
    path << File.basename(filename, ".js")

    file = File.new(filename)
    path << file.read
  end

Using a tail-recursive method, I can then build the desired data structure:

  def build_doc(list)
    key = list.first
    if (list.length > 2)
      { key => build_doc(list[1,list.length]) }
    else
      { key => list.last }
    end
  end

Trying this out in and IRB session, I am getting my desired results:

>>   def build_doc(list)
>>       key = list.first
>>       if (list.length > 2)
>>           { key => build_doc(list[1,list.length]) }
>>         else
?>             { key => list.last }
>>         end
>>     end
=> nil
>> build_doc(%w{1 2 3 4 5})
=> {"1"=>{"2"=>{"3"=>{"4"=>"5"}}}}

So I feel safe to give this a try with my existing design docs being added in the Cucumber features/support/env.rb file. I add this to my Before block:

  def build_doc(list)
    key = list.first
    if (list.length > 2)
      { key => build_doc(list[1,list.length]) }
    else
      { key => list.last }
    end
  end

  docs = { }
  DESIGN_DOC_DIR = "couch/_design"
  Dir["#{DESIGN_DOC_DIR}/**/*.js"].each do |filename|
    path = File.dirname(filename).
      gsub(/#{DESIGN_DOC_DIR}\//, '').
      split(/\//)
    path << File.basename(filename, ".js")

    file = File.new(filename)
    path << file.read

    docs.merge!(build_doc(path))
  end

  docs.each do |document_name, doc|
    RestClient.put "#{@@db}/_design/#{document_name}",
      doc.to_json,
      :content_type => 'application/json'
  end

To make sure that everything is still working, I run one of the recipe search Cucumber scenarios (which would fail without a good lucene design document):

cstrom@jaynestown:~/repos/eee-code$ cucumber features -n -s "Matching a word in the recipe summary"
Sinatra::Test is deprecated; use Rack::Test instead.
Feature: Search for recipes

  So that I can find one recipe among many
  As a web user
  I want to be able search recipes

  Scenario: Matching a word in the recipe summary
    Given a "pancake" recipe with a "Yummy!" summary
    And a "french toast" recipe with a "Delicious" summary
    And a 0.5 second wait to allow the search index to be updated
    When I search for "yummy"
    Then I should see the "pancake" recipe in the search results
    And I should not see the "french toast" recipe in the search results

1 scenario
6 passed steps

Yay!

I rather like that. It is especially nice that the transform.js file is pure javascript, making is easily tested. I may very well delete my prototype and implement for real tomorrow.