Building and Merging Deep Hashes

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Today, I continue work on my couch_design_docs gem. The basic idea remains the same, a javascript file in couch/_design/lucene/transform.js should describe a CouchDB design document:

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

So far, I have a Directory class that converts the directory path, file basename and file contents into an array. Next up, I need a way to convert arrays into deep hashes:

  it "should convert arrays into deep hashes" do
    Directory.
      a_to_hash(%w{a b c d}).
      should == {
      'a' => {
        'b' => {
          'c' => 'd'
        }
      }
    }
  end

Running the RSpec example, I get a failure:

cstrom@jaynestown:~/repos/couch_design_docs$ spec ./spec/couch_design_docs_spec.rb
.F.

1)
NoMethodError in 'CouchDesignDocs::Directory should convert arrays into deep hashes'
undefined method `a_to_hash' for CouchDesignDocs::Directory:Class
./spec/couch_design_docs_spec.rb:17:

Finished in 0.006896 seconds

3 examples, 1 failure

I change the message by defining an empty a_to_hash class method:

cstrom@jaynestown:~/repos/couch_design_docs$ spec ./spec/couch_design_docs_spec.rb 
.F.

1)
'CouchDesignDocs::Directory should convert arrays into deep hashes' FAILED
expected: {"a"=>{"b"=>{"c"=>"d"}}},
     got: nil (using ==)
./spec/couch_design_docs_spec.rb:25:

Finished in 0.007203 seconds

3 examples, 1 failure

No more changing the message is needed, now I can make it pass. I still rather fancy tail recursion, so I end up with something similar to the other day:

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

With that, I am ready to assemble all javascript files into a design docs structure. First up, I make sure that I have more than one javascript file in the fixtures directory:

cstrom@jaynestown:~/repos/couch_design_docs$ find fixtures/
fixtures/
fixtures/a
fixtures/a/b
fixtures/a/b/d.js
fixtures/a/b/c.js

Then I write my example to drive things along:

    it "should assemble all documents into a single docs structure" do
      @it.to_hash.
        should == {
        'a' => {
          'b' => {
            'c' => 'function(doc) { return true; }',
            'd' => 'function(doc) { return true; }'
          }
        }

      }
    end

The code that implements this iterates over each .js file in the design docs directory, using the previously built methods to build hashes to be merged together:

    def to_hash
      Dir["#{couch_view_dir}/**/*.js"].inject({}) do |memo, filename|
        hash = Directory.a_to_hash(expand_file(filename))
        memo.merge(hash)
      end
    end

Simple enough, but wrong:

cstrom@jaynestown:~/repos/couch_design_docs$ spec ./spec/couch_design_docs_spec.rb 
...F

1)
'CouchDesignDocs::Directory a valid directory should assemble all documents into a single docs structure' FAILED
expected: {"a"=>{"b"=>{"c"=>"function(doc) { return true; }", "d"=>"function(doc) { return true; }"}}},
     got: {"a"=>{"b"=>{"c"=>"function(doc) { return true; }"}}} (using ==)
./spec/couch_design_docs_spec.rb:48:

Finished in 0.008113 seconds

4 examples, 1 failure

The problem is the merge, which merges at the top level (the "a" key). I want the deepest hashes (the values of "b") to be merged together. Something like this does the trick:

    def deep_hash_merge(h1, h2)
      h2.each_key do |k|
        if h1.key? k
          deep_hash_merge(h1[k], h2[k])
        else
          h1[k] = h2[k]
        end
      end
      h1
    end

It works, but it is not side-effect free. I will likely retry that bit of code tomorrow. My brain is not working so well at this point, so it is a good time to call it a day. Before I quit, leave myself a note:

    it "should assemble all documents into a single docs structure" do
      pending "you can do a better job with deep hash merging than that"
      @it.to_hash.
        should == {
        'a' => {
          'b' => {
            'c' => 'function(doc) { return true; }',
            'd' => 'function(doc) { return true; }'
          }
        }

      }
    end

Hopefully my tomorrow self will not be too offended.