an implementation of JSON Schema's fragment resolution algorithm

json_fragment_resolution.rb

require 'uri'

    ##
    # Attempts to resolve a fragment identifier (`identifier`) in a deserialized
    # JSON object (`object`).
    #
    # This method always returns a pair of values.  The first value is the
    # resolution result, and the second value is the portion of the identifier
    # that was successfully resolved.  The characteristics of these values
    # therefore directly depend on resolution success:
    #
    #     | Resolution | First value  | Second value                           |
    #     | Success    | a JSON value | the full identifier                    |
    #     | Failure    | :error       | the identifier up to the failure point |
    #
    # @param [String] identifier the fragment identifier
    # @param [Hash] object a deserialized JSON object
    # @return [Array<[Object, :error], String>] the resolution result and
    #   resolved fragment
    # @see http://tools.ietf.org/html/draft-zyp-json-schema-03#section-6.2
    #   JSON Schema, draft 03, section 6.2
    def resolve(identifier, object)
      fragment, path = identifier.split('/').inject([object, []]) do |(fragment, path), component|
        if component == '#'
          [object, path << '#']
        else
          decoded = URI.decode(component)
          next_fragment = case fragment
                          when Hash
                            fragment.has_key?(decoded) ? fragment[decoded] : :error
                          when Array
                            index = decoded.to_i

                            if decoded =~ /\d+/ && index >= 0 && index < fragment.length
                              fragment[index]
                            else
                              :error
                            end
                          else
                            :error
                          end

          if next_fragment == :error
            break [:error, path]
          else
            [next_fragment, path << component]
          end
        end
      end

      [fragment, path.join('/')]
    end

# ---
# demo
# ---

require 'json'
require 'yaml' # better pretty-printing

object = JSON.parse(%Q{
  {
      "foo": {
          "bar": [
              {
                  "baz": {
                      "qux": 5
                  }
              }
          ]
      }, 
      "metasyntactic names suck": [
          "yeah, they do", 
          "but I lack imagination"
      ]
  }
})

def print(identifier, object)
  fragment, path = resolve(identifier, object)

  y({ 'expected' => identifier,
      'actual' => path,
      'fragment' => fragment,
      'fragment class' => fragment.class.to_s })

  puts
  puts '*' * 80
  puts
end

print('#', object)
print('#/foo/bar/0/baz/qux', object)
print('#/foo/bar/1/baz/qux', object)
print('#/metasyntactic%20names%20suck/1', object)