JSONPath like access to Oj JSON structs.

path.rb

require 'delegate'
require 'oj'
require 'strscan'

# JSONPath like access to Oj JSON structs.
#
# @note Array access is identical to Array#[]
class Oj::Path < SimpleDelegator
  class Error < StandardError; end
  class MissingError < Error;  end
  class ParseError < Error;    end
  class TypeError < Error;     end

  attr_reader :root

  def self.load string, options = {}
    new Oj.load(string, {symbol_keys: true}.merge(options))
  end

  def initialize json, document = nil
    @root = document || self
    super json
  end

  def path path, *default
    scanner = StringScanner.new(path)
    context = self
    while not scanner.eos?
      if token = scanner.scan(/\$/)
        context = root
      elsif token = scanner.scan(/\./)
        # noop, current context
      elsif token = scanner.scan(/[a-z0-9_]+/i)
        raise TypeError.new("Attempt to access Hash key :#{token} on a #{context.class} for '#{path}' position #{scanner.pos}.") \
          unless context.respond_to?(:key)
        unless context.key?(token.to_sym)
          raise MissingError.new("Unknown Hash key :#{token} for '#{path}' position #{scanner.pos}.") if default.empty?
          context = default.first
          break
        end
        context = context[token.to_sym]
      elsif token = scanner.scan(/\[([\-0-9](?:(?:(?:\.\.)|(?:\,\s))[\-0-9])?)\]/)
        raise TypeError.new("Attempt to access Array #{token} on a #{context.class} for '#{path}' postion #{scanner.pos}.") \
          unless context.respond_to?(:[])
        context = eval("context#{token}")
        if context.nil?
          raise MissingError.new("Array access #{path} returned nil for '#{path}' position #{scanner.pos}.") if default.empty?
          context = default.first
          break
        end
      else
        raise ParseError.new("Invalid token '#{scanner.peek(1)}' for '#{path}' position #{scanner.pos}.")
      end
    end

    self.class.new(context, root)
  end
end # Oj::Path