module Mongoid::Positional

def positionally(selector, operations, processed = {})
  if selector.size == 1 || selector.values.any? { |val| val.nil? }
    return operations
  end
  keys = selector.keys.map{ |m| m.sub('._id','') } - ['_id']
  keys = keys.sort_by { |s| s.length*-1 }
  process_operations(keys, operations, processed)
end

def process_operations(keys, operations, processed)
  operations.each_pair do |operation, update|
    processed[operation] = process_updates(keys, update)
  end
  processed
end

def process_updates(keys, update, updates = {})
  update.each_pair do |position, value|
    updates[replace_index(keys, position)] = value
  end
  updates
end

def replace_index(keys, position)
  # replace index with $ only if that key is in the selector and it is only
  # nested a single level deep.
  matches = position.scan(/\.\d+\./)
  if matches.size == 1
    keys.each do |kk|
      if position =~ /\A#{kk}\.\d+\.(.*)\z/
        return "#{kk}.$.#{$1}"
      end
    end
  end
  position
end