module Jekyll::Filters::GroupingFilters

def group_by(input, property)

"items" => [...] } # all the items where `property` == "larry"
{"name" => "larry"
Returns an array of Hashes, each looking something like this:

property - the property
input - the inputted Enumerable

Group an array of items by a property 
def group_by(input, property)
  if groupable?(input)
    groups = input.group_by { |item| item_property(item, property).to_s }
    grouped_array(groups)
  else
    input
  end
end

def group_by_exp(input, variable, expression)

Returns the filtered array of objects

expression -a Liquid comparison expression passed in as a string
variable - the variable to assign each item to in the expression
input - the object array

Group an array of items by an expression 
def group_by_exp(input, variable, expression)
  return input unless groupable?(input)
  parsed_expr = parse_expression(expression)
  @context.stack do
    groups = input.group_by do |item|
      @context[variable] = item
      parsed_expr.render(@context)
    end
    grouped_array(groups)
  end
end

def groupable?(element) 

def groupable?(element)
  element.respond_to?(:group_by)
end

def grouped_array(groups) 

def grouped_array(groups)
  groups.each_with_object([]) do |item, array|
    array << {
      "name"  => item.first,
      "items" => item.last,
      "size"  => item.last.size,
    }
  end
end

def parse_expression(str) 

def parse_expression(str)
  Liquid::Variable.new(str, Liquid::ParseContext.new)
end