module Liquid

  # "For" iterates over an array or collection.
  # Several useful variables are available to you within the loop.
  #
  # == Basic usage:
  #    {% for item in collection %}
  #      {{ forloop.index }}: {{ item.name }}
  #    {% endfor %}
  #
  # == Advanced usage:
  #    {% for item in collection %}
  #      <div {% if forloop.first %}class="first"{% endif %}>
  #        Item {{ forloop.index }}: {{ item.name }}
  #      </div>
  #    {% endfor %}
  #
  # You can also define a limit and offset much like SQL.  Remember
  # that offset starts at 0 for the first item.
  #
  #    {% for item in collection limit:5 offset:10 %}
  #      {{ item.name }}
  #    {% end %}
  #
  #  To reverse the for loop simply use {% for item in collection reversed %}
  #
  # == Available variables:
  #
  # forloop.name:: 'item-collection'
  # forloop.length:: Length of the loop
  # forloop.index:: The current item's position in the collection;
  #                 forloop.index starts at 1.
  #                 This is helpful for non-programmers who start believe
  #                 the first item in an array is 1, not 0.
  # forloop.index0:: The current item's position in the collection
  #                  where the first item is 0
  # forloop.rindex:: Number of items remaining in the loop
  #                  (length - index) where 1 is the last item.
  # forloop.rindex0:: Number of items remaining in the loop
  #                   where 0 is the last item.
  # forloop.first:: Returns true if the item is the first item.
  # forloop.last:: Returns true if the item is the last item.
  #
  class For < Block
    Syntax = /(\w+)\s+in\s+(#{QuotedFragment}+)\s*(reversed)?/

    def initialize(tag_name, markup, tokens, context)
      if markup =~ Syntax
        @variable_name = $1
        @collection_name = $2
        @name = "#{$1}-#{$2}"
        @reversed = $3
        @attributes = {}
        markup.scan(TagAttributes) do |key, value|
          @attributes[key] = value
        end
      else
        raise SyntaxError.new("Syntax Error in 'for loop' - Valid syntax: for [item] in [collection]")
      end

      super
    end

    def render(context)
      context.registers[:for] ||= Hash.new(0)

      collection = context[@collection_name]
      collection = collection.to_a if collection.is_a?(Range)

      return '' unless collection.respond_to?(:each)

      from = if @attributes['offset'] == 'continue'
        context.registers[:for][@name].to_i
      else
        context[@attributes['offset']].to_i
      end

      limit = context[@attributes['limit']]
      to    = limit ? limit.to_i + from : nil


      segment = slice_collection_using_each(collection, from, to)

      return '' if segment.empty?

      segment.reverse! if @reversed

      result = []

      length = segment.length

      # Store our progress through the collection for the continue flag
      context.registers[:for][@name] = from + segment.length

      context.stack do
        segment.each_with_index do |item, index|
          context[@variable_name] = item
          context['forloop'] = {
            'name'    => @name,
            'length'  => length,
            'index'   => index + 1,
            'index0'  => index,
            'rindex'  => length - index,
            'rindex0' => length - index -1,
            'first'   => (index == 0),
            'last'    => (index == length - 1) }

          result << render_all(@nodelist, context)
        end
      end
      result
    end

    def slice_collection_using_each(collection, from, to)
      segments = []
      index = 0
      yielded = 0
      collection.each do |item|

        if to && to <= index
          break
        end

        if from <= index
          segments << item
        end

        index += 1
      end

      segments
    end
  end

  Template.register_tag('for', For)
end