diff --git a/src/advent/day3_test.rb b/src/advent/day3_test.rb
index ac471d7..e196b16 100644
--- a/src/advent/day3_test.rb
+++ b/src/advent/day3_test.rb
@@ -33,6 +33,8 @@ class Day3Test < Test::Unit::TestCase
 
     assert_true 0 <= 10 and 10 < 100
 
+    assert_equal [1, 2] + [3, 4], [1,2,3,4]
+
 
 
   end
diff --git a/src/advent/day4.rb b/src/advent/day4.rb
index 069cb0e..bfb80d3 100644
--- a/src/advent/day4.rb
+++ b/src/advent/day4.rb
@@ -7,8 +7,12 @@ class Card
     @winning_numbers = winning_numbers
   end
 
+  def matching_number
+    @selected_numbers.filter {|s| @winning_numbers.index(s) != nil}.length
+  end
+
   def score
-    n = @selected_numbers.filter {|s| @winning_numbers.index(s) != nil}.length
+    n = matching_number
     n == 0 ? 0 : 2 ** (n - 1)
   end
 
@@ -29,6 +33,7 @@ end
 class Cards
   def initialize(cards)
     @cards = cards
+    @scratchcards_generated_by_card_cache = {}
   end
 
   def self.deserialize(ser)
@@ -43,12 +48,38 @@ class Cards
     @cards.map {|card| card.score}
   end
 
+  def scratchcards_generated_by_card(card)
+    if @scratchcards_generated_by_card_cache.has_key? card.id
+      return @scratchcards_generated_by_card_cache[card.id]
+    end
+    ret = [card]
+    score = card.matching_number
+    if score > 0
+      sub_cards = @cards.filter {|sub_card| sub_card.id > card.id and sub_card.id <= card.id + score }
+      ret += sub_cards
+                  .map {|sub_card| scratchcards_generated_by_card(sub_card)}
+                  .flatten
+    end
+    @scratchcards_generated_by_card_cache[card.id] = ret
+    ret
+  end
+
+  def all_scratchcards
+    ret = []
+    @cards.each { |card| ret +=  scratchcards_generated_by_card card }
+    ret
+
+  end
+
   attr_accessor :cards
 
 end
 
-input_file_name = File.join(File.dirname(__FILE__), "day_4_input.txt")
-input_content = File.open(input_file_name).read
-cards = Cards.deserialize input_content
+if __FILE__ == $0
+  input_file_name = File.join(File.dirname(__FILE__), "day_4_input.txt")
+  input_content = File.open(input_file_name).read
+  cards = Cards.deserialize input_content
 
-puts "Solution to part 1: #{cards.scores.sum}"
\ No newline at end of file
+  puts "Solution to part 1: #{cards.scores.sum}"
+  puts "Solution to part 2: #{cards.all_scratchcards.length}"
+end
diff --git a/src/advent/day4_test.rb b/src/advent/day4_test.rb
index 41a21c8..bd16f8a 100644
--- a/src/advent/day4_test.rb
+++ b/src/advent/day4_test.rb
@@ -49,6 +49,40 @@ Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11"
   def test_day_4_part_1
     cards = Cards.deserialize @@easy_input
     assert_equal 13, cards.scores.sum
-
   end
-end
\ No newline at end of file
+
+  def test_scratchcards_generated_by_card
+    cards = Cards.deserialize @@easy_input
+    ret = cards.scratchcards_generated_by_card cards.cards[0]
+    expt = [1, 2, 3, 4, 5, 5, 4, 5, 3, 4, 5, 5, 4, 5, 5]
+    assert_equal expt, ret.map {|card| card.id}
+  end
+
+  def counter(lst)
+    ret = {}
+    lst.each do |e|
+      if ret.has_key? e
+        ret[e] += 1
+      else
+        ret[e] = 1
+      end
+    end
+    ret
+  end
+
+  def test_day_4_part_2
+    cards = Cards.deserialize @@easy_input
+    assert_equal( 30, cards.all_scratchcards.length)
+  end
+end
+
+#Once all of the originals and copies have been processed, you end up with
+# 1 instance of card 1
+# , 2 instances of card 2,
+# 4 instances of card 3,
+# 8 instances of card 4,
+# 14 instances of card 5,
+# and 1 instance of card 6.
+#
+#
+# In total, this example pile of scratchcards causes you to ultimately have 30 scratchcards!
\ No newline at end of file