2023 Day 4 Part 2

2023/day04.nim

@@ -28,8 +28,11 @@ proc parseCards(text: string): seq[Card] =
   for line in lines:
     result.add parseCard(line)
 
+func matches(card: Card): int =
+  len(card.winning * card.mine)
+
 func score(card: Card): int =
-  let matches = len(card.winning * card.mine)
+  let matches = card.matches
   if matches > 0:
     2 ^ (matches - 1)
   else:
@@ -42,4 +45,23 @@ proc part1(): int =
   for card in cards:
     result.inc card.score
 
+proc part2(): int =
+  let text = readFile("input/day04.txt")
+  let cards = parseCards(text)
+
+  var multiples: CountTable[int]
+
+  for card in cards:
+    multiples.inc card.id
+
+    let extras = card.matches
+    let scale = multiples[card.id]
+
+    for id in (card.id + 1) .. (card.id + extras):
+      multiples.inc(id, scale)
+
+  for (id, count) in multiples.pairs:
+    result += count
+
 echo part1()
+echo part2()