complete day 4 part 2

day04.go

@@ -69,24 +69,42 @@ func (d day04) solve1(r io.Reader) string {
 	return fmt.Sprintf("%d", sum)
 }
 
+func expand(cursor, matches int, duplicates []int) []int {
+	if cursor+matches > len(duplicates) {
+		tmp := make([]int, cursor+matches)
+		for i := range duplicates {
+			tmp[i] = duplicates[i]
+		}
+		duplicates = tmp
+	}
+
+	for i := cursor; i < cursor+matches; i++ {
+		duplicates[i] = duplicates[i] + 1
+
+		// only count duplicates if we are past the first card
+		if cursor > 0 {
+			duplicates[i] = duplicates[i] + duplicates[cursor-1]
+		}
+	}
+
+	return duplicates
+}
+
 func (d day04) solve2(r io.Reader) string {
 	var (
-		current int
+		cursor     int
-		sum     = 1
+		duplicates = make([]int, 0)
-		cards   = make([]int, 185)
+		scanner    = bufio.NewScanner(r)
-		scanner = bufio.NewScanner(r)
 	)
 
-	for matches := -1; scanner.Scan() && matches != 0; current++ {
+	for matches := -1; scanner.Scan() && matches != 0; cursor++ {
 		_, line, _ := strings.Cut(scanner.Text(), ": ")
-		sum++
+		matches := countwinings(line)
-		matches = countwinings(line)
+		duplicates = expand(cursor, matches, duplicates)
-		for i := 1; i <= matches; i++ {
-			cards[current+i] = cards[current+i] + 1 + cards[current]
-		}
 	}
 
-	for _, c := range cards {
+	sum := cursor
+	for _, c := range duplicates {
 		sum += c
 	}
 	return fmt.Sprintf("%d", sum)