1 files changed, 71 insertions, 0 deletions
diff --git a/aoc23/day04_test.go b/aoc23/day04_test.go
new file mode 100644
index 0000000..e9dd190
--- /dev/null
+++ b/aoc23/day04_test.go
@@ -0,0 +1,71 @@
+package aoc23
+
+import (
+	"io"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+type Card struct {
+	id      int
+	winners Set[int]
+	got     []int
+}
+
+func parseCard(s string) Card {
+	re := regexp.MustCompile(": | \\| ")
+	parts := re.Split(s, -1)
+	id := requireInt(strings.Fields(parts[0])[1])
+	got := map2(strings.Fields(parts[2]), requireInt)
+	winners := Set[int]{}
+	winners.AddAll(map2(strings.Fields(parts[1]), requireInt))
+	return Card{id, winners, got}
+}
+
+func countWinners(card Card) int {
+	return frequency(card.got, func(g int) bool { return card.winners.Contains(g) })
+}
+
+func cardPoints(r io.Reader) int {
+	score := 0
+	cards := parseLines(r, parseCard)
+	for _, card := range cards {
+		winners := countWinners(card)
+		if winners > 0 {
+			score += pow(2, winners-1)
+		}
+	}
+	return score
+}
+
+func countCards(cards []Card) int {
+	sum := 0
+	for i, card := range cards {
+		winners := countWinners(card)
+		next := i + 1
+		sum += 1 + countCards(cards[next:next+winners])
+	}
+	return sum
+}
+
+func cardCount(r io.Reader) int {
+	cards := parseLines(r, parseCard)
+	return countCards(cards)
+}
+
+func TestDay04(t *testing.T) {
+	example := `
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+`
+	assert(t, 13, run(cardPoints, inputString(example)))
+	assert(t, 32001, run(cardPoints, inputFile(4)))
+
+	assert(t, 30, run(cardCount, inputString(example)))
+	assert(t, 5037841, run(cardCount, inputFile(4)))
+}