From 8900459fc61b0a7ee9328f442f8f9a6b126025cb Mon Sep 17 00:00:00 2001
From: Nick Krichevsky <nick@ollien.com>
Date: Mon, 4 Dec 2023 08:54:23 -0500
Subject: [PATCH] Add day 4 part 2

---
 day4/main.go | 111 +++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 95 insertions(+), 16 deletions(-)

diff --git a/day4/main.go b/day4/main.go
index 505442c..330d251 100644
--- a/day4/main.go
+++ b/day4/main.go
@@ -6,31 +6,48 @@ import (
 	"io"
 	"os"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 )
 
 type Card struct {
+	id             int
 	winningNumbers []int
 	ourNumbers     []int
 }
 
-func (card Card) Score() int {
-	score := 0
+func (card Card) NumMatchingNumbers() int {
+	matchingNumbers := 0
+
 	winningNumbers := makeSet(card.winningNumbers)
 	for _, ourNumber := range card.ourNumbers {
-		if _, ok := winningNumbers[ourNumber]; !ok {
-			continue
-		}
-
-		if score == 0 {
-			score = 1
-		} else {
-			score *= 2
+		if _, ok := winningNumbers[ourNumber]; ok {
+			matchingNumbers++
 		}
 	}
 
-	return score
+	return matchingNumbers
+}
+
+func (card Card) Score() int {
+	numMatchingNumbers := card.NumMatchingNumbers()
+	if numMatchingNumbers == 0 {
+		return 0
+	}
+
+	return pow2(numMatchingNumbers)
+}
+
+func (card Card) WinsCardsWithIDs() []int {
+	numMatchingNumbers := card.NumMatchingNumbers()
+
+	wonCards := make([]int, numMatchingNumbers)
+	for i := 0; i < numMatchingNumbers; i++ {
+		wonCards[i] = card.id + i + 1
+	}
+
+	return wonCards
 }
 
 func main() {
@@ -60,6 +77,7 @@ func main() {
 	}
 
 	fmt.Printf("Part 1: %d\n", part1(cards))
+	fmt.Printf("Part 2: %d\n", part2(cards))
 }
 
 func part1(cards []Card) int {
@@ -71,28 +89,76 @@ func part1(cards []Card) int {
 	return score
 }
 
+func part2(cards []Card) int {
+	if len(cards) == 0 {
+		return 0
+	}
+
+	cardsByID := map[int]Card{}
+	for _, card := range cards {
+		cardsByID[card.id] = card
+	}
+
+	// This solution is a bit naive; I didn't get particularly clever with
+	// the number of cards we had and treated it like a tree problem (I could
+	// have just made this faster by doing
+	// visitedCardIds[wonCard] += visitedCardIds[card.id]
+	// which would have been equivalent, and faster, but meh, I like this
+	// solution even if it's slow)
+	visitedCardIDs := map[int]int{}
+	cardsInPlay := slices.Clone(cards)
+
+	for len(cardsInPlay) > 0 {
+		card := cardsInPlay[0]
+		cardsInPlay = cardsInPlay[1:]
+
+		visitedCardIDs[card.id]++
+
+		wonCardIDs := card.WinsCardsWithIDs()
+		for _, wonCardID := range wonCardIDs {
+			cardsInPlay = append(cardsInPlay, cardsByID[wonCardID])
+		}
+	}
+
+	totalCards := 0
+	for _, numVisited := range visitedCardIDs {
+		totalCards += numVisited
+	}
+
+	return totalCards
+}
+
 func parseCards(inputLines []string) ([]Card, error) {
 	return tryParse(inputLines, parseCard)
 }
 
 func parseCard(inputLine string) (Card, error) {
-	pattern := regexp.MustCompile(`^Card\s+\d+: ((?:\s*\d+\s*?)+) \| ((?:\s*\d+\s*)+)$`)
+	pattern := regexp.MustCompile(`^Card\s+(\d+): ((?:\s*\d+\s*?)+) \| ((?:\s*\d+\s*)+)$`)
 	matches := pattern.FindStringSubmatch(inputLine)
 	if matches == nil {
 		return Card{}, errors.New("did not match line pattern")
 	}
 
-	winningNumbers, err := parseCardNumbers(matches[1])
+	id, err := strconv.Atoi(matches[1])
+	if err != nil {
+		return Card{}, fmt.Errorf("parse id: %w", err)
+	}
+
+	winningNumbers, err := parseCardNumbers(matches[2])
 	if err != nil {
 		return Card{}, fmt.Errorf("parse winning numbers: %w", err)
 	}
 
-	ourNumbers, err := parseCardNumbers(matches[2])
+	ourNumbers, err := parseCardNumbers(matches[3])
 	if err != nil {
-		return Card{}, fmt.Errorf("parse winning numbers: %w", err)
+		return Card{}, fmt.Errorf("parse our numbers: %w", err)
 	}
 
-	return Card{winningNumbers: winningNumbers, ourNumbers: ourNumbers}, nil
+	return Card{
+		id:             id,
+		winningNumbers: winningNumbers,
+		ourNumbers:     ourNumbers,
+	}, nil
 }
 
 func normalizeSeparatingSpaces(s string) string {
@@ -131,3 +197,16 @@ func makeSet[T comparable, S ~[]T](items S) map[T]struct{} {
 
 	return set
 }
+
+func pow2(exp int) int {
+	if exp == 0 {
+		return 1
+	}
+
+	res := 1
+	for i := 1; i < exp; i++ {
+		res *= 2
+	}
+
+	return res
+}