Solution for 2024, day 22

Author: lucianoq

2024/22/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2024/day/22/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2024/22/common.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bufio"
+	"os"
+	"strconv"
+)
+
+func parseInput() []int {
+	var list []int
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		line := scanner.Text()
+		num, _ := strconv.Atoi(line)
+		list = append(list, num)
+	}
+	return list
+}
+
+func compute(secret int) int {
+	secret = (secret ^ (secret << 6)) % 16777216
+	secret = (secret ^ (secret >> 5)) % 16777216
+	secret = (secret ^ (secret << 11)) % 16777216
+	return secret
+}

2024/22/main1.go

@@ -0,0 +1,17 @@
+package main
+
+import "fmt"
+
+func main() {
+	secrets := parseInput()
+
+	sum := 0
+	for _, secret := range secrets {
+		for i := 0; i < 2000; i++ {
+			secret = compute(secret)
+		}
+		sum += secret
+	}
+
+	fmt.Println(sum)
+}

2024/22/main2.go

@@ -0,0 +1,66 @@
+package main
+
+import "fmt"
+
+type Changes [4]int
+
+func main() {
+	secrets := parseInput()
+	priceAfter := map[int]map[Changes]int{}
+	discovered := map[Changes]map[int]struct{}{}
+
+	for buyerID, secret := range secrets {
+
+		// Initialise map
+		if _, ok := priceAfter[buyerID]; !ok {
+			priceAfter[buyerID] = map[Changes]int{}
+		}
+
+		// ring data structure, to reduce allocations
+		ring, i := Changes{}, 0
+
+		oldPrice := secret % 10
+		for t := 1; t <= 2000; t++ {
+
+			// generate a new secret
+			secret = compute(secret)
+
+			newPrice := secret % 10
+			delta := newPrice - oldPrice
+			oldPrice = newPrice
+
+			// add the delta to the ring
+			ring[i], i = delta, (i+1)%4
+
+			// skip the first 3 elements, as there are
+			// not enough deltas for a full change
+			if t >= 4 {
+				change := Changes{ring[i], ring[(i+1)%4], ring[(i+2)%4], ring[(i+3)%4]}
+
+				// Initialise map
+				if _, ok := discovered[change]; !ok {
+					discovered[change] = map[int]struct{}{}
+				}
+				discovered[change][buyerID] = struct{}{}
+
+				// insert only the first occurrence, ignore the next ones
+				if _, ok := priceAfter[buyerID][change]; !ok {
+					priceAfter[buyerID][change] = newPrice
+				}
+			}
+		}
+	}
+
+	maxBananas := 0
+	for change, affectedIDs := range discovered {
+		bananas := 0
+		for buyerID := range affectedIDs {
+			bananas += priceAfter[buyerID][change]
+		}
+		if bananas > maxBananas {
+			maxBananas = bananas
+		}
+	}
+
+	fmt.Println(maxBananas)
+}