Day 22

day22/go.mod

@@ -0,0 +1,3 @@
+module stevenlr.com/aoc2024/day22
+
+go 1.23.3

day22/main.go

@@ -0,0 +1,80 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"slices"
+	"strconv"
+)
+
+func main() {
+	fmt.Println(part1([]int64{1, 10, 100, 2024}), 37327623)
+	fmt.Println(part1(readData("data.txt")), 13004408787)
+	fmt.Println(part2([]int64{1, 2, 3, 2024}), 23)
+	fmt.Println(part2(readData("data.txt")))
+}
+
+func part1(init []int64) (result int64) {
+	for _, n := range init {
+		for range 2000 {
+			n = next(n)
+		}
+		result += n
+	}
+	return
+}
+
+func makeChangeId(memory [4]int, i int) (result int) {
+	for j := range 4 {
+		result = result*19 + memory[(i+j)%4] + 9
+	}
+	return
+}
+
+func part2(init []int64) int {
+	priceForChangeId := make([]int, 19*19*19*19)
+	for _, n := range init {
+		seenChange := make([]bool, 19*19*19*19)
+		prevPrice := n % 10
+		memory := [4]int{0, 0, 0, 0}
+		for i := range 2000 {
+			n2 := next(n)
+			price := n2 % 10
+			change := price - prevPrice
+			memory[i%4] = int(change)
+			if i >= 3 {
+				changeId := makeChangeId(memory, i)
+				if !seenChange[changeId] {
+					seenChange[changeId] = true
+					priceForChangeId[changeId] += int(price)
+				}
+			}
+			n, prevPrice = n2, price
+		}
+	}
+	return slices.Max(priceForChangeId)
+}
+
+func prune(a int64) int64 {
+	return a % 16777216
+}
+
+func next(a int64) int64 {
+	a = prune((a << 6) ^ a)
+	a = prune((a >> 5) ^ a)
+	a = prune((a << 11) ^ a)
+	return a
+}
+
+func readData(fileName string) (numbers []int64) {
+	fp, _ := os.Open(fileName)
+	scanner := bufio.NewScanner(fp)
+
+	for scanner.Scan() {
+		n, _ := strconv.ParseInt(scanner.Text(), 10, 64)
+		numbers = append(numbers, n)
+	}
+
+	return
+}

go.work

@@ -14,6 +14,7 @@ use (
 	./day19
 	./day20
 	./day21
+	./day22
 	./day2
 	./day3
 	./day4