Solution for 2024, day 11

Author: lucianoq

2024/11/Makefile

@@ -0,0 +1,19 @@
+input:
+	http "https://adventofcode.com/2024/day/11/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go utils.go
+
+main2:
+	go build -o main2 main2.go common.go utils.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input

2024/11/common.go

@@ -0,0 +1,49 @@
+package main
+
+import "fmt"
+
+func main() {
+	sum := 0
+	for _, num := range readInput() {
+		sum += numChildren(num, Blinks)
+	}
+	fmt.Println(sum)
+}
+
+func blink(n int) []int {
+	if n == 0 {
+		return []int{1}
+	}
+
+	digits := numDigits(n)
+	if digits&1 == 0 {
+		pow := intPow(10, digits/2)
+		return []int{n / pow, n % pow}
+	}
+
+	return []int{n * 2024}
+}
+
+// table for memoization
+var cache = map[args]int{}
+
+func numChildren(n, blinks int) int {
+
+	if res, ok := cache[args{n, blinks}]; ok {
+		return res
+	}
+
+	if blinks == 0 {
+		return 1
+	}
+
+	sum := 0
+	for _, child := range blink(n) {
+		sum += numChildren(child, blinks-1)
+	}
+
+	cache[args{n, blinks}] = sum
+	return sum
+}
+
+type args struct{ a, b int }

2024/11/main1.go

@@ -0,0 +1,3 @@
+package main
+
+const Blinks = 25

2024/11/main2.go

@@ -0,0 +1,3 @@
+package main
+
+const Blinks = 75

2024/11/utils.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"bufio"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func numDigits(n int) int {
+	if n == 0 {
+		return 1
+	}
+
+	count := 0
+	for n != 0 {
+		n /= 10
+		count++
+	}
+	return count
+}
+
+func intPow(n, m int) int {
+	if m == 0 {
+		return 1
+	}
+
+	if m == 1 {
+		return n
+	}
+
+	result := n
+	for i := 2; i <= m; i++ {
+		result *= n
+	}
+	return result
+}
+
+func readInput() []int {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Scan()
+	nums := []int{}
+	for _, num := range strings.Fields(scanner.Text()) {
+		n, _ := strconv.Atoi(num)
+		nums = append(nums, n)
+	}
+	return nums
+}