Day 11

day11/go.mod

@@ -0,0 +1,3 @@
+module stevenlr.com/aoc2024/day11
+
+go 1.23.3

day11/main.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+)
+
+func main() {
+	fmt.Println(expand([]int64{125, 17}, 25), 55312)
+	fmt.Println(expand([]int64{6, 11, 33023, 4134, 564, 0, 8922422, 688775}, 25), 220999)
+	fmt.Println(expand([]int64{6, 11, 33023, 4134, 564, 0, 8922422, 688775}, 75))
+}
+
+func evenLength(v int64) bool {
+	return int(math.Ceil(math.Log10(float64(v)+0.5)))%2 == 0
+}
+
+func splitEven(v int64) (a, b int64) {
+	s := strconv.FormatInt(v, 10)
+	sa, sb := s[:len(s)/2], s[len(s)/2:]
+	a, err := strconv.ParseInt(sa, 10, 64)
+	if err != nil {
+		panic(err)
+	}
+	b, err = strconv.ParseInt(sb, 10, 64)
+	if err != nil {
+		panic(err)
+	}
+	return
+}
+
+type Entry struct {
+	Value int64
+	Depth int
+}
+
+func expand(initial []int64, depth int) (result int) {
+	memo := make(map[Entry]int)
+	for _, n := range initial {
+		result += expandOne(memo, n, depth)
+	}
+	return
+}
+
+func expandOne(memo map[Entry]int, v int64, depth int) int {
+	if depth == 0 {
+		return 1
+	}
+
+	entry := Entry{v, depth}
+	n, ok := memo[entry]
+	if ok {
+		return n
+	}
+
+	if v == 0 {
+		n = expandOne(memo, 1, depth-1)
+	} else if evenLength(v) {
+		a, b := splitEven(v)
+		n = expandOne(memo, a, depth-1) + expandOne(memo, b, depth-1)
+	} else {
+		n = expandOne(memo, v*2024, depth-1)
+	}
+
+	memo[entry] = n
+	return n
+}

go.work

@@ -3,6 +3,7 @@ go 1.23.3
 use (
 	./day1
 	./day10
+	./day11
 	./day2
 	./day3
 	./day4