This is even better, but still not fast enough.

2018/day12-2.go

@@ -12,13 +12,14 @@ const NumGenerations = 50000000000
 func main() {
 	data := util.ReadInput()
 	plants, rules := day12.FastParseInput(data)
-	rootIndex := 0
 
+	runner := day12.FastGenerationRunner()
+	rootIndex := 0
 	for i := 0; i < NumGenerations; i++ {
-		plants, rootIndex = day12.FastRunGeneration(plants, rules, rootIndex)
+		plants, rootIndex = runner(plants, rules, rootIndex)
 
 		// debug
-		if i%1000000 == 0 {
+		if i%1000000000 == 0 {
 			fmt.Println("DEBUG: Generation: ", i)
 		}
 	}

2018/internal/day12/fastplants.go

@@ -1,26 +1,39 @@
+// Unused solution: still not fast enough
 package day12
 
-func FastRunGeneration(plants []byte, rules map[string]byte, rootIndex int) ([]byte, int) {
-	newGen := make([]byte, len(plants), len(plants)+4)
+func FastGenerationRunner() func([]byte, map[string]byte, int) ([]byte, int) {
+	cache := make(map[string]string)
 
-	for i := 0; i < len(plants); i++ {
-		newGen[i] = rules[getKey(plants, i)]
+	return func(current []byte, rules map[string]byte, rootIndex int) ([]byte, int) {
+		// check the cache
+		if cache[string(current)] != "" {
+			return []byte(cache[string(current)]), rootIndex
 		}
 
-	result1 := rules[getKey(plants, -2)]
-	result2 := rules[getKey(plants, -1)]
+		newGen := make([]byte, len(current), len(current)+4)
+
+		for i := 0; i < len(current); i++ {
+			newGen[i] = rules[getKey(current, i)]
+		}
+
+		result1 := rules[getKey(current, -2)]
+		result2 := rules[getKey(current, -1)]
 		if result1 == '#' || result2 == '#' {
 			newGen = append([]byte{result1, result2}, newGen...)
 			rootIndex += 2
 		}
 
-	result1 = rules[getKey(plants, len(plants))]
-	result2 = rules[getKey(plants, len(plants)+1)]
+		result1 = rules[getKey(current, len(current))]
+		result2 = rules[getKey(current, len(current)+1)]
 		if result1 == '#' || result2 == '#' {
 			newGen = append(newGen, result1, result2)
 		}
 
+		// cache result
+		cache[string(current)] = string(newGen)
+
 		return newGen, rootIndex
+	}
 }
 
 func getKey(plants []byte, i int) string {

2018/internal/day12/plants.go

@@ -1,3 +1,4 @@
+// This works fine for 12.1, but it's way too slow for 12.2
 package day12
 
 type Rule struct {