First attempt at a faster / more efficient implementation. This is orders of magnitude faster than the first, but still not nearly fast enough.

2018/internal/day12/fastplants.go

@@ -0,0 +1,56 @@
+package day12
+
+func FastRunGeneration(plants []byte, rules map[string]byte, rootIndex int) ([]byte, int) {
+	newGen := make([]byte, len(plants), len(plants)+4)
+
+	for i := 0; i < len(plants); i++ {
+		newGen[i] = rules[getKey(plants, i)]
+	}
+
+	result1 := rules[getKey(plants, -2)]
+	result2 := rules[getKey(plants, -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)]
+	if result1 == '#' || result2 == '#' {
+		newGen = append(newGen, result1, result2)
+	}
+
+	return newGen, rootIndex
+}
+
+func getKey(plants []byte, i int) string {
+	if i < 2 {
+		key := ""
+		for j := 0; j < 2-i; j++ {
+			key += "."
+		}
+		key += string(plants[0 : i+2])
+		return key
+	}
+
+	if i > len(plants)-2 {
+		key := string(plants[i-2 : len(plants)-1])
+		for j := 0; j < len(plants)-1-i; j++ {
+			key += "."
+		}
+		key += string(plants[0 : i+2])
+		return key
+	}
+
+	return string(plants[i-2 : i+2])
+}
+
+func FastSumPlants(plants []byte, rootIndex int) int {
+	sum := 0
+	for index, value := range plants {
+		if value == '#' {
+			sum += index - rootIndex
+		}
+	}
+	return sum
+}