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/input.go

@@ -40,3 +40,21 @@ func ParseInput(data []string) (map[int]bool, []*Rule) {
 
 	return plants, rules
 }
+
+// The function isn't fast, the resulting data is.
+func FastParseInput(data []string) ([]byte, map[string]byte) {
+	stateStrings := strings.Split(data[0], " ")
+	ruleStrings := data[2:]
+
+	// Populate plant list
+	plants := []byte(stateStrings[len(stateStrings)-1])
+
+	// Create rules
+	rules := make(map[string]byte)
+	for _, ruleStr := range ruleStrings {
+		ruleData := strings.Split(ruleStr, " ")
+		rules[ruleData[0]] = []byte(ruleData[2])[0]
+	}
+
+	return plants, rules
+}