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/plants.go

@@ -53,6 +53,7 @@ func RunGeneration(plants map[int]bool, rules []*Rule) map[int]bool {
 		for _, rule := range rules {
 			if checkRule(plants, rule, i) {
 				newGen[i] = rule.Result
+				break // only one rule will match a given slice
 			}
 		}
 	}