adventofcode/2018/internal/day12/fastplants.go

// Unused solution: still not fast enough
package day12

func FastGenerationRunner() func([]byte, map[string]byte, int) ([]byte, int) {
	cache := make(map[string]string)

	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
		}

		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(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 {
	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
}