aoc2024/day-11/internal/two/solve.go

package two

import (
	"bytes"

	log "github.com/sirupsen/logrus"
)

const blinks = 75
const magic = 2024

func Solve(buf []byte) (int, error) {
	r := bytes.NewReader(buf)
	numbers, err := parseLines(r)
	if err != nil {
		return 0, err
	}

	old := make(map[int]int)
	for _, n := range numbers {
		old[n]++
	}
	log.Debug(old)

	memo := make(map[int]result)
	for range blinks {
		next := make(map[int]int)

		for stone, count := range old {
			var r result
			r, ok := memo[stone]
			if !ok {
				r = applyRules(stone)
				memo[stone] = r
			}

			switch r.kind {
			case flip, multiply:
				next[r.right] += count
			case split:
				next[r.left] += count
				next[r.right] += count
			}
		}

		old = next
		log.Debug(old)
	}

	var totalCount int
	for _, count := range old {
		totalCount += count
	}

	return totalCount, nil
}

func applyRules(num int) result {
	switch num := num; {
	case num == 0:
		return result{flip, 0, 1}
	case lenItoa(num)%2 == 0:
		left, right := splitNumber(num)
		return result{split, left, right}
	default:
		return result{multiply, 0, num * magic}
	}
}