aoc2024/day-19/internal/two/solve.go

package two

import (
	"bytes"
	"cmp"
	"slices"
	"strings"

	log "github.com/sirupsen/logrus"
)

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

	conc := len(designs)
	countChan := make(chan int)
	for _, design := range designs {
		go func() {
			count, ok := numPossibleArrangement(design, stripes, make(map[string]int))
			if ok {
				log.Debugf("design: %s is possible in %d ways", design, count)
				countChan <- count
				return
			}
			countChan <- 0
		}()
	}

	var totalCount int
	for range conc {
		count := <-countChan
		totalCount += count
	}

	return totalCount, nil
}

func numPossibleArrangement(design string, stripes []string, memo map[string]int) (int, bool) {
	if len(design) == 0 {
		return 1, true
	}

	greedy := []string{}
	for _, stripe := range stripes {
		if strings.HasPrefix(design, stripe) {
			greedy = append(greedy, stripe)
		}
	}

	slices.SortFunc(greedy, func(a, b string) int {
		return cmp.Compare(len(b), len(a))
	})

	var totalCount int
	for _, g := range greedy {
		if count, ok := memo[design[len(g):]]; ok {
			totalCount += count
			continue
		}

		count, ok := numPossibleArrangement(design[len(g):], stripes, memo)
		if ok {
			totalCount += count
		}
		memo[design[len(g):]] = count
	}
	return totalCount, true
}