package main
import (
	"bufio"
	"log"
	"os"
	"strings"
)
type routes map[string][]string
type seen map[string]int
func main() {
	if err := myMain(); err != nil {
		log.Println(err)
	}
}
func myMain() error {
	r, err := parseInput("input.txt")
	if err != nil {
		return err
	}
	log.Println(solveFirst(r))
	return nil
}
func solveFirst(r routes) int {
	s := seen{}
	return r.genRoutes(s, "start")
}
func (r routes) genRoutes(s seen, root string) int {
	var sum int
	if root == "end" {
		sum++
		return sum
	}
	if s[root] > 0 {
		return 0
	}
	if strings.ToLower(root) == root && root != "end" {
		// lowercase, only visit once
		s[root]++
	}
	for _, item := range r[root] {
		sum += r.genRoutes(s, item)
	}
	delete(s, root)
	return sum
}
func parseInput(fileName string) (routes, error) {
	var ret = routes{}
	fd, err := os.Open(fileName)
	if err != nil {
		return ret, err
	}
	defer fd.Close()
	buf := bufio.NewScanner(fd)
	for buf.Scan() {
		tmp := strings.Split(buf.Text(), "-")
		if tmp[1] != "start" && tmp[0] != "end" {
			ret[tmp[0]] = append(ret[tmp[0]], tmp[1])
		}
		if tmp[0] != "start" && tmp[1] != "end" {
			ret[tmp[1]] = append(ret[tmp[1]], tmp[0])
		}
	}
	return ret, nil
}

pf-pk
ZQ-iz
iz-NY
ZQ-end
pf-gx
pk-ZQ
ZQ-dc
NY-start
NY-pf
NY-gx
ag-ZQ
pf-start
start-gx
BN-ag
iz-pf
ag-FD
pk-NY
gx-pk
end-BN
ag-pf
iz-pk
pk-ag
iz-end
iz-BN

start-A
start-b
A-c
A-b
b-d
A-end
b-end

module main
go 1.17