package main
import (
	"bufio"
	"log"
	"os"
	"strings"
)
type insert map[string]byte
type polimer struct {
	template string
	inserts  insert
}
func main() {
	if err := myMain(); err != nil {
		log.Println(err)
	}
}
func myMain() error {
	p, err := parseInput("input.txt")
	if err != nil {
		return err
	}
	log.Println(solveFirst(p))
	return nil
}
func solveFirst(p polimer) int {
	tmp := strings.Builder{}
	for i := 0; i < 10; i++ {
		for ch := range p.template {
			if ch == len(p.template)-1 {
				tmp.WriteByte(p.template[ch])
				break
			}
			tmp.WriteByte(p.template[ch])
			tmp.WriteByte(p.inserts[p.template[ch:ch+2]])
		}
		p.template = tmp.String()
		tmp.Reset()
	}
	chars := map[rune]int{}
	for _, ch := range p.template {
		chars[ch]++
	}
	var min, max int
	for _, v := range chars {
		if min == 0 {
			min = v
		}
		if v > max {
			max = v
		}
		if v < min {
			min = v
		}
	}
	return max - min
}
func parseInput(fileName string) (polimer, error) {
	var ret = polimer{}
	ret.inserts = insert{}
	fd, err := os.Open(fileName)
	if err != nil {
		return ret, err
	}
	defer fd.Close()
	buf := bufio.NewScanner(fd)
	for buf.Scan() {
		if buf.Text() == "" {
			continue
		}
		tmp := strings.Split(buf.Text(), " -> ")
		if len(tmp) == 2 {
			ret.inserts[tmp[0]] = tmp[1][0]
		} else { // polymer template
			ret.template = tmp[0]
		}
	}
	return ret, nil
}

SNPVPFCPPKSBNSPSPSOF
CF -> N
NK -> B
SF -> B
HV -> P
FN -> S
VV -> F
FO -> F
VN -> V
PV -> P
FF -> P
ON -> S
PB -> S
PK -> P
OO -> P
SP -> F
VF -> H
OV -> C
BN -> P
OH -> H
NC -> F
BH -> N
CS -> C
BC -> N
OF -> N
SN -> B
FP -> F
FV -> K
HP -> H
VB -> P
FH -> F
HF -> P
BB -> O
HH -> S
PC -> O
PP -> B
VS -> B
HC -> H
NS -> N
KF -> S
BO -> V
NP -> S
NF -> K
BS -> O
KK -> O
VC -> V
KP -> K
CK -> P
HN -> F
KN -> H
KH -> N
SB -> S
NO -> K
HK -> H
BF -> V
SV -> B
CV -> P
CO -> P
FC -> O
CP -> H
CC -> N
CN -> P
SK -> V
SS -> V
VH -> B
OS -> N
FB -> H
NB -> N
SC -> K
NV -> H
HO -> S
SO -> P
PH -> C
VO -> O
OB -> O
FK -> S
PN -> P
VK -> O
NH -> N
OC -> B
BP -> O
PF -> F
KB -> K
KV -> B
PO -> N
NN -> K
CH -> O
KC -> P
OP -> V
VP -> F
OK -> P
FS -> K
CB -> S
HB -> N
KS -> O
BK -> C
BV -> O
SH -> H
PS -> N
HS -> K
KO -> N

NNCB
CH -> B
HH -> N
CB -> H
NH -> C
HB -> C
HC -> B
HN -> C
NN -> C
BH -> H
NC -> B
NB -> B
BN -> B
BB -> N
BC -> B
CC -> N
CN -> C

module main
go 1.17