package cnpj

import (
	"fmt"
	"math/rand"
	"strings"
)

var (
	cnpjP1 = []int{5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2}
	cnpjP2 = []int{6, 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2}
)

const alphaChars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"

func Generate() string {
	return generate(false)
}

func GenerateAlpha() string {
	return generate(true)
}

func generate(alpha bool) string {
	for {
		var b strings.Builder
		b.Grow(14)
		for range 12 {
			if alpha {
				b.WriteByte(alphaChars[rand.Intn(len(alphaChars))])
			} else {
				b.WriteByte(byte('0' + rand.Intn(10)))
			}
		}
		s := b.String()
		if isRepetitive(s) {
			continue
		}
		dv1 := computeDV(s, cnpjP1, alpha)
		s2 := s + string(byte('0'+dv1))
		dv2 := computeDV(s2, cnpjP2, alpha)
		return s + string(byte('0'+dv1)) + string(byte('0'+dv2))
	}
}

func computeDV(s string, table []int, alpha bool) int {
	total := 0
	for i, w := range table {
		c := s[i]
		var val int
		if alpha {
			val = alphaValue(rune(c))
		} else {
			val = int(c - '0')
		}
		total += val * w
	}
	r := total % 11
	if r >= 2 {
		return 11 - r
	}
	return 0
}

func alphaValue(r rune) int {
	return int(r) - 48
}

func isRepetitive(s string) bool {
	if len(s) == 0 {
		return false
	}
	c := s[0]
	for i := 1; i < len(s); i++ {
		if s[i] != c {
			return false
		}
	}
	return true
}

func Format(cnpj string) string {
	if len(cnpj) != 14 {
		return cnpj
	}
	return fmt.Sprintf("%s.%s.%s/%s-%s",
		cnpj[:2], cnpj[2:5], cnpj[5:8], cnpj[8:12], cnpj[12:])
}