package deeplink

import (
	"github.com/go-faster/errors"

	"go.mau.fi/mautrix-telegram/pkg/gotd/ascii"
)

// ValidateDomain validate given domain (user) name
func ValidateDomain(domain string) error {
	return checkDomainSymbols(domain)
}

// checkDomainSymbols check that domain contains only a-z, A-Z, 0-9 and '_'
// symbols.
func checkDomainSymbols(domain string) error {
	switch {
	case domain == "":
		return errors.New("is empty")
	case len(domain) > 32:
		return errors.New("is too big")
	case !ascii.IsLatinLower(rune(domain[0])):
		return errors.New("must start with lower letter")
	case domain[len(domain)-1] == '_':
		return errors.New("must not end with '_'")
	}

	for i, r := range domain {
		switch {
		case !ascii.IsLatinLetter(r) && !ascii.IsDigit(r) && r != '_':
		case i > 0 && domain[i] == '_' && domain[i] == domain[i-1]:
		default:
			continue
		}

		return errors.Errorf("unexpected %c at %d", r, i)
	}

	return nil
}
