package phone

import (
	"errors"
	"regexp"
	"strings"
)

var nonDigit = regexp.MustCompile(`[^\d]`)
var e164Pattern = regexp.MustCompile(`^\+63\d{10}$`)

// ToLocal converts a phone number to Philippine local format (09XXXXXXXXX).
func ToLocal(e164 string) (string, error) {
	if e164 == "" {
		return "", errors.New("invalid Philippine phone number: empty input")
	}
	digits := nonDigit.ReplaceAllString(e164, "")
	if digits == "" {
		return "", errors.New("invalid Philippine phone number: no digits found")
	}

	var local string
	switch {
	case strings.HasPrefix(digits, "63") && len(digits) == 12:
		local = "0" + digits[2:]
	case strings.HasPrefix(digits, "9") && len(digits) == 10:
		local = "0" + digits
	case strings.HasPrefix(digits, "0") && len(digits) == 11:
		local = digits
	default:
		return "", errors.New("invalid Philippine phone number")
	}

	if len(local) != 11 || !strings.HasPrefix(local, "09") {
		return "", errors.New("invalid Philippine phone number")
	}
	return local, nil
}

// ToE164 converts a phone number to E.164 format (+63XXXXXXXXXX).
func ToE164(local string) (string, error) {
	if local == "" {
		return "", errors.New("invalid Philippine phone number: empty input")
	}

	// preserve leading + before stripping
	hasPlus := strings.HasPrefix(local, "+")
	digits := nonDigit.ReplaceAllString(local, "")
	if digits == "" {
		return "", errors.New("invalid Philippine phone number: no digits found")
	}

	var e164 string
	switch {
	case hasPlus && strings.HasPrefix(digits, "63") && len(digits) == 12:
		e164 = "+" + digits
	case !hasPlus && strings.HasPrefix(digits, "63") && len(digits) == 12:
		e164 = "+" + digits
	case strings.HasPrefix(digits, "0") && len(digits) == 11:
		e164 = "+63" + digits[1:]
	case strings.HasPrefix(digits, "9") && len(digits) == 10:
		e164 = "+63" + digits
	default:
		return "", errors.New("invalid Philippine phone number")
	}

	if !e164Pattern.MatchString(e164) {
		return "", errors.New("invalid Philippine phone number")
	}
	return e164, nil
}