package domain

import (
	"fmt"
	"math"
	"strings"
)

// officialUpdatesChannelMention is the public @username of the updates channel
// linked from the welcome message. It is carried both in the template text and
// in a MessageEntityMention so clients render it as a tappable link.
const officialUpdatesChannelMention = "@ziodotsh"

const officialWelcomeMessageTemplate = "👋 Welcome to OwpenGram!\n\nYou just signed in via %s.\n\nIf this wasn't you, revoke this session from \"Settings > Privacy and Security > Active sessions\" immediately.\n\nIf you haven't already, feel free to join " + officialUpdatesChannelMention + " for all the latest updates!"

// OfficialWelcomeMessage builds the account-visible incoming message sent
// from the official system account on every completed sign-in (SignUp and
// every subsequent SignIn/SignInWithEmail), regardless of delivery channel.
// Unlike OfficialLoginCodeMessage this never embeds a secret, so it is safe
// to send unconditionally — it exists to give the account owner (and, on a
// self-hosted single-admin server, that's usually also "the admin") a
// visible record of every session start.
func OfficialWelcomeMessage(userID int64, method string, date int) (Message, error) {
	method = strings.TrimSpace(method)
	if userID <= 0 || IsSystemUserID(userID) || method == "" || date < 0 || date > math.MaxInt32 {
		return Message{}, fmt.Errorf("%w: user=%d method=%q date=%d", ErrLoginCodeDeliveryInvalid, userID, method, date)
	}
	body := fmt.Sprintf(officialWelcomeMessageTemplate, method)
	msg := Message{
		OwnerUserID: userID,
		Peer:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
		From:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
		Date:        date,
		Body:        body,
	}
	if ent, ok := usernameMentionEntity(body, officialUpdatesChannelMention); ok {
		msg.Entities = []MessageEntity{ent}
	}
	return msg, nil
}

// usernameMentionEntity locates mention ("@name") in text and returns a
// MessageEntityMention spanning it, with the UTF-16 offset/length clients expect.
func usernameMentionEntity(text, mention string) (MessageEntity, bool) {
	before, _, found := strings.Cut(text, mention)
	if !found {
		return MessageEntity{}, false
	}
	return MessageEntity{
		Type:   MessageEntityMention,
		Offset: utf16Len(before),
		Length: utf16Len(mention),
	}, true
}

func utf16Len(s string) int {
	n := 0
	for _, r := range s {
		n++
		if r > 0xffff {
			n++
		}
	}
	return n
}

// SignInMethodLabel returns the human-readable method name embedded in
// OfficialWelcomeMessage. Email-signup accounts are identified by
// SignupEmail (see NewEmailSignupDisplayPhone) rather than by their stored
// Phone, which — once assigned — is an ordinary-looking short number that
// carries no information about the signup method.
func SignInMethodLabel(u User) string {
	if u.SignupEmail != "" {
		return "email"
	}
	return "phone number"
}