// Package volcsubtitle implements the native RTS subtitle callback documented at
// https://docs.volcengine.com/docs/6348/1337284 (verified 2026-09-06).
package volcsubtitle

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/binary"
	"encoding/json"
	"errors"
	"unicode/utf8"
)

var ErrInvalid = errors.New("invalid native subtitle packet")

type Envelope struct {
	Message   string `json:"message"`
	Signature string `json:"signature"`
}
type Segment struct {
	UserID    string `json:"userId"`
	RoundID   *int64 `json:"roundId"`
	Sequence  *int64 `json:"sequence"`
	Text      string `json:"text"`
	Definite  bool   `json:"definite"`
	Paragraph bool   `json:"paragraph"`
}
type Packet struct {
	Type       string    `json:"type"`
	Data       []Segment `json:"data"`
	DeviceName string    `json:"deviceName,omitempty"`
	ProductKey string    `json:"productKey,omitempty"`
}

// Bind authentication to the platform conversation. A delayed callback can
// never be reassigned by looking up the device's newest active conversation.
func Signature(secret []byte, conversationID string) string {
	mac := hmac.New(sha256.New, secret)
	mac.Write([]byte("volc-subtitle-v1\n" + conversationID))
	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
}

func Decode(message string) (Packet, error) {
	var p Packet
	if len(message) > 350000 {
		return p, ErrInvalid
	}
	data, err := base64.StdEncoding.DecodeString(message)
	if err != nil || len(data) == 0 {
		return p, ErrInvalid
	}
	if data[0] != '{' {
		if len(data) < 8 || string(data[:4]) != "subv" || uint64(binary.BigEndian.Uint32(data[4:8])) != uint64(len(data)-8) {
			return p, ErrInvalid
		}
		data = data[8:]
	}
	if !utf8.Valid(data) {
		return p, ErrInvalid
	}
	if json.Unmarshal(data, &p) != nil || p.Type != "subtitle" || len(p.Data) == 0 || len(p.Data) > 100 || len(p.DeviceName) > 128 || len(p.ProductKey) > 200 {
		return Packet{}, ErrInvalid
	}
	for _, s := range p.Data {
		if s.UserID == "" || len(s.UserID) > 200 || s.RoundID == nil || s.Sequence == nil ||
			*s.RoundID < 0 || *s.Sequence < 0 || len([]rune(s.Text)) > 20000 {
			return Packet{}, ErrInvalid
		}
	}
	return p, nil
}