package controlplane

import (
	"encoding/base64"
	"encoding/json"
	"errors"
	"strings"
	"time"
)

const pageCursorVersion = 1

type pageCursorPayload struct {
	Version   int    `json:"v"`
	Kind      string `json:"kind"`
	ChatID    string `json:"chat_id,omitempty"`
	SortTime  string `json:"sort_time"`
	MessageID string `json:"message_id,omitempty"`
}

type MessagePageCursor struct {
	ChatID     string
	SourceTime string
	MessageID  string
}

type ConversationPageCursor struct {
	SortTime string
	ChatID   string
}

func EncodeMessagePageCursor(chatID string, sourceTime time.Time, messageID string) string {
	return encodePageCursor(pageCursorPayload{
		Version: pageCursorVersion, Kind: "messages", ChatID: chatID,
		SortTime: sourceTime.UTC().Format(time.RFC3339Nano), MessageID: messageID,
	})
}

func DecodeMessagePageCursor(raw, chatID string) (*MessagePageCursor, error) {
	payload, err := decodePageCursor(raw, "messages")
	if err != nil || payload.ChatID != chatID || payload.MessageID == "" {
		return nil, errors.New("invalid message cursor")
	}
	if _, err := time.Parse(time.RFC3339Nano, payload.SortTime); err != nil {
		return nil, errors.New("invalid message cursor")
	}
	return &MessagePageCursor{ChatID: payload.ChatID, SourceTime: payload.SortTime, MessageID: payload.MessageID}, nil
}

func EncodeConversationPageCursor(sortTime time.Time, chatID string) string {
	return encodePageCursor(pageCursorPayload{
		Version: pageCursorVersion, Kind: "conversations", ChatID: chatID,
		SortTime: sortTime.UTC().Format(time.RFC3339Nano),
	})
}

func DecodeConversationPageCursor(raw string) (*ConversationPageCursor, error) {
	payload, err := decodePageCursor(raw, "conversations")
	if err != nil || payload.ChatID == "" {
		return nil, errors.New("invalid conversation cursor")
	}
	if _, err := time.Parse(time.RFC3339Nano, payload.SortTime); err != nil {
		return nil, errors.New("invalid conversation cursor")
	}
	return &ConversationPageCursor{SortTime: payload.SortTime, ChatID: payload.ChatID}, nil
}

func encodePageCursor(payload pageCursorPayload) string {
	encoded, _ := json.Marshal(payload)
	return base64.RawURLEncoding.EncodeToString(encoded)
}

func decodePageCursor(raw, kind string) (pageCursorPayload, error) {
	if strings.TrimSpace(raw) == "" {
		return pageCursorPayload{}, errors.New("cursor is empty")
	}
	decoded, err := base64.RawURLEncoding.DecodeString(raw)
	if err != nil {
		return pageCursorPayload{}, errors.New("cursor is not base64url")
	}
	var payload pageCursorPayload
	if err := json.Unmarshal(decoded, &payload); err != nil || payload.Version != pageCursorVersion || payload.Kind != kind || payload.SortTime == "" {
		return pageCursorPayload{}, errors.New("cursor payload is invalid")
	}
	return payload, nil
}