package httpapi

import (
	"context"
	"encoding/json"
	"errors"
	"net/http"
	"time"

	"commilitia.net/cdrop/internal/hub"
	"commilitia.net/cdrop/internal/jwtauth"
	"commilitia.net/cdrop/internal/push"
)

// 4 KB caps DoS-via-paste; longer payloads should use file transfer instead.
const maxMessageBytes = 4 * 1024

// maxMessageBodyBytes bounds the raw request body BEFORE decode (R5): the 4 KB
// limit above checks the decoded Go string, so on its own the whole body is
// still read into memory first. The wire form can be larger than the decoded
// text — JSON escaping inflates quotes/control chars (worst case ~6×) — so the
// body cap is the content cap with headroom, not equal to it, to avoid falsely
// rejecting a legitimate 4 KB message while still bounding memory per request.
const maxMessageBodyBytes = maxMessageBytes * 8

type messageReq struct {
	To   string `json:"to"`
	Text string `json:"text"`
}

// handleMessage forwards a one-shot text payload to a peer device. Reuses the
// SSE Hub the same way /api/hub/signal does — no DB row, no transfer state
// machine. Receiver sees a `message` event; the frontend keeps it in memory
// only until the tab closes (per user spec).
//
// 204 if delivered live, 202 if the peer is offline but a Web Push was sent
// (the notification carries the text — the message itself is not persisted),
// 410 if offline with no push subscription, 413 if oversized.
func (s *Server) handleMessage(w http.ResponseWriter, r *http.Request) {
	claims, _ := jwtauth.ClaimsFromContext(r.Context())
	from, _ := jwtauth.DeviceNameFromContext(r.Context())

	r.Body = http.MaxBytesReader(w, r.Body, maxMessageBodyBytes)
	var req messageReq
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		var mbe *http.MaxBytesError
		if errors.As(err, &mbe) {
			writeJSON(w, http.StatusRequestEntityTooLarge,
				map[string]string{"error": "message exceeds 4 KB"})
			return
		}
		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "invalid json"})
		return
	}
	if req.To == "" {
		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "missing 'to'"})
		return
	}
	if req.Text == "" {
		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "empty text"})
		return
	}
	if len(req.Text) > maxMessageBytes {
		writeJSON(w, http.StatusRequestEntityTooLarge,
			map[string]string{"error": "message exceeds 4 KB"})
		return
	}
	if req.To == from {
		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "cannot send to self"})
		return
	}

	delivered := s.hub.SendTo(claims.UserID, req.To, hub.Event{
		Type: "message",
		Data: map[string]any{
			"from":    from,
			"text":    req.Text,
			"sent_at": time.Now().Unix(),
		},
	})
	if !delivered {
		// Peer's page is closed (no live SSE). The message has no DB row, so the
		// only delivery left is a Web Push carrying the text itself. If the peer
		// has a subscription, send it and report 202; otherwise it's truly gone.
		if s.push.Enabled() {
			go s.push.Notify(context.Background(), claims.UserID, req.To, push.Notification{
				Type:   push.KindMessage,
				Params: map[string]string{"sender": from, "text": req.Text},
			})
			w.WriteHeader(http.StatusAccepted)
			return
		}
		writeJSON(w, http.StatusGone, map[string]string{"error": "peer offline"})
		return
	}
	w.WriteHeader(http.StatusNoContent)
}
