package webrtc

import (
	"fmt"
	"io"
	"net/http"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"github.com/labstack/echo/v4"
	"github.com/pion/webrtc/v4"

	corewebrtc "github.com/datarhei/core/v16/core/webrtc"
)

// Default per-stream peer cap when the caller passes 0. The total cap
// (passed to NewHandler) is enforced separately and takes precedence.
const defaultMaxPeersPerStream = 8

// Handler exposes the subsystem's WHEP Echo handlers. Wire them into
// the /api/v3 group (or a sibling group) via Handler.Register.
//
// Lifecycle: peers are tracked in a streamID→resourceID→Peer index.
// On every Subscribe we spin a tiny goroutine watching the new peer's
// Done() channel; when ICE fails or Close() runs the index entry is
// removed and the counters tick back down — no leaks if the browser
// rage-quits.
type Handler struct {
	sub *Subsystem

	mu            sync.Mutex
	peersByStream map[string]map[string]*corewebrtc.Peer // streamID -> resource -> peer
	peerStream    map[string]string                      // resource -> streamID (reverse index)
	count         int64                                  // atomic
	maxCapTotal   int64
	maxCapPerStrm int64

	met *webrtcMetrics
}

// NewHandler wraps the subsystem in an Echo-compatible HTTP handler.
// The maxPeers argument caps concurrent subscribers across all streams;
// pass 0 to use a generous default (matches corewebrtc.DefaultConfig).
// The per-stream cap is taken from the corewebrtc default; pass a
// non-zero value to override via NewHandlerWithCaps.
func NewHandler(s *Subsystem, maxPeers int) *Handler {
	return NewHandlerWithCaps(s, maxPeers, 0)
}

// NewHandlerWithCaps is NewHandler plus an explicit per-stream cap.
// maxPeersPerStream <= 0 falls back to defaultMaxPeersPerStream.
func NewHandlerWithCaps(s *Subsystem, maxPeers, maxPeersPerStream int) *Handler {
	total := int64(maxPeers)
	if total <= 0 {
		total = int64(corewebrtc.DefaultConfig().MaxPeersTotal)
	}
	perStream := int64(maxPeersPerStream)
	if perStream <= 0 {
		perStream = defaultMaxPeersPerStream
	}
	h := &Handler{
		sub:           s,
		peersByStream: make(map[string]map[string]*corewebrtc.Peer),
		peerStream:    make(map[string]string),
		maxCapTotal:   total,
		maxCapPerStrm: perStream,
	}
	// Subsystem broadcasts process-stop via this hook so the handler
	// can yank stale peer entries before their Sources close out
	// from underneath them.
	if s != nil {
		s.SetTeardownHook(h.tearDownStreamPeers)
	}
	return h
}

// Register mounts the WHEP routes on the provided Echo group.
//
// CORS preflights are answered on every WHEP path; regular WHEP
// responses also carry the Access-Control-* headers so browser-side
// players living on a different origin can subscribe.
func (h *Handler) Register(g *echo.Group) {
	g.OPTIONS("/whep/:id", h.preflight)
	g.OPTIONS("/whep/:id/:resource", h.preflight)
	g.POST("/whep/:id", h.Subscribe)
	g.DELETE("/whep/:id/:resource", h.Unsubscribe)
	g.PATCH("/whep/:id/:resource", h.Trickle)
}

// Subscribe handles POST /whep/:id. Request body is an SDP offer,
// response is an SDP answer with a Location header pointing at the
// DELETE/PATCH resource.
//
// @Summary Subscribe to a WebRTC stream via WHEP
// @Description Subscribe to a process's WebRTC egress stream. Body is the SDP offer (Content-Type: application/sdp). Response is the SDP answer; the Location header points at the DELETE/PATCH resource for teardown and trickle ICE.
// @Tags v16.16.0
// @ID webrtc-3-whep-subscribe
// @Accept application/sdp
// @Produce application/sdp
// @Param id path string true "Process ID with config.webrtc.enabled=true"
// @Success 201 {string} string "SDP answer"
// @Failure 400 {string} string "missing stream id, malformed body, or invalid SDP"
// @Failure 404 {string} string "no stream registered for this process id"
// @Failure 406 {string} string "offer SDP missing required H264 / Opus rtpmap"
// @Failure 503 {string} string "peer cap reached (per-stream or total)"
// @Failure 504 {string} string "ICE gathering timeout"
// @Security ApiKeyAuth
// @Router /api/v3/whep/{id} [post]
func (h *Handler) Subscribe(c echo.Context) error {
	addCORS(c)
	t0 := time.Now()

	id := c.Param("id")
	if id == "" {
		h.recordRequest("subscribe", "", http.StatusBadRequest, t0)
		return c.String(http.StatusBadRequest, "missing stream id")
	}

	// Total cap: cheap atomic check before doing real work.
	if atomic.LoadInt64(&h.count) >= h.maxCapTotal {
		if h.met != nil {
			h.met.capRejections.WithLabelValues("", "global").Inc()
		}
		h.recordRequest("subscribe", id, http.StatusServiceUnavailable, t0)
		return c.String(http.StatusServiceUnavailable, corewebrtc.ErrPeerCapReached.Error())
	}

	stream, ok := h.sub.lookup(id)
	if !ok {
		h.recordRequest("subscribe", id, http.StatusNotFound, t0)
		return c.String(http.StatusNotFound, corewebrtc.ErrStreamNotFound.Error())
	}

	// Per-stream cap: needs the lock since we're indexing per stream.
	h.mu.Lock()
	if int64(len(h.peersByStream[id])) >= h.maxCapPerStrm {
		h.mu.Unlock()
		if h.met != nil {
			h.met.capRejections.WithLabelValues(id, "stream").Inc()
		}
		h.recordRequest("subscribe", id, http.StatusServiceUnavailable, t0)
		return c.String(http.StatusServiceUnavailable, "webrtc: per-stream peer cap reached")
	}
	h.mu.Unlock()

	body, err := io.ReadAll(c.Request().Body)
	if err != nil {
		h.recordRequest("subscribe", id, http.StatusBadRequest, t0)
		return c.String(http.StatusBadRequest, "read body: "+err.Error())
	}
	if len(body) == 0 || !strings.HasPrefix(string(body), "v=") {
		h.recordRequest("subscribe", id, http.StatusBadRequest, t0)
		return c.String(http.StatusBadRequest, corewebrtc.ErrInvalidSDP.Error())
	}
	if err := requireH264AndOpus(string(body)); err != nil {
		if h.met != nil {
			if cme, ok2 := err.(*codecMismatchError); ok2 {
				for _, kind := range cme.missing {
					h.met.codecMismatches.WithLabelValues(id, strings.ToLower(kind)).Inc()
				}
			}
		}
		h.recordRequest("subscribe", id, http.StatusNotAcceptable, t0)
		return c.String(http.StatusNotAcceptable, err.Error())
	}

	offer := webrtc.SessionDescription{Type: webrtc.SDPTypeOffer, SDP: string(body)}
	peer, err := h.sub.factory.CreatePeerFromSources(c.Request().Context(), stream.video, stream.audio, offer)
	if err != nil {
		// Surface the design's error matrix.
		switch err {
		case corewebrtc.ErrICETimeout:
			h.recordRequest("subscribe", id, http.StatusGatewayTimeout, t0)
			return c.String(http.StatusGatewayTimeout, err.Error())
		case corewebrtc.ErrCodecMismatch:
			h.recordRequest("subscribe", id, http.StatusNotAcceptable, t0)
			return c.String(http.StatusNotAcceptable, err.Error())
		default:
			h.recordRequest("subscribe", id, http.StatusInternalServerError, t0)
			return c.String(http.StatusInternalServerError, "create peer: "+err.Error())
		}
	}

	rid := peer.ResourceID()
	h.mu.Lock()
	if h.peersByStream[id] == nil {
		h.peersByStream[id] = make(map[string]*corewebrtc.Peer)
	}
	h.peersByStream[id][rid] = peer
	h.peerStream[rid] = id
	h.mu.Unlock()
	atomic.AddInt64(&h.count, 1)

	// Auto-cleanup: when Pion's OnConnectionStateChange triggers
	// peer.Close() (ICE failed/disconnected), the Done channel
	// closes and we yank the index entry. Without this the map
	// leaks for the lifetime of the handler.
	go h.awaitPeerClose(rid, peer)

	// Track ICE establishment duration asynchronously.
	go h.trackICE(id, peer, time.Now())

	h.recordRequest("subscribe", id, http.StatusCreated, t0)

	c.Response().Header().Set("Content-Type", "application/sdp")
	c.Response().Header().Set("Location", "/whep/"+id+"/"+rid)
	c.Response().Header().Set("ETag", `"`+rid+`"`)
	return c.String(http.StatusCreated, peer.Answer().SDP)
}

// Unsubscribe handles DELETE /whep/:id/:resource. Per WHEP spec we
// return 204 even when the resource is unknown — DELETE is idempotent
// and a re-issued tear-down should never error out.
//
// @Summary Tear down a WHEP subscription
// @Description Idempotent peer teardown by resource id (returned in the Location header by Subscribe). Returns 204 even when the resource is unknown, per the WHEP spec.
// @Tags v16.16.0
// @ID webrtc-3-whep-unsubscribe
// @Param id path string true "Process ID"
// @Param resource path string true "Resource ID from the Subscribe Location header"
// @Success 204 "no content"
// @Failure 400 {string} string "missing resource id"
// @Security ApiKeyAuth
// @Router /api/v3/whep/{id}/{resource} [delete]
func (h *Handler) Unsubscribe(c echo.Context) error {
	addCORS(c)
	t0 := time.Now()

	resource := c.Param("resource")
	if resource == "" {
		h.recordRequest("unsubscribe", "", http.StatusBadRequest, t0)
		return c.String(http.StatusBadRequest, "missing resource id")
	}

	h.mu.Lock()
	streamID := h.peerStream[resource]
	var peer *corewebrtc.Peer
	if streamID != "" {
		peer = h.peersByStream[streamID][resource]
		delete(h.peersByStream[streamID], resource)
		if len(h.peersByStream[streamID]) == 0 {
			delete(h.peersByStream, streamID)
		}
		delete(h.peerStream, resource)
	}
	h.mu.Unlock()

	if peer != nil {
		_ = peer.Close()
	}
	if streamID != "" {
		atomic.AddInt64(&h.count, -1)
	}

	h.recordRequest("unsubscribe", streamID, http.StatusNoContent, t0)
	return c.NoContent(http.StatusNoContent)
}

// Trickle handles PATCH /whep/:id/:resource — adds ICE candidates
// from a trickle-ice-sdpfrag body. Empty body is a no-op (clients
// signal end-of-candidates via an a=end-of-candidates line, which
// AddICECandidate accepts).
//
// @Summary Trickle ICE candidates for a WHEP subscription
// @Description Add ICE candidates to an existing WebRTC peer. Body is application/trickle-ice-sdpfrag.
// @Tags v16.16.0
// @ID webrtc-3-whep-trickle
// @Accept application/trickle-ice-sdpfrag
// @Param id path string true "Process ID"
// @Param resource path string true "Resource ID from the Subscribe Location header"
// @Success 204 "no content"
// @Failure 400 {string} string "missing resource id or unreadable body"
// @Failure 404 {string} string "peer not found"
// @Security ApiKeyAuth
// @Router /api/v3/whep/{id}/{resource} [patch]
func (h *Handler) Trickle(c echo.Context) error {
	addCORS(c)
	t0 := time.Now()

	resource := c.Param("resource")
	if resource == "" {
		h.recordRequest("trickle", "", http.StatusBadRequest, t0)
		return c.String(http.StatusBadRequest, "missing resource id")
	}

	h.mu.Lock()
	streamID := h.peerStream[resource]
	var peer *corewebrtc.Peer
	if streamID != "" {
		peer = h.peersByStream[streamID][resource]
	}
	h.mu.Unlock()
	if peer == nil {
		h.recordRequest("trickle", streamID, http.StatusNotFound, t0)
		return c.NoContent(http.StatusNotFound)
	}

	body, err := io.ReadAll(c.Request().Body)
	if err != nil {
		h.recordRequest("trickle", streamID, http.StatusBadRequest, t0)
		return c.String(http.StatusBadRequest, "read body: "+err.Error())
	}
	for _, line := range strings.Split(string(body), "\n") {
		line = strings.TrimSpace(line)
		if !strings.HasPrefix(line, "a=candidate:") {
			continue
		}
		cand := strings.TrimPrefix(line, "a=")
		_ = peer.AddICECandidate(webrtc.ICECandidateInit{Candidate: cand})
	}

	h.recordRequest("trickle", streamID, http.StatusNoContent, t0)
	return c.NoContent(http.StatusNoContent)
}

// recordRequest records whepRequests counter and whepRequestDuration histogram
// for any WHEP route outcome. Silently no-ops if metrics are not initialised.
func (h *Handler) recordRequest(route, streamID string, code int, t0 time.Time) {
	if h.met == nil {
		return
	}
	codeStr := fmt.Sprintf("%d", code)
	h.met.whepRequests.WithLabelValues(route, codeStr, streamID).Inc()
	h.met.whepRequestDuration.WithLabelValues(route, streamID).Observe(time.Since(t0).Seconds())
}

// preflight answers a CORS OPTIONS request; the headers are also
// echoed on every other response.
func (h *Handler) preflight(c echo.Context) error {
	addCORS(c)
	return c.NoContent(http.StatusNoContent)
}

// Close tears down every active peer (e.g., during Core shutdown).
func (h *Handler) Close() {
	h.mu.Lock()
	peers := make([]*corewebrtc.Peer, 0)
	for _, m := range h.peersByStream {
		for _, p := range m {
			peers = append(peers, p)
		}
	}
	h.peersByStream = make(map[string]map[string]*corewebrtc.Peer)
	h.peerStream = make(map[string]string)
	h.mu.Unlock()

	for _, p := range peers {
		if p != nil {
			_ = p.Close()
		}
	}
	atomic.StoreInt64(&h.count, 0)
}

// awaitPeerClose blocks on peer.Done() and yanks the index entry when
// the peer self-closes (ICE failed/disconnected). Idempotent with
// the Unsubscribe path: if Unsubscribe ran first the index is already
// empty and we just decrement the counter once on first arrival.
func (h *Handler) awaitPeerClose(resource string, peer *corewebrtc.Peer) {
	<-peer.Done()
	h.mu.Lock()
	streamID := h.peerStream[resource]
	_, present := h.peerStream[resource]
	if present {
		delete(h.peerStream, resource)
		if streamID != "" {
			delete(h.peersByStream[streamID], resource)
			if len(h.peersByStream[streamID]) == 0 {
				delete(h.peersByStream, streamID)
			}
		}
	}
	h.mu.Unlock()
	if present {
		atomic.AddInt64(&h.count, -1)
	}
}

// tearDownStreamPeers is the callback the Subsystem runs in its
// onProcessStop hook. It closes every peer subscribed to that
// stream and records FFmpeg leg failures if any peers were active,
// which indicates the process died unexpectedly.
func (h *Handler) tearDownStreamPeers(streamID string) {
	h.mu.Lock()
	bucket := h.peersByStream[streamID]
	hadPeers := len(bucket) > 0
	peers := make([]*corewebrtc.Peer, 0, len(bucket))
	for _, p := range bucket {
		peers = append(peers, p)
	}
	h.mu.Unlock()

	for _, p := range peers {
		if p != nil {
			_ = p.Close()
		}
	}

	if hadPeers && h.met != nil {
		h.met.ffmpegLegFailures.WithLabelValues(streamID, "video").Inc()
		h.met.ffmpegLegFailures.WithLabelValues(streamID, "audio").Inc()
	}
}

// addCORS emits the response headers a browser-side WHEP player
// expects. WHEP's Location and ETag headers must be exposed for
// fetch() to read them across origins.
func addCORS(c echo.Context) {
	hh := c.Response().Header()
	hh.Set("Access-Control-Allow-Origin", "*")
	hh.Set("Access-Control-Allow-Methods", "POST, DELETE, PATCH, OPTIONS")
	hh.Set("Access-Control-Allow-Headers", "Content-Type, Authorization, If-Match, If-None-Match")
	hh.Set("Access-Control-Expose-Headers", "Location, ETag")
}

// requireH264AndOpus does a coarse SDP scan to confirm the offer
// includes both an H.264 video rtpmap and an Opus audio rtpmap. The
// design treats codec mismatch as a 406, never a silent black frame.
//
// This is intentionally a string scan rather than a full SDP parse:
// every modern browser advertises H.264 and Opus by name, and a
// dependency on a real SDP parser for one validation step is
// disproportionate. M4 may swap this for pion/sdp.v3 when other
// surfaces also need parsing.
func requireH264AndOpus(sdp string) error {
	lower := strings.ToLower(sdp)
	hasH264 := strings.Contains(lower, "h264/90000") || strings.Contains(lower, " h264/")
	hasOpus := strings.Contains(lower, "opus/48000") || strings.Contains(lower, " opus/")
	if hasH264 && hasOpus {
		return nil
	}
	missing := []string{}
	if !hasH264 {
		missing = append(missing, "H264")
	}
	if !hasOpus {
		missing = append(missing, "Opus")
	}
	return &codecMismatchError{missing: missing}
}

type codecMismatchError struct{ missing []string }

func (e *codecMismatchError) Error() string {
	return "webrtc: codec mismatch — offer is missing: " + strings.Join(e.missing, ", ")
}