/**
 * WebRTC DataChannel transport for low-latency game updates
 */

class WebRTCTransport {
    constructor(signalingWs, onStateUpdate, playerId) {
        this.signalingWs = signalingWs;  // WebSocket for signaling
        this.onStateUpdate = onStateUpdate;
        this.playerId = playerId;

        this.peerConnection = null;
        this.dataChannel = null;
        this.connected = false;
        this.fallbackToWebSocket = false;

        this.sequenceTracker = new SequenceTracker();
        this.partialTracker = new PartialStateTracker();

        // Statistics
        this.packetsReceived = 0;
        this.packetsLost = 0;
        this.lastUpdateId = -1;
    }

    /**
     * Check if browser supports WebRTC
     */
    static isSupported() {
        return typeof RTCPeerConnection !== 'undefined';
    }

    /**
     * Initialize WebRTC connection
     */
    async init() {
        if (!WebRTCTransport.isSupported()) {
            console.log('WebRTC not supported, using WebSocket');
            this.fallbackToWebSocket = true;
            return false;
        }

        try {
            // Create peer connection
            this.peerConnection = new RTCPeerConnection({
                iceServers: [
                    { urls: 'stun:stun.l.google.com:19302' }
                ]
            });

            // Create data channel with UDP-like behavior
            this.dataChannel = this.peerConnection.createDataChannel('game-updates', {
                ordered: false,           // Unordered delivery
                maxRetransmits: 0        // No retransmissions (UDP-like)
            });

            this.setupDataChannel();

            // Handle ICE candidates
            this.peerConnection.onicecandidate = (event) => {
                if (event.candidate) {
                    // Send ICE candidate to server via WebSocket
                    this.signalingWs.send(JSON.stringify({
                        type: 'webrtc_ice',
                        player_id: this.playerId,
                        candidate: event.candidate
                    }));
                }
            };

            // Create and send offer
            const offer = await this.peerConnection.createOffer();
            await this.peerConnection.setLocalDescription(offer);

            // Send offer to server via WebSocket
            this.signalingWs.send(JSON.stringify({
                type: 'webrtc_offer',
                player_id: this.playerId,
                sdp: offer.sdp
            }));

            return true;

        } catch (error) {
            console.error('WebRTC initialization failed:', error);
            this.fallbackToWebSocket = true;
            return false;
        }
    }

    /**
     * Setup data channel event handlers
     */
    setupDataChannel() {
        this.dataChannel.binaryType = 'arraybuffer';

        this.dataChannel.onopen = () => {
            console.log('WebRTC DataChannel opened');
            this.connected = true;
        };

        this.dataChannel.onclose = () => {
            console.log('WebRTC DataChannel closed');
            this.connected = false;
        };

        this.dataChannel.onerror = (error) => {
            console.error('WebRTC DataChannel error:', error);
            this.fallbackToWebSocket = true;
        };

        this.dataChannel.onmessage = (event) => {
            this.handleMessage(new Uint8Array(event.data));
        };
    }

    /**
     * Handle WebRTC signaling messages from server
     */
    async handleSignaling(message) {
        try {
            if (message.type === 'webrtc_answer') {
                // Received SDP answer from server
                await this.peerConnection.setRemoteDescription({
                    type: 'answer',
                    sdp: message.sdp
                });
            }
            else if (message.type === 'webrtc_ice') {
                // Received ICE candidate from server
                if (message.candidate) {
                    await this.peerConnection.addIceCandidate(message.candidate);
                }
            }
        } catch (error) {
            console.error('Error handling WebRTC signaling:', error);
        }
    }

    /**
     * Handle incoming binary message from DataChannel
     */
    async handleMessage(data) {
        // Parse UDP-style packet
        const result = await UDPProtocol.parsePacket(data);
        if (!result) {
            return;
        }

        const { seqNum, msgType, updateId, payload } = result;

        // Check sequence
        if (!this.sequenceTracker.shouldAccept(seqNum)) {
            // Old or duplicate packet
            return;
        }

        // Update statistics
        this.packetsReceived++;

        // Check for lost packets
        if (this.lastUpdateId !== -1) {
            const expectedId = UDPProtocol.nextUpdateId(this.lastUpdateId);
            if (updateId !== expectedId && updateId !== this.lastUpdateId) {
                // Detect loss (accounting for wrapping)
                const gap = (updateId - expectedId) & 0xFFFF;
                if (gap < 100) {
                    this.packetsLost += gap;
                }
            }
        }

        this.lastUpdateId = updateId;

        // Check fallback condition
        if (this.packetsReceived > 100) {
            const lossRate = this.packetsLost / (this.packetsReceived + this.packetsLost);
            if (lossRate > 0.2) {
                console.log(`High packet loss (${(lossRate * 100).toFixed(1)}%), suggesting WebSocket fallback`);
                this.fallbackToWebSocket = true;
            }
        }

        // Process packet
        if (msgType === BinaryMessageType.PARTIAL_STATE_UPDATE ||
            msgType === BinaryMessageType.GAME_META_UPDATE) {
            // Process partial update
            const ready = this.partialTracker.processPacket(updateId, payload);

            if (ready) {
                // Get assembled state
                const gameState = this.partialTracker.getGameState();
                this.onStateUpdate(gameState);
            }
        }
    }

    /**
     * Check if should fallback to WebSocket
     */
    shouldFallback() {
        return this.fallbackToWebSocket;
    }

    /**
     * Check if connected
     */
    isConnected() {
        return this.connected;
    }

    /**
     * Close WebRTC connection
     */
    close() {
        if (this.dataChannel) {
            this.dataChannel.close();
        }
        if (this.peerConnection) {
            this.peerConnection.close();
        }
        this.connected = false;
    }
}

/**
 * Sequence tracker for WebRTC (mirrors Python/UDP version)
 */
class SequenceTracker {
    constructor() {
        this.lastSeq = 0;
        this.receivedSeqs = new Set();
    }

    shouldAccept(seqNum) {
        // Check if newer
        if (!UDPProtocol.isSeqNewer(seqNum, this.lastSeq)) {
            return false;
        }

        // Check for duplicate
        if (this.receivedSeqs.has(seqNum)) {
            return false;
        }

        // Accept packet
        this.lastSeq = seqNum;
        this.receivedSeqs.add(seqNum);

        // Clean up old sequences
        if (this.receivedSeqs.size > 1000) {
            const minSeq = (this.lastSeq - 1000) & 0xFFFFFFFF;
            this.receivedSeqs = new Set(
                Array.from(this.receivedSeqs).filter(s =>
                    UDPProtocol.isSeqNewer(s, minSeq)
                )
            );
        }

        return true;
    }

    reset() {
        this.lastSeq = 0;
        this.receivedSeqs.clear();
    }
}

/**
 * UDP Protocol utilities (JavaScript version)
 */
class UDPProtocol {
    static SEQUENCE_WINDOW = 1000;
    static MAX_SEQUENCE = 0xFFFFFFFF;
    static MAX_UPDATE_ID = 0xFFFF;

    /**
     * Check if new_seq is newer than last_seq (with wrapping)
     */
    static isSeqNewer(newSeq, lastSeq, window = UDPProtocol.SEQUENCE_WINDOW) {
        const diff = (newSeq - lastSeq) & 0xFFFFFFFF;

        if (diff === 0) {
            return false;  // Duplicate
        }

        // Treat as signed: if diff > 2^31, it wrapped backwards
        if (diff > 0x7FFFFFFF) {
            return false;  // Old packet
        }

        if (diff > window) {
            return false;  // Too far ahead
        }

        return true;
    }

    /**
     * Parse UDP-style packet
     * Returns {seqNum, msgType, updateId, payload} or null
     */
    static async parsePacket(packet) {
        if (packet.length < 7) {
            return null;
        }

        const dv = new DataView(packet.buffer, packet.byteOffset, packet.byteLength);

        const seqNum = dv.getUint32(0, false);  // Big endian
        let msgType = dv.getUint8(4);
        const updateId = dv.getUint16(5, false);  // Big endian

        let payload = packet.slice(7);

        // Check compression flag
        const compressed = (msgType & 0x80) !== 0;
        msgType &= 0x7F;  // Clear compression flag

        // Decompress if needed
        if (compressed && payload.length > 0) {
            try {
                payload = await BinaryCodec.decompress(payload);
            } catch (e) {
                console.error('Decompression failed:', e);
                return null;
            }
        }

        return { seqNum, msgType, updateId, payload };
    }

    /**
     * Get next update ID with wrapping
     */
    static nextUpdateId(updateId) {
        return (updateId + 1) & 0xFFFF;
    }
}