/**
 * @file server.js
 * @brief Node.js Chunk Transfer Server
 */

const http = require('http');
const fs = require('fs');
const path = require('path');
const EventEmitter = require('events');

class ChunkServer extends EventEmitter {
    constructor(config) {
        super();
        this.config = {
            port: 8080,
            rootPath: '.',
            chunkSize: 1024,
            ...config
        };
        this.server = null;
        this.running = false;
    }
    
    /**
     * Calculate CRC32 (IEEE 802.3)
     */
    static crc32(data) {
        const table = ChunkServer.crc32Table;
        let crc = 0xFFFFFFFF;
        
        for (let i = 0; i < data.length; i++) {
            const index = (crc ^ data[i]) & 0xFF;
            crc = (crc >>> 8) ^ table[index];
        }
        
        return crc ^ 0xFFFFFFFF;
    }
    
    /**
     * CRC32 lookup table
     */
    static get crc32Table() {
        if (!ChunkServer._crc32Table) {
            const table = new Uint32Array(256);
            const poly = 0xEDB88320;
            
            for (let i = 0; i < 256; i++) {
                let crc = i;
                for (let j = 0; j < 8; j++) {
                    crc = (crc & 1) ? ((crc >>> 1) ^ poly) : (crc >>> 1);
                }
                table[i] = crc >>> 0;
            }
            ChunkServer._crc32Table = table;
        }
        return ChunkServer._crc32Table;
    }
    
    /**
     * Start server
     */
    start() {
        return new Promise((resolve, reject) => {
            this.server = http.createServer((req, res) => {
                this.handleRequest(req, res).catch(err => {
                    console.error('Request error:', err);
                    res.writeHead(500);
                    res.end('Internal Server Error');
                });
            });
            
            this.server.listen(this.config.port, () => {
                console.log(`Chunk server running on port ${this.config.port}`);
                console.log(`Root path: ${this.config.rootPath}`);
                console.log(`Chunk size: ${this.config.chunkSize} bytes`);
                this.running = true;
                resolve();
            });
            
            this.server.on('error', reject);
        });
    }
    
    /**
     * Stop server
     */
    stop() {
        return new Promise((resolve, reject) => {
            if (!this.server) {
                resolve();
                return;
            }
            
            this.running = false;
            this.server.close(err => {
                if (err) reject(err);
                else {
                    console.log('Server stopped');
                    resolve();
                }
            });
        });
    }
    
    /**
     * Handle HTTP request
     */
    async handleRequest(req, res) {
        // Parse URL
        const url = new URL(req.url, `http://${req.headers.host}`);
        
        // Only handle /ota endpoint
        if (url.pathname !== '/ota') {
            res.writeHead(404);
            res.end('Not Found');
            return;
        }
        
        const filename = url.searchParams.get('file');
        const chunkParam = url.searchParams.get('chunk');
        
        if (!filename) {
            res.writeHead(400, { 'Content-Type': 'text/plain' });
            res.end('Missing file parameter');
            return;
        }
        
        // Security: Prevent path traversal
        const safeFilename = path.basename(filename);
        const filePath = path.join(this.config.rootPath, safeFilename);
        
        // Check file exists
        let stat;
        try {
            stat = fs.statSync(filePath);
            if (!stat.isFile()) {
                res.writeHead(404);
                res.end('File not found');
                return;
            }
        } catch (err) {
            res.writeHead(404);
            res.end('File not found');
            return;
        }
        
        const fileSize = stat.size;
        const totalChunks = Math.ceil(fileSize / this.config.chunkSize);
        
        // Calculate chunk index
        let chunkIdx;
        if (chunkParam !== null) {
            chunkIdx = parseInt(chunkParam, 10);
            if (isNaN(chunkIdx) || chunkIdx < 0 || chunkIdx >= totalChunks) {
                res.writeHead(416, { 
                    'Content-Type': 'text/plain',
                    'Content-Range': `chunks 0-${totalChunks-1}/${totalChunks}` 
                });
                res.end('Range Not Satisfiable');
                return;
            }
        } else {
            chunkIdx = 0;
        }
        
        // Read chunk data
        const offset = chunkIdx * this.config.chunkSize;
        const length = Math.min(this.config.chunkSize, fileSize - offset);
        
        const buffer = Buffer.alloc(length);
        const fd = fs.openSync(filePath, 'r');
        try {
            fs.readSync(fd, buffer, 0, length, offset);
        } finally {
            fs.closeSync(fd);
        }
        
        // Calculate CRC32
        const crc32 = ChunkServer.crc32(buffer);
        
        // Build response: [chunk_idx: 2B][payload: N B][crc32: 4B]
        const responseBuffer = Buffer.alloc(2 + length + 4);
        responseBuffer.writeUInt16LE(chunkIdx, 0);
        buffer.copy(responseBuffer, 2);
        responseBuffer.writeUInt32LE(crc32 >>> 0, 2 + length);  // Ensure unsigned
        
        // Set headers
        const headers = {
            'Content-Type': 'application/octet-stream',
            'Content-Length': responseBuffer.length,
            'X-Chunk-Index': chunkIdx,
            'X-Chunk-Size': length,
            'X-Total-Chunks': totalChunks,
            'X-File-Size': fileSize
        };
        
        // Response status
        if (chunkIdx > 0) {
            res.writeHead(206, headers);  // Partial Content for resumed chunks
        } else {
            res.writeHead(200, headers);  // OK for first chunk
        }
        
        res.end(responseBuffer);
        
        // Emit event
        this.emit('chunk-sent', {
            file: filename,
            chunkIdx: chunkIdx,
            chunkSize: length,
            totalChunks: totalChunks,
            fileSize: fileSize
        });
    }
}

// Export
module.exports = ChunkServer;

// CLI usage
if (require.main === module) {
    const port = process.argv[2] || 8080;
    const rootPath = process.argv[3] || '.';
    
    const server = new ChunkServer({ port: parseInt(port), rootPath });
    
    server.on('chunk-sent', (info) => {
        console.log(`Sent chunk ${info.chunkIdx}/${info.totalChunks} for ${info.file} (${info.chunkSize}B)`);
    });
    
    server.start().then(() => {
        console.log(`Server started on port ${port}`);
        console.log(`Root path: ${rootPath}`);
    }).catch(err => {
        console.error('Failed to start server:', err);
        process.exit(1);
    });
    
    process.on('SIGINT', async () => {
        await server.stop();
        process.exit(0);
    });
}