package protocol

import (
	"bufio"
	"encoding/binary"
	"fmt"
	"io"
	"net"
)

// Handler processes Kafka protocol requests from clients
type Handler struct {
}

func NewHandler() *Handler {
	return &Handler{}
}

// HandleConn processes a single client connection
func (h *Handler) HandleConn(conn net.Conn) error {
	defer conn.Close()

	r := bufio.NewReader(conn)
	w := bufio.NewWriter(conn)
	defer w.Flush()

	for {
		// Read message size (4 bytes)
		var sizeBytes [4]byte
		if _, err := io.ReadFull(r, sizeBytes[:]); err != nil {
			if err == io.EOF {
				return nil // clean disconnect
			}
			return fmt.Errorf("read size: %w", err)
		}

		size := binary.BigEndian.Uint32(sizeBytes[:])
		if size == 0 || size > 1024*1024 { // 1MB limit
			return fmt.Errorf("invalid message size: %d", size)
		}

		// Read the message
		messageBuf := make([]byte, size)
		if _, err := io.ReadFull(r, messageBuf); err != nil {
			return fmt.Errorf("read message: %w", err)
		}

		// Parse at least the basic header to get API key and correlation ID
		if len(messageBuf) < 8 {
			return fmt.Errorf("message too short")
		}

		apiKey := binary.BigEndian.Uint16(messageBuf[0:2])
		apiVersion := binary.BigEndian.Uint16(messageBuf[2:4])
		correlationID := binary.BigEndian.Uint32(messageBuf[4:8])

		// Handle the request based on API key
		var response []byte
		var err error

		switch apiKey {
		case 18: // ApiVersions
			response, err = h.handleApiVersions(correlationID)
		case 3: // Metadata
			response, err = h.handleMetadata(correlationID, messageBuf[8:]) // skip header
		default:
			err = fmt.Errorf("unsupported API key: %d (version %d)", apiKey, apiVersion)
		}

		if err != nil {
			return fmt.Errorf("handle request: %w", err)
		}

		// Write response size and data
		responseSizeBytes := make([]byte, 4)
		binary.BigEndian.PutUint32(responseSizeBytes, uint32(len(response)))

		if _, err := w.Write(responseSizeBytes); err != nil {
			return fmt.Errorf("write response size: %w", err)
		}
		if _, err := w.Write(response); err != nil {
			return fmt.Errorf("write response: %w", err)
		}

		if err := w.Flush(); err != nil {
			return fmt.Errorf("flush response: %w", err)
		}
	}
}

func (h *Handler) handleApiVersions(correlationID uint32) ([]byte, error) {
	// Build ApiVersions response manually
	// Response format: correlation_id(4) + error_code(2) + num_api_keys(4) + api_keys + throttle_time(4)
	
	response := make([]byte, 0, 64)
	
	// Correlation ID
	correlationIDBytes := make([]byte, 4)
	binary.BigEndian.PutUint32(correlationIDBytes, correlationID)
	response = append(response, correlationIDBytes...)
	
	// Error code (0 = no error)
	response = append(response, 0, 0)
	
	// Number of API keys (compact array format in newer versions, but using basic format for simplicity)
	response = append(response, 0, 0, 0, 2) // 2 API keys
	
	// API Key 18 (ApiVersions): api_key(2) + min_version(2) + max_version(2)
	response = append(response, 0, 18) // API key 18
	response = append(response, 0, 0)  // min version 0
	response = append(response, 0, 3)  // max version 3
	
	// API Key 3 (Metadata): api_key(2) + min_version(2) + max_version(2)
	response = append(response, 0, 3)  // API key 3
	response = append(response, 0, 0)  // min version 0  
	response = append(response, 0, 7)  // max version 7
	
	// Throttle time (4 bytes, 0 = no throttling)
	response = append(response, 0, 0, 0, 0)
	
	return response, nil
}

func (h *Handler) handleMetadata(correlationID uint32, requestBody []byte) ([]byte, error) {
	// For now, ignore the request body content (topics filter, etc.)
	// Build minimal Metadata response
	// Response format: correlation_id(4) + throttle_time(4) + brokers + cluster_id + controller_id + topics
	
	response := make([]byte, 0, 256)
	
	// Correlation ID
	correlationIDBytes := make([]byte, 4)
	binary.BigEndian.PutUint32(correlationIDBytes, correlationID)
	response = append(response, correlationIDBytes...)
	
	// Throttle time (4 bytes, 0 = no throttling)
	response = append(response, 0, 0, 0, 0)
	
	// Brokers array length (4 bytes) - 1 broker (this gateway)
	response = append(response, 0, 0, 0, 1)
	
	// Broker 0: node_id(4) + host + port(4) + rack
	response = append(response, 0, 0, 0, 0) // node_id = 0
	
	// Host string: length(2) + "localhost"
	host := "localhost"
	response = append(response, 0, byte(len(host)))
	response = append(response, []byte(host)...)
	
	// Port (4 bytes) - 9092 (standard Kafka port)
	response = append(response, 0, 0, 0x23, 0x84) // 9092 in big-endian
	
	// Rack - nullable string, using null (-1 length)
	response = append(response, 0xFF, 0xFF) // null rack
	
	// Cluster ID - nullable string, using null
	response = append(response, 0xFF, 0xFF) // null cluster_id
	
	// Controller ID (4 bytes) - -1 (no controller)
	response = append(response, 0xFF, 0xFF, 0xFF, 0xFF)
	
	// Topics array length (4 bytes) - 0 topics for now
	response = append(response, 0, 0, 0, 0)
	
	return response, nil
}