// Package proxy provides reverse proxy functionality for Konduktor
package proxy

import (
	"context"
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"strings"
	"time"

	"github.com/konduktor/konduktor/internal/logging"
)

type Config struct {
	// Target is the backend server URL
	Target string

	// Timeout is the request timeout (default: 30s)
	Timeout time.Duration

	// Headers are additional headers to add to requests
	Headers map[string]string

	// StripPrefix removes this prefix from the request path
	StripPrefix string

	// PreserveHost keeps the original Host header
	PreserveHost bool

	// IgnoreRequestPath ignores the request path and uses only the target path
	// This is useful for exact match routes where target URL should be used as-is
	IgnoreRequestPath bool
}

type ReverseProxy struct {
	config     *Config
	targetURL  *url.URL
	httpClient *http.Client
	logger     *logging.Logger
}

func New(cfg *Config, logger *logging.Logger) (*ReverseProxy, error) {
	if cfg.Target == "" {
		return nil, fmt.Errorf("proxy target is required")
	}

	targetURL, err := url.Parse(cfg.Target)
	if err != nil {
		return nil, fmt.Errorf("invalid proxy target URL: %w", err)
	}

	timeout := cfg.Timeout
	if timeout == 0 {
		timeout = 30 * time.Second
	}

	transport := &http.Transport{
		Proxy: http.ProxyFromEnvironment,
		DialContext: (&net.Dialer{
			Timeout:   10 * time.Second,
			KeepAlive: 30 * time.Second,
		}).DialContext,
		MaxIdleConns:          100,
		MaxIdleConnsPerHost:   10,
		IdleConnTimeout:       90 * time.Second,
		TLSHandshakeTimeout:   10 * time.Second,
		ExpectContinueTimeout: 1 * time.Second,
		ResponseHeaderTimeout: timeout,
	}

	return &ReverseProxy{
		config:    cfg,
		targetURL: targetURL,
		httpClient: &http.Client{
			Transport: transport,
			Timeout:   timeout,
			CheckRedirect: func(req *http.Request, via []*http.Request) error {
				return http.ErrUseLastResponse // Don't follow redirects
			},
		},
		logger: logger,
	}, nil
}

func (rp *ReverseProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	rp.ProxyRequest(w, r, nil)
}

func (rp *ReverseProxy) ProxyRequest(w http.ResponseWriter, r *http.Request, params map[string]string) {
	ctx := r.Context()

	// Build target URL
	targetURL := rp.buildTargetURL(r)

	// Create proxy request
	proxyReq, err := rp.createProxyRequest(ctx, r, targetURL)
	if err != nil {
		rp.handleError(w, http.StatusInternalServerError, "Failed to create proxy request", err)
		return
	}

	// Add custom headers with parameter substitution
	rp.addCustomHeaders(proxyReq, r, params)

	// Execute request
	resp, err := rp.httpClient.Do(proxyReq)
	if err != nil {
		rp.handleProxyError(w, err)
		return
	}
	defer resp.Body.Close()

	// Copy response
	rp.copyResponse(w, resp)
}

func (rp *ReverseProxy) buildTargetURL(r *http.Request) *url.URL {
	targetURL := *rp.targetURL

	// If ignoring request path, use target URL path as-is
	if rp.config.IgnoreRequestPath {
		// Preserve query string only
		targetURL.RawQuery = r.URL.RawQuery
		return &targetURL
	}

	// Strip prefix if configured
	path := r.URL.Path
	if rp.config.StripPrefix != "" {
		path = strings.TrimPrefix(path, rp.config.StripPrefix)
		if path == "" || path[0] != '/' {
			path = "/" + path
		}
	}

	// If target URL has a non-empty path, combine it with the request path
	if rp.targetURL.Path != "" && rp.targetURL.Path != "/" {
		// Combine target path with request path
		targetURL.Path = strings.TrimSuffix(rp.targetURL.Path, "/") + path
	} else {
		// No path in target, use request path as-is
		targetURL.Path = path
	}

	// Preserve query string
	targetURL.RawQuery = r.URL.RawQuery

	return &targetURL
}

func (rp *ReverseProxy) createProxyRequest(ctx context.Context, r *http.Request, targetURL *url.URL) (*http.Request, error) {
	proxyReq, err := http.NewRequestWithContext(ctx, r.Method, targetURL.String(), r.Body)
	if err != nil {
		return nil, err
	}

	// Copy ContentLength
	proxyReq.ContentLength = r.ContentLength

	// Copy headers
	for key, values := range r.Header {
		for _, value := range values {
			proxyReq.Header.Add(key, value)
		}
	}

	// Set/update Host header
	if rp.config.PreserveHost {
		proxyReq.Host = r.Host
	} else {
		proxyReq.Host = targetURL.Host
	}

	// Remove hop-by-hop headers
	removeHopByHopHeaders(proxyReq.Header)

	return proxyReq, nil
}

func (rp *ReverseProxy) addCustomHeaders(proxyReq *http.Request, originalReq *http.Request, params map[string]string) {
	// Add X-Forwarded headers
	clientIP := getClientIP(originalReq)
	if prior := originalReq.Header.Get("X-Forwarded-For"); prior != "" {
		clientIP = prior + ", " + clientIP
	}
	proxyReq.Header.Set("X-Forwarded-For", clientIP)
	proxyReq.Header.Set("X-Forwarded-Proto", getScheme(originalReq))
	proxyReq.Header.Set("X-Forwarded-Host", originalReq.Host)

	// Add custom headers from config
	for key, value := range rp.config.Headers {
		// Substitute parameters like {version}
		substituted := value
		for paramKey, paramValue := range params {
			substituted = strings.ReplaceAll(substituted, "{"+paramKey+"}", paramValue)
		}
		// Substitute $remote_addr
		substituted = strings.ReplaceAll(substituted, "$remote_addr", clientIP)
		proxyReq.Header.Set(key, substituted)
	}
}

func (rp *ReverseProxy) copyResponse(w http.ResponseWriter, resp *http.Response) {
	// Copy headers
	for key, values := range resp.Header {
		for _, value := range values {
			w.Header().Add(key, value)
		}
	}

	// Remove hop-by-hop headers from response
	removeHopByHopHeaders(w.Header())

	// Write status code
	w.WriteHeader(resp.StatusCode)

	// Copy body
	io.Copy(w, resp.Body)
}

func (rp *ReverseProxy) handleError(w http.ResponseWriter, status int, message string, err error) {
	if rp.logger != nil {
		rp.logger.Error(message, "error", err)
	}
	http.Error(w, message, status)
}

func (rp *ReverseProxy) handleProxyError(w http.ResponseWriter, err error) {
	if rp.logger != nil {
		rp.logger.Error("Proxy request failed", "error", err)
	}

	// Check for timeout
	if err, ok := err.(net.Error); ok && err.Timeout() {
		http.Error(w, "504 Gateway Timeout", http.StatusGatewayTimeout)
		return
	}

	// Check for connection errors
	if isConnectionError(err) {
		http.Error(w, "502 Bad Gateway", http.StatusBadGateway)
		return
	}

	// Context cancelled (client disconnected)
	if err == context.Canceled {
		return
	}

	http.Error(w, "502 Bad Gateway", http.StatusBadGateway)
}

// Helper functions

func singleJoiningSlash(a, b string) string {
	aslash := strings.HasSuffix(a, "/")
	bslash := strings.HasPrefix(b, "/")
	switch {
	case aslash && bslash:
		return a + b[1:]
	case !aslash && !bslash:
		return a + "/" + b
	}
	return a + b
}

func removeHopByHopHeaders(h http.Header) {
	hopByHopHeaders := []string{
		"Connection",
		"Proxy-Connection",
		"Keep-Alive",
		"Proxy-Authenticate",
		"Proxy-Authorization",
		"Te",
		"Trailer",
		"Transfer-Encoding",
		"Upgrade",
	}

	for _, header := range hopByHopHeaders {
		h.Del(header)
	}
}

func getClientIP(r *http.Request) string {
	// Check X-Real-IP first
	if ip := r.Header.Get("X-Real-IP"); ip != "" {
		return ip
	}

	// Get from RemoteAddr
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return host
}

func getScheme(r *http.Request) string {
	if r.TLS != nil {
		return "https"
	}
	if scheme := r.Header.Get("X-Forwarded-Proto"); scheme != "" {
		return scheme
	}
	return "http"
}

func isConnectionError(err error) bool {
	if err == nil {
		return false
	}

	errStr := err.Error()
	connectionErrors := []string{
		"connection refused",
		"no such host",
		"network is unreachable",
		"connection reset",
		"broken pipe",
	}

	for _, connErr := range connectionErrors {
		if strings.Contains(strings.ToLower(errStr), connErr) {
			return true
		}
	}

	return false
}