package websrv

import (
	"fmt"
	"net"
	"net/http"
	"net/http/httputil"
	"net/url"
	"os"
	"path"
	"path/filepath"
	"strings"
)

// NewHandler serves SPA files from webDir and reverse-proxies /api and /health to apiURL.
func NewHandler(webDir string, apiURL string) (http.Handler, error) {
	absoluteWebDir, err := filepath.Abs(webDir)
	if err != nil {
		return nil, fmt.Errorf("resolve webdir: %w", err)
	}

	info, err := os.Stat(absoluteWebDir)
	if err != nil {
		return nil, fmt.Errorf("webdir: %w", err)
	}
	if !info.IsDir() {
		return nil, fmt.Errorf("webdir is not a directory: %s", absoluteWebDir)
	}

	parsedAPIURL, err := url.Parse(apiURL)
	if err != nil {
		return nil, fmt.Errorf("parse API_URL: %w", err)
	}
	if parsedAPIURL.Scheme == "" || parsedAPIURL.Host == "" {
		return nil, fmt.Errorf("API_URL must include scheme and host: %q", apiURL)
	}

	apiProxy := httputil.NewSingleHostReverseProxy(parsedAPIURL)
	defaultDirector := apiProxy.Director
	apiProxy.Director = func(request *http.Request) {
		originalHost := request.Host
		clientIP := clientIPFromRequest(request)
		defaultDirector(request)
		// Preserve the browser-facing host so the API can enforce public_hostname.
		if request.Header.Get("X-Forwarded-Host") == "" && originalHost != "" {
			request.Header.Set("X-Forwarded-Host", originalHost)
		}
		if request.Header.Get("X-Forwarded-Proto") == "" {
			if request.TLS != nil {
				request.Header.Set("X-Forwarded-Proto", "https")
			} else {
				request.Header.Set("X-Forwarded-Proto", "http")
			}
		}
		if clientIP != "" {
			prior := strings.TrimSpace(request.Header.Get("X-Forwarded-For"))
			if prior == "" {
				request.Header.Set("X-Forwarded-For", clientIP)
			} else {
				request.Header.Set("X-Forwarded-For", prior+", "+clientIP)
			}
		}
	}
	fileServer := http.FileServer(http.Dir(absoluteWebDir))

	return http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
		requestPath := request.URL.Path
		if requestPath == "/health" || strings.HasPrefix(requestPath, "/api/") || requestPath == "/api" {
			apiProxy.ServeHTTP(writer, request)
			return
		}

		serveSPA(writer, request, absoluteWebDir, fileServer)
	}), nil
}

func clientIPFromRequest(request *http.Request) string {
	host, _, err := net.SplitHostPort(request.RemoteAddr)
	if err != nil {
		return request.RemoteAddr
	}
	return host
}

func serveSPA(
	writer http.ResponseWriter,
	request *http.Request,
	webDir string,
	fileServer http.Handler,
) {
	cleanedPath := path.Clean("/" + request.URL.Path)
	if cleanedPath == "/" {
		http.ServeFile(writer, request, filepath.Join(webDir, "index.html"))
		return
	}

	candidatePath := filepath.Join(webDir, filepath.FromSlash(cleanedPath))
	if !isWithinDir(webDir, candidatePath) {
		http.NotFound(writer, request)
		return
	}

	fileInfo, err := os.Stat(candidatePath)
	if err != nil || fileInfo.IsDir() {
		http.ServeFile(writer, request, filepath.Join(webDir, "index.html"))
		return
	}

	fileServer.ServeHTTP(writer, request)
}

func isWithinDir(rootDir string, candidatePath string) bool {
	relativePath, err := filepath.Rel(rootDir, candidatePath)
	if err != nil {
		return false
	}
	return relativePath != ".." && !strings.HasPrefix(relativePath, ".."+string(os.PathSeparator))
}