package httpx

import (
	"io"
	"io/fs"
	"net/http"
	"path"
	"strings"
)

// SPA 提供后台静态页：存在的文件原样返回；无扩展名的前端路由回退 index.html；
// 有扩展名但不存在返回 404；访问目录不列清单，同样回退 index.html。
func SPA(fsys fs.FS) http.Handler {
	files := http.FileServer(http.FS(fsys))
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet && r.Method != http.MethodHead {
			w.WriteHeader(http.StatusMethodNotAllowed)
			return
		}
		cleaned := path.Clean("/" + r.URL.Path)
		rel := strings.TrimPrefix(cleaned, "/")
		if rel == "" {
			rel = "."
		}
		fi, err := fs.Stat(fsys, rel)
		if err == nil && !fi.IsDir() {
			files.ServeHTTP(w, r)
			return
		}
		if path.Ext(cleaned) != "" {
			http.NotFound(w, r)
			return
		}
		serveIndex(w, r, fsys)
	})
}

func serveIndex(w http.ResponseWriter, r *http.Request, fsys fs.FS) {
	f, err := fsys.Open("index.html")
	if err != nil {
		http.NotFound(w, r)
		return
	}
	defer func() { _ = f.Close() }()
	st, err := f.Stat()
	if err != nil || st.IsDir() {
		http.NotFound(w, r)
		return
	}
	w.Header().Set("Cache-Control", "no-cache")
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.WriteHeader(http.StatusOK)
	if r.Method == http.MethodHead {
		return
	}
	_, _ = io.Copy(w, f)
}