package main

import (
	"crypto/sha256"
	"encoding/hex"
	"io/fs"
	"net/http"
	"strings"
)

// staticFiles serves the embedded UI with an ETag per file (a hash of its
// content, computed at startup). Pages, scripts and styles are revalidated on
// every load ("no-cache"): an unchanged file costs a 304 without body, and the
// files of a new version are always taken after an update. Fonts are kept for
// 30 days.
func staticFiles(static fs.FS) http.Handler {
	etags := map[string]string{}
	_ = fs.WalkDir(static, ".", func(path string, d fs.DirEntry, err error) error {
		if err != nil || d.IsDir() {
			return err
		}
		b, err := fs.ReadFile(static, path)
		if err != nil {
			return err
		}
		sum := sha256.Sum256(b)
		etags["/"+path] = `"` + hex.EncodeToString(sum[:8]) + `"`
		return nil
	})
	etags["/"] = etags["/index.html"]
	files := http.FileServer(http.FS(static))
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if tag, ok := etags[r.URL.Path]; ok {
			// http.FileServer answers If-None-Match with 304 from this header.
			w.Header().Set("ETag", tag)
			if strings.HasPrefix(r.URL.Path, "/fonts/") {
				w.Header().Set("Cache-Control", "public, max-age=2592000")
			} else {
				w.Header().Set("Cache-Control", "no-cache")
			}
		}
		files.ServeHTTP(w, r)
	})
}