package web

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

// The pages, scripts and styles are embedded, so they only change with a new build. They're served
// with Cache-Control: no-cache and an ETag (a hash of the file): browsers, the Android app and
// Cloudflare check each time and get a quick 304 while the file is unchanged, and a new deploy
// shows up at once. Without these headers Cloudflare tells browsers to keep them for 4 hours.

var assetTags = sync.OnceValue(func() map[string]string {
	tags := map[string]string{}
	static, _ := fs.Sub(assets, "assets")
	_ = fs.WalkDir(static, ".", func(path string, d fs.DirEntry, err error) error {
		if err != nil || d.IsDir() {
			return err
		}
		data, err := fs.ReadFile(static, path)
		if err != nil {
			return err
		}
		sum := sha256.Sum256(data)
		tags[path] = `"` + hex.EncodeToString(sum[:8]) + `"`
		return nil
	})
	return tags
})

// revalidate sets the caching headers for an embedded file. http.ServeContent answers a matching
// If-None-Match with 304 from the ETag set here.
func revalidate(w http.ResponseWriter, name string) {
	w.Header().Set("Cache-Control", "no-cache")
	if tag, ok := assetTags()[name]; ok {
		w.Header().Set("ETag", tag)
	}
}

// serveAsset serves one embedded file, see revalidate.
func serveAsset(w http.ResponseWriter, r *http.Request, static fs.FS, name string) {
	revalidate(w, name)
	http.ServeFileFS(w, r, static, name)
}