server.go

package main

import (
	"compress/gzip"
	"io"
	"net/http"
	"strings"
)

type gzipResponseWriter struct {
	io.Writer
	http.ResponseWriter
}

func (w gzipResponseWriter) Write(b []byte) (int, error) {
	return w.Writer.Write(b)
}

func wrapHandler(h http.Handler) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
			h.ServeHTTP(w, r)
			return
		}
		w.Header().Set("Content-Encoding", "gzip")
		gz := gzip.NewWriter(w)
		defer gz.Close()
		gzw := gzipResponseWriter{Writer: gz, ResponseWriter: w}
		h.ServeHTTP(gzw, r)
		// NOTE: we do not know if the request completed successfully (i.e., if the user did not cancel
		//      the download in middle)
	}
}

func main() {
	http.ListenAndServeTLS(":443", "fullchain.pem", "privkey.pem", wrapHandler(http.FileServer(http.Dir("./static"))))
	//http.ListenAndServe(":8000", wrapHandler(http.FileServer(http.Dir("./static"))))
}