package middleware

import (
	"net/http"
	"time"
)

var epoch = time.Unix(0, 0).Format(time.RFC1123)

var etagHeaders = []string{
	"ETag",
	"If-Modified-Since",
	"If-Match",
	"If-None-Match",
	"If-Range",
	"If-Unmodified-Since",
}

var noCacheHeaders = map[string]string{
	"Expires":         epoch,
	"Cache-Control":   "no-cache, no-store, no-transform, must-revalidate, private, max-age=0",
	"Pragma":          "no-cache",
	"X-Accel-Expires": "0",
}

// NoCache sets a number of HTTP headers to prevent a router from being cached
func NoCache(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		for _, v := range etagHeaders {
			if r.Header.Get(v) != "" {
				r.Header.Del(v)
			}
		}

		for k, v := range noCacheHeaders {
			w.Header().Set(k, v)
		}

		next.ServeHTTP(w, r)
	})
}