package main

import (
	"crypto/subtle"
	"log"
	"net/http"
	"os"
	"regexp"
	"time"
)

// versionLine matches the userscript metadata block's @version directive.
var versionLine = regexp.MustCompile(`(?m)^// @version[ \t]+.*$`)

// stampVersion replaces the served @version with one derived from the file's
// mtime, discarding whatever the file body says.
//
// Violentmonkey only updates when the served version sorts higher than the
// installed one. Deriving it from the body means one accidental downgrade or
// typo freezes updates forever; an mtime-derived version is monotonic by
// construction, so any later write always outranks any earlier one.
//
// A file with no @version line is returned untouched: such a script never
// auto-updates anyway, and inventing a metadata block is not this handler's job.
func stampVersion(src []byte, mod time.Time) []byte {
	return versionLine.ReplaceAll(src, []byte("// @version      "+mod.UTC().Format("2006.01.02.1504")))
}

// userscriptHandler serves the userscript to Violentmonkey's updater.
//
// The token lives in the path because the update poll sends no Authorization
// header, and the file embeds API_TOKEN in plain text, so an open path would
// hand that token to anyone who guessed the URL. A mismatch answers 404 rather
// than 401: a prober learns nothing about whether the route exists.
//
// The file is read per request — that is what lets a bindmounted copy be edited
// on the host without a restart. It is ~50 KB and polled about once a day.
func userscriptHandler(token, path string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if subtle.ConstantTimeCompare([]byte(r.PathValue("token")), []byte(token)) != 1 {
			http.NotFound(w, r)
			return
		}
		info, err := os.Stat(path)
		if err != nil {
			log.Printf("userscript: stat %s: %v", path, err)
			http.NotFound(w, r)
			return
		}
		src, err := os.ReadFile(path)
		if err != nil {
			log.Printf("userscript: read %s: %v", path, err)
			http.NotFound(w, r)
			return
		}
		w.Header().Set("Content-Type", "text/javascript; charset=utf-8")
		w.Header().Set("Cache-Control", "no-cache")
		w.Write(stampVersion(src, info.ModTime()))
	}
}