Substringing more efficient and go-like.

main.go

@@ -46,6 +46,7 @@ import (
 var (
 	hostname  string
 	logger    *log.Logger
+	lenPrefix int
 	address   = flag.String("address", ":7357", "listen address")
 	logF      = flag.String("log", "", "log file (default stdout)")
 	setPrefix = flag.String("set_prefix", "X-Set-",
@@ -81,7 +82,7 @@ func handle(resp http.ResponseWriter, req *http.Request) {
 		if !strings.HasPrefix(reqHdr, *setPrefix) {
 			continue
 		}
-		newHdr := string([]byte(reqHdr)[len(*setPrefix):])
+		newHdr := reqHdr[lenPrefix:]
 		for _, val := range vals {
 			respHdr.Add(newHdr, val)
 		}
@@ -129,6 +130,7 @@ func main() {
 		hostname = "unknown"
 	}
 
+	lenPrefix = len(*setPrefix)
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", handle)
 	logger.Println("listening at:", *address)