From 04f3ad3a1c19c0a19ffdf1c8a508a976f4ea437c Mon Sep 17 00:00:00 2001
From: Daniel Rocha <daniel@rocha.io>
Date: Sun, 25 Mar 2018 19:09:26 +0200
Subject: [PATCH] Initial commit

---
 htpdate.go | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 htpdate.go

diff --git a/htpdate.go b/htpdate.go
new file mode 100644
index 0000000..795fd68
--- /dev/null
+++ b/htpdate.go
@@ -0,0 +1,48 @@
+package main
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"log"
+	"net/http"
+	"time"
+)
+
+func timeDiff(url string) (float64, float64, error) {
+	t0 := time.Now()
+	resp, err := http.Head(url)
+	delta := time.Since(t0).Seconds()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	date := resp.Header.Get("Date")
+	if date == "" {
+		return 0, 0, errors.New("Date header is missing")
+	}
+	t1, err := time.Parse(time.RFC1123, date)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	theta := t1.Sub(t0).Seconds() + 0.5 - delta/2
+	return theta, delta, nil
+}
+
+func main() {
+	n := flag.Uint("n", 10, "Number of requests")
+	h := flag.String("u", "https://google.com", "Host URL")
+	flag.Parse()
+
+	var sumT, sumD float64
+	for i := uint(0); i < *n; i++ {
+		theta, delta, err := timeDiff(*h)
+		if err != nil {
+			log.Fatal(err)
+		}
+		sumT += theta / delta
+		sumD += 1 / delta
+	}
+	fmt.Printf("offset %.5f sec\n", sumT/sumD)
+}