Revert "Don't use modules"

This reverts commit bffe99aead.

internal/tai64n/tai64n.go

@@ -0,0 +1,26 @@
+package tai64n
+
+import (
+	"bytes"
+	"encoding/binary"
+	"time"
+)
+
+const TimestampSize = 12
+const base = uint64(4611686018427387914)
+
+type Timestamp [TimestampSize]byte
+
+func Now() Timestamp {
+	var tai64n Timestamp
+	now := time.Now()
+	secs := base + uint64(now.Unix())
+	nano := uint32(now.UnixNano())
+	binary.BigEndian.PutUint64(tai64n[:], secs)
+	binary.BigEndian.PutUint32(tai64n[8:], nano)
+	return tai64n
+}
+
+func (t1 Timestamp) After(t2 Timestamp) bool {
+	return bytes.Compare(t1[:], t2[:]) > 0
+}

internal/tai64n/tai64n_test.go

@@ -0,0 +1,21 @@
+package tai64n
+
+import (
+	"testing"
+	"time"
+)
+
+/* Testing the essential property of the timestamp
+ * as used by WireGuard.
+ */
+func TestMonotonic(t *testing.T) {
+	old := Now()
+	for i := 0; i < 10000; i++ {
+		time.Sleep(time.Nanosecond)
+		next := Now()
+		if !next.After(old) {
+			t.Error("TAI64N, not monotonically increasing on nano-second scale")
+		}
+		old = next
+	}
+}