Add timeattack detector

timeattack/detector.go

@@ -0,0 +1,36 @@
+package timeattack
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/9seconds/mtg/v2/mtglib"
+)
+
+type detector struct {
+	time.Duration
+}
+
+func (d detector) Valid(then time.Time) error {
+	now := time.Now()
+
+	diff := now.Sub(then)
+	if diff < 0 {
+		diff = -diff
+	}
+
+	if diff > d.Duration {
+		return fmt.Errorf("time is invalid. now=%d, then=%d, diff=%v",
+			now.Unix(),
+			then.Unix(),
+			diff)
+	}
+
+	return nil
+}
+
+func NewDetector(duration time.Duration) mtglib.TimeAttackDetector {
+	return detector{
+		Duration: duration,
+	}
+}