Add Schedule Interface + Fixed Intervals scheduler

Signed-off-by: Sherif Abdel-Naby <sherifabdlnaby@gmail.com>

fixed.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+type Fixed struct {
+	duration time.Duration
+	next     time.Time
+}
+
+func NewFixed(duration time.Duration) (*Fixed, error) {
+	if duration < 0 {
+		return nil, fmt.Errorf("invalid duration, must be >= 0")
+	}
+	return &Fixed{
+		duration: duration,
+		next:     time.Now().Add(duration),
+	}, nil
+}
+
+func (f *Fixed) Next() time.Time {
+	now := time.Now()
+	if now.After(f.next) {
+		f.next = f.next.Add(f.duration)
+	}
+	return f.next
+}

schedule.go

@@ -0,0 +1,7 @@
+package main
+
+import "time"
+
+type Schedule interface {
+	Next() time.Time
+}