1 files changed, 163 insertions, 0 deletions
diff --git a/chronos/chronos.go b/chronos/chronos.go
new file mode 100644
index 0000000..18bf775
--- /dev/null
+++ b/chronos/chronos.go
@@ -0,0 +1,163 @@
+package chronos
+
+import (
+	"math"
+	"time"
+)
+
+type TimeUnit int
+
+const (
+	Minute TimeUnit = iota
+	Hour
+	Day
+	Week
+	Month
+	Year
+)
+
+var NilTime time.Time
+
+func (tu TimeUnit) String() string {
+	switch tu {
+	case Minute:
+		return "Minute"
+	case Hour:
+		return "Hour"
+	case Day:
+		return "Day"
+	case Week:
+		return "Week"
+	case Month:
+		return "Month"
+	case Year:
+		return "Year"
+	default:
+		return "(Unknown TimeUnit)"
+	}
+}
+
+func (tu TimeUnit) minApprox() time.Duration {
+	const (
+		maMinute = time.Minute
+		maHour   = time.Hour
+		maDay    = 24*time.Hour - time.Second
+		maWeek   = 7 * maDay
+		maMonth  = 28 * maDay
+		maYear   = 365 * maDay
+	)
+
+	switch tu {
+	case Minute:
+		return maMinute
+	case Hour:
+		return maHour
+	case Day:
+		return maDay
+	case Week:
+		return maWeek
+	case Month:
+		return maMonth
+	case Year:
+		return maYear
+	default:
+		return 0
+	}
+}
+
+func (tu TimeUnit) maxApprox() time.Duration {
+	const (
+		maMinute = time.Minute
+		maHour   = time.Hour
+		maDay    = 24*time.Hour + time.Second
+		maWeek   = 7 * maDay
+		maMonth  = 31 * maDay
+		maYear   = 366 * maDay
+	)
+
+	switch tu {
+	case Minute:
+		return maMinute
+	case Hour:
+		return maHour
+	case Day:
+		return maDay
+	case Week:
+		return maWeek
+	case Month:
+		return maMonth
+	case Year:
+		return maYear
+	default:
+		return 0
+	}
+}
+
+type Frequency struct {
+	Unit  TimeUnit
+	Count uint
+}
+
+func (f Frequency) addTo(t time.Time, mul uint) time.Time {
+	sec := t.Second()
+	min := t.Minute()
+	hour := t.Hour()
+	day := t.Day()
+	month := t.Month()
+	year := t.Year()
+	loc := t.Location()
+
+	fq := int(f.Count * mul)
+
+	switch f.Unit {
+	case Minute:
+		return t.Add(time.Minute * time.Duration(fq))
+	case Hour:
+		return t.Add(time.Hour * time.Duration(fq))
+	case Day:
+		return time.Date(year, month, day+fq, hour, min, sec, 0, loc)
+	case Week:
+		return time.Date(year, month, day+fq*7, hour, min, sec, 0, loc)
+	case Month:
+		return time.Date(year, month+time.Month(fq), day, hour, min, sec, 0, loc)
+	case Year:
+		return time.Date(year+fq, month, day, hour, min, sec, 0, loc)
+	default:
+		return NilTime
+	}
+}
+
+func (f Frequency) minApprox() time.Duration { return time.Duration(f.Count) * f.Unit.minApprox() }
+func (f Frequency) maxApprox() time.Duration { return time.Duration(f.Count) * f.Unit.maxApprox() }
+
+type Chronos struct {
+	Start, End time.Time
+	Freq       Frequency
+}
+
+func (c Chronos) NextAfter(t time.Time) time.Time {
+	if !t.After(c.Start) {
+		return c.Start
+	}
+	if c.Freq.Count == 0 {
+		return NilTime
+	}
+
+	d := t.Sub(c.Start)
+
+	fmin := uint(math.Floor(float64(d) / float64(c.Freq.maxApprox())))
+	fmax := uint(math.Ceil(float64(d) / float64(c.Freq.minApprox())))
+
+	for f := fmin; f <= fmax; f++ {
+		t2 := c.Freq.addTo(c.Start, f)
+		if t2.Before(c.Start) || t2.Before(t) {
+			continue
+		}
+		if (!c.End.IsZero()) && t2.After(c.End) {
+			return NilTime
+		}
+		return t2
+	}
+
+	return NilTime // Should actually never happen...
+}