refactor: introduce concept of durations (resolve #261)

models/durations.go

@@ -0,0 +1,38 @@
+package models
+
+import "sort"
+
+type Durations []*Duration
+
+func (d Durations) Len() int {
+	return len(d)
+}
+
+func (d Durations) Less(i, j int) bool {
+	return d[i].Time.T().Before(d[j].Time.T())
+}
+
+func (d Durations) Swap(i, j int) {
+	d[i], d[j] = d[j], d[i]
+}
+
+func (d *Durations) Sorted() *Durations {
+	sort.Sort(d)
+	return d
+}
+
+func (d *Durations) First() *Duration {
+	// assumes slice to be sorted
+	if d.Len() == 0 {
+		return nil
+	}
+	return (*d)[0]
+}
+
+func (d *Durations) Last() *Duration {
+	// assumes slice to be sorted
+	if d.Len() == 0 {
+		return nil
+	}
+	return (*d)[d.Len()-1]
+}