Class tag support, add Adabelle

tag.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+type TagChange struct {
+	Start string
+	Value string
+}
+
+type TagChangeList []TagChange
+type TagKey string
+type TagMap map[TagKey]TagChangeList
+
+func getTagAt(key string, t time.Time) (string, error) {
+	// Guaranteed to be in local time
+	date := t.Format("2006-01-02")
+
+	tagChanges := tags[TagKey(key)]
+
+	if tagChanges == nil {
+		return "", fmt.Errorf("tag key not found: %s", key)
+	}
+
+	value := ""
+
+	for _, change := range tagChanges {
+		if strings.Compare(change.Start, date) > 0 {
+			break
+		}
+		value = change.Value
+	}
+
+	if value == "" {
+		return "", fmt.Errorf("tag key '%s' has no value yet", key)
+	}
+
+	return value, nil
+}