1 files changed, 128 insertions, 0 deletions
diff --git a/operations/label_change.go b/operations/label_change.go
new file mode 100644
index 00000000..551b8be0
--- /dev/null
+++ b/operations/label_change.go
@@ -0,0 +1,128 @@
+package operations
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"sort"
+
+	"github.com/MichaelMure/git-bug/bug"
+)
+
+var _ bug.Operation = LabelChangeOperation{}
+
+// LabelChangeOperation define a Bug operation to add or remove labels
+type LabelChangeOperation struct {
+	bug.OpBase
+	Added   []bug.Label
+	Removed []bug.Label
+}
+
+// Apply apply the operation
+func (op LabelChangeOperation) Apply(snapshot bug.Snapshot) bug.Snapshot {
+	// Add in the set
+AddLoop:
+	for _, added := range op.Added {
+		for _, label := range snapshot.Labels {
+			if label == added {
+				// Already exist
+				continue AddLoop
+			}
+		}
+
+		snapshot.Labels = append(snapshot.Labels, added)
+	}
+
+	// Remove in the set
+	for _, removed := range op.Removed {
+		for i, label := range snapshot.Labels {
+			if label == removed {
+				snapshot.Labels[i] = snapshot.Labels[len(snapshot.Labels)-1]
+				snapshot.Labels = snapshot.Labels[:len(snapshot.Labels)-1]
+			}
+		}
+	}
+
+	// Sort
+	sort.Slice(snapshot.Labels, func(i, j int) bool {
+		return string(snapshot.Labels[i]) < string(snapshot.Labels[j])
+	})
+
+	return snapshot
+}
+
+func NewLabelChangeOperation(author bug.Person, added, removed []bug.Label) LabelChangeOperation {
+	return LabelChangeOperation{
+		OpBase:  bug.NewOpBase(bug.LabelChangeOp, author),
+		Added:   added,
+		Removed: removed,
+	}
+}
+
+// ChangeLabels is a convenience function to apply the operation
+func ChangeLabels(out io.Writer, b bug.Interface, author bug.Person, add, remove []string) error {
+	// TODO: return a channel of result (like MergeAll) instead of formatting the result for the upper layers
+	var added, removed []bug.Label
+
+	if out == nil {
+		out = ioutil.Discard
+	}
+
+	snap := b.Compile()
+
+	for _, str := range add {
+		label := bug.Label(str)
+
+		// check for duplicate
+		if labelExist(added, label) {
+			fmt.Fprintf(out, "label \"%s\" is a duplicate\n", str)
+			continue
+		}
+
+		// check that the label doesn't already exist
+		if labelExist(snap.Labels, label) {
+			fmt.Fprintf(out, "label \"%s\" is already set on this bug\n", str)
+			continue
+		}
+
+		added = append(added, label)
+	}
+
+	for _, str := range remove {
+		label := bug.Label(str)
+
+		// check for duplicate
+		if labelExist(removed, label) {
+			fmt.Fprintf(out, "label \"%s\" is a duplicate\n", str)
+			continue
+		}
+
+		// check that the label actually exist
+		if !labelExist(snap.Labels, label) {
+			fmt.Fprintf(out, "label \"%s\" doesn't exist on this bug\n", str)
+			continue
+		}
+
+		removed = append(removed, label)
+	}
+
+	if len(added) == 0 && len(removed) == 0 {
+		return fmt.Errorf("no label added or removed")
+	}
+
+	labelOp := NewLabelChangeOperation(author, added, removed)
+
+	b.Append(labelOp)
+
+	return nil
+}
+
+func labelExist(labels []bug.Label, label bug.Label) bool {
+	for _, l := range labels {
+		if l == label {
+			return true
+		}
+	}
+
+	return false
+}