This is the v1 version, had the v2 before.

backend/backendutil/flags.go

@@ -0,0 +1,73 @@
+package backendutil
+
+import (
+	"github.com/emersion/go-imap"
+)
+
+// UpdateFlags executes a flag operation on the flag set current.
+func UpdateFlags(current []string, op imap.FlagsOp, flags []string) []string {
+	// Don't modify contents of 'flags' slice.  Only modify 'current'.
+	// See https://github.com/golang/go/wiki/SliceTricks
+
+	// Re-use current's backing store
+	newFlags := current[:0]
+	switch op {
+	case imap.SetFlags:
+		hasRecent := false
+		// keep recent flag
+		for _, flag := range current {
+			if flag == imap.RecentFlag {
+				newFlags = append(newFlags, imap.RecentFlag)
+				hasRecent = true
+				break
+			}
+		}
+		// append new flags
+		for _, flag := range flags {
+			if flag == imap.RecentFlag {
+				// Make sure we don't add the recent flag multiple times.
+				if hasRecent {
+					// Already have the recent flag, skip.
+					continue
+				}
+				hasRecent = true
+			}
+			// append new flag
+			newFlags = append(newFlags, flag)
+		}
+	case imap.AddFlags:
+		// keep current flags
+		newFlags = current
+		// Only add new flag if it isn't already in current list.
+		for _, addFlag := range flags {
+			found := false
+			for _, flag := range current {
+				if addFlag == flag {
+					found = true
+					break
+				}
+			}
+			// new flag not found, add it.
+			if !found {
+				newFlags = append(newFlags, addFlag)
+			}
+		}
+	case imap.RemoveFlags:
+		// Filter current flags
+		for _, flag := range current {
+			remove := false
+			for _, removeFlag := range flags {
+				if removeFlag == flag {
+					remove = true
+				}
+			}
+			if !remove {
+				newFlags = append(newFlags, flag)
+			}
+		}
+	default:
+		// Unknown operation, return current flags unchanged
+		newFlags = current
+	}
+	return newFlags
+}