message: add support for labels + tests

Labels are strings associated with messages, which can be used for searching
them.

lib/message/mu-labels.hh

@@ -0,0 +1,88 @@
+/*
+** Copyright (C) 2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+#ifndef MU_LABELS_HH
+#define MU_LABELS_HH
+
+#include <utils/mu-result.hh>
+
+#include <string>
+#include <vector>
+#include <utility>
+
+namespace Mu {
+namespace Labels {
+
+using LabelVec = std::vector<std::string>;
+
+enum struct Delta { Add='+', Remove='-'};
+using DeltaLabel = std::pair<Delta, std::string>;
+using DeltaLabelVec = std::vector<DeltaLabel>;
+
+/**
+ * Parse a label expression, i.e., a label prefixed with '+' or '-'
+ *
+ * This also validates the label, as per valid_label()
+ *
+ * @param expr expression
+ *
+ * @return a result with either a DeltaLabel or an error
+ */
+Result<DeltaLabel> parse_delta_label(const std::string& expr);
+
+/**
+ * Is the label (without +/- prefix) valid?
+ *
+ * @param label some label
+ *
+ * @return either Ok or some error
+ */
+Result<void> validate_label(const std::string &label);
+
+/**
+ * Apply deltas to labels and return the result as well as the
+ * effective changes.
+ *
+ * The deltas are handled in order; 'last one wins', hence:
+ *      { +foo, -foo }    ==> no foo in the result
+ *  and
+ *      { -foo, +foo }   ==> foo in results
+ *
+ * The result labels do not contain duplicates. Order is not necessarily
+ * maintained.
+ *
+ * The result is a pair, the first element is LabelVec with the results
+ * as explained.
+ *
+ * The second is a DeltaVec with the _effective_ changes; this the input
+ * DeltaVec but without any +<label> if label was already in labels; and without
+ * -<label> if <label> was not in labels.
+ *
+ * @param labels existing labels
+ * @param deltas deltas for these labels
+ *
+ * @return updated labels
+ */
+std::pair<LabelVec, DeltaLabelVec> updated_labels(const LabelVec& labels, const DeltaLabelVec& delta);
+
+} // Labels
+} // Mu
+
+
+#endif /*MU_LABELS_HH*/