store: add support for modifying and listing labels and caching

Add methods update_labels, clear_labels which update or clear the labels for a
message in the store, and update the cache with the overall counts of labels.

Add a LabelsCache to keep track of the counts and labels_map() to retrieve that
map.

lib/mu-labels-cache.hh

@@ -0,0 +1,156 @@
+/*
+** 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_CACHE_HH
+#define MU_LABELS_CACHE_HH
+
+#include <set>
+#include <string>
+#include <unordered_map>
+
+#include "utils/mu-utils.hh"
+#include "message/mu-labels.hh"
+
+namespace Mu {
+
+/**
+ * The cache keeps track of what labels are being used. This can be used
+ * for completion etc. and `mu label list`
+ */
+class LabelsCache {
+public:
+	// maps a label to a number of occurrences
+	using Map = std::unordered_map<std::string, size_t>;
+	/**
+	 * CTOR
+	 *
+	 * Deserialize the map from a string
+	 *
+	 * @param serialized serialization string
+	 */
+	LabelsCache(const std::string serialized = {}): label_map_{deserialize(serialized)} {
+	}
+
+	/**
+	 * Construct a new ContactsCache object
+	 *
+	 * @param config db configuration database object
+	 */
+	LabelsCache(Config& config) {
+
+
+	}
+
+
+	/**
+	 * Add a label occurrence to the cache
+	 *
+	 * @param label
+	 */
+	void add(const std::string& label) {
+		if (auto it = label_map_.find(label); it == label_map_.end())
+			label_map_.insert({label, 1});
+		else
+			++it->second;
+	}
+	/**
+	 * Remove label occurrence from the cache
+	 *
+	 * @param label
+	 */
+	void remove(const std::string& label) {
+		if (auto it = label_map_.find(label); it != label_map_.end()) {
+			if (it->second == 1)
+				label_map_.erase(it);
+			else
+				--it->second;
+		}
+	}
+
+	/**
+	 * Update the cache with the the label changes
+	 *
+	 * @param updates a vector of delta-labels
+	 */
+	void update(const Labels::DeltaLabelVec& updates) {
+		for(const auto& [delta, label]: updates) {
+			switch(delta) {
+			case Labels::Delta::Add:
+				add(label);
+				break;
+			case Labels::Delta::Remove:
+				remove(label);
+				break;
+			}
+		}
+	}
+
+	/**
+	 * Return a copy of the label-map
+	 *
+	 * @return the label-map
+	 */
+	Map label_map() const { return label_map_; }
+
+
+	// serialization/deserialization could be optimized, but is not super
+	// time-critical
+
+	/**
+	 * Serialize the cache into a string.
+	 *
+	 * @return serialized cache
+	 */
+	std::string serialize() const {
+		std::string s;
+		for (const auto&[label, n]: label_map_)
+			s += mu_format("{}{}{}\n", label, SepaChar2, n);
+
+		return s;
+	}
+
+
+	/**
+	 * Deserialize the cache into a Map
+	 *
+	 * @return serialized cache
+	 */
+	Map deserialize(const std::string& serialized) const {
+
+		Map       map;
+		std::stringstream ss{serialized, std::ios_base::in};
+		std::string       line;
+
+		while (std::getline(ss, line)) {
+			if (const auto parts = Mu::split(line, SepaChar2); parts.size() != 2)
+				mu_warning("error: '{}'", line);
+			else
+				map.emplace(std::move(parts[0]),
+					    static_cast<std::size_t>(g_ascii_strtoll(parts[1].c_str(),{}, 10)));
+		}
+		return map;
+	}
+
+private:
+	Map label_map_;
+};
+
+} // namespace Mux
+#endif /*MU_LABELS_CACHE_HH*/