SongFilter: add ToExpression()

src/SongFilter.cxx

@@ -73,6 +73,27 @@ SongFilter::Item::Item(unsigned _tag,
 {
 }
 
+std::string
+SongFilter::Item::ToExpression() const noexcept
+{
+	switch (tag) {
+	case LOCATE_TAG_FILE_TYPE:
+		return "(" LOCATE_TAG_FILE_KEY " == \"" + value + "\")";
+
+	case LOCATE_TAG_BASE_TYPE:
+		return "(base \"" + value + "\")";
+
+	case LOCATE_TAG_MODIFIED_SINCE:
+		return "(modified-since \"" + value + "\")";
+
+	case LOCATE_TAG_ANY_TYPE:
+		return "(" LOCATE_TAG_ANY_KEY " == \"" + value + "\")";
+
+	default:
+		return std::string("(") + tag_item_names[tag] + " == \"" + value + "\")";
+	}
+}
+
 bool
 SongFilter::Item::StringMatchNN(const char *s) const noexcept
 {
@@ -177,6 +198,27 @@ SongFilter::~SongFilter()
 	/* this destructor exists here just so it won't get inlined */
 }
 
+std::string
+SongFilter::ToExpression() const noexcept
+{
+	auto i = items.begin();
+	const auto end = items.end();
+
+	if (std::next(i) == end)
+		return i->ToExpression();
+
+	std::string e("(");
+	e += i->ToExpression();
+
+	for (++i; i != end; ++i) {
+		e += " AND ";
+		e += i->ToExpression();
+	}
+
+	e.push_back(')');
+	return e;
+}
+
 static std::chrono::system_clock::time_point
 ParseTimeStamp(const char *s)
 {

src/SongFilter.hxx

@@ -70,6 +70,12 @@ public:
 		Item(unsigned tag, std::string &&_value, bool fold_case=false);
 		Item(unsigned tag, std::chrono::system_clock::time_point time);
 
+		/**
+		 * Convert this object into an "expression".  This is
+		 * only useful for debugging.
+		 */
+		std::string ToExpression() const noexcept;
+
 		unsigned GetTag() const {
 			return tag;
 		}
@@ -129,6 +135,12 @@ public:
 
 	~SongFilter();
 
+	/**
+	 * Convert this object into an "expression".  This is
+	 * only useful for debugging.
+	 */
+	std::string ToExpression() const noexcept;
+
 private:
 	gcc_nonnull(2,3)
 	void Parse(const char *tag, const char *value, bool fold_case=false);