output/snapcast: calculate the latency for TIME responses

src/output/plugins/snapcast/Client.cxx

@@ -22,6 +22,7 @@
 #include "Timestamp.hxx"
 #include "Internal.hxx"
 #include "tag/RiffFormat.hxx"
+#include "event/Loop.hxx"
 #include "net/SocketError.hxx"
 #include "net/UniqueSocketDescriptor.hxx"
 #include "util/StringView.hxx"
@@ -146,7 +147,8 @@ SendTime(SocketDescriptor s, const PackedBE16 id,
 	 const SnapcastBase &request_header,
 	 const SnapcastTime &request_payload) noexcept
 {
-	SnapcastTime payload = request_payload; // TODO
+	SnapcastTime payload = request_payload;
+	payload.latency = request_header.received - request_header.sent;
 
 	SnapcastBase base{};
 	base.type = uint16_t(SnapcastMessageType::TIME);
@@ -196,12 +198,14 @@ SnapcastClient::SendWireChunk(ConstBuffer<void> payload,
 BufferedSocket::InputResult
 SnapcastClient::OnSocketInput(void *data, size_t length) noexcept
 {
-	const auto &base = *(const SnapcastBase *)data;
+	auto &base = *(SnapcastBase *)data;
 
 	if (length < sizeof(base) ||
 	    length < sizeof(base) + base.size)
 		return InputResult::MORE;
 
+	base.received = ToSnapcastTimestamp(GetEventLoop().SteadyNow());
+
 	ConsumeInput(sizeof(base) + base.size);
 
 	const ConstBuffer<void> payload{&base + 1, base.size};

src/output/plugins/snapcast/Protocol.hxx

@@ -35,6 +35,21 @@ enum class SnapcastMessageType : uint16_t {
 
 struct SnapcastTimestamp {
 	PackedLE32 sec, usec;
+
+	constexpr SnapcastTimestamp operator-(SnapcastTimestamp other) const noexcept {
+		const uint32_t a_sec = sec, a_usec = usec;
+		const uint32_t b_sec = other.sec, b_usec = other.usec;
+
+		uint32_t result_sec = a_sec - b_sec;
+		uint32_t result_usec = a_usec - b_usec;
+
+		if (a_usec < b_usec) {
+			--result_sec;
+			result_usec += 1'000'0000;
+		}
+
+		return {result_sec, result_usec};
+	}
 };
 
 struct SnapcastBase {