Clean up AIS lat/lon types and formatting.

firmware/application/ais_app.cpp

@@ -33,11 +33,34 @@ using namespace portapack;
 namespace ais {
 namespace format {
 
-static std::string latlon_normalized(const int32_t normalized) {
-	const int32_t t = (normalized * 5) / 3;
-	const int32_t degrees = t / (100 * 10000);
-	const int32_t fraction = std::abs(t) % (100 * 10000);
-	return to_string_dec_int(degrees) + "." + to_string_dec_int(fraction, 6, '0');
+static std::string latlon_abs_normalized(const int32_t normalized) {
+	const uint32_t normalized_abs = std::abs(normalized);
+	const uint32_t t = (normalized_abs * 5) / 3;
+	const uint32_t degrees = t / (100 * 10000);
+	const uint32_t fraction = t % (100 * 10000);
+	return to_string_dec_uint(degrees) + "." + to_string_dec_uint(fraction, 6, '0');
+}
+
+static std::string latitude(const Latitude value) {
+	if( value.is_not_available() ) {
+		return "not available";
+	} else if( value.is_valid() ) {
+		const auto normalized = value.normalized();
+		return latlon_abs_normalized(normalized) + ((normalized < 0) ? "S" : "N");
+	} else {
+		return "invalid";
+	}
+}
+
+static std::string longitude(const Longitude value) {
+	if( value.is_not_available() ) {
+		return "not available";
+	} else if( value.is_valid() ) {
+		const auto normalized = value.normalized();
+		return latlon_abs_normalized(normalized) + ((normalized < 0) ? "W" : "E");
+	} else {
+		return "invalid";
+	}
 }
 
 static std::string mmsi(
@@ -379,8 +402,8 @@ void AISRecentEntryDetailView::paint(Painter& painter) {
 	field_rect = draw_field(painter, field_rect, s, "Name", entry_.name);
 	field_rect = draw_field(painter, field_rect, s, "Call", entry_.call_sign);
 	field_rect = draw_field(painter, field_rect, s, "Dest", entry_.destination);
-	field_rect = draw_field(painter, field_rect, s, "Lat ", ais::format::latlon_normalized(entry_.last_position.latitude) + "N");
-	field_rect = draw_field(painter, field_rect, s, "Lon ", ais::format::latlon_normalized(entry_.last_position.longitude) + "E");
+	field_rect = draw_field(painter, field_rect, s, "Lat ", ais::format::latitude(entry_.last_position.latitude));
+	field_rect = draw_field(painter, field_rect, s, "Lon ", ais::format::longitude(entry_.last_position.longitude));
 	field_rect = draw_field(painter, field_rect, s, "Stat", ais::format::navigational_status(entry_.navigational_status));
 	field_rect = draw_field(painter, field_rect, s, "Rx #", to_string_dec_uint(entry_.received_count));
 	field_rect = draw_field(painter, field_rect, s, "RoT ", ais::format::rate_of_turn(entry_.last_position.rate_of_turn));

firmware/application/ais_app.hpp

@@ -40,8 +40,8 @@ using namespace lpc43xx;
 
 struct AISPosition {
 	rtc::RTC timestamp { };
-	ais::Latitude latitude { 0 };
-	ais::Longitude longitude { 0 };
+	ais::Latitude latitude;
+	ais::Longitude longitude;
 	ais::RateOfTurn rate_of_turn { -128 };
 	ais::SpeedOverGround speed_over_ground { 1023 };
 	ais::CourseOverGround course_over_ground { 3600 };

firmware/common/ais_packet.cpp

@@ -178,13 +178,13 @@ DateTime Packet::datetime(const size_t start_bit) const {
 Latitude Packet::latitude(const size_t start_bit) const {
 	// Shifting and dividing is to sign-extend the source field.
 	// TODO: There's probably a more elegant way to do it.
-	return static_cast<int32_t>(field_.read(start_bit, 27) << 5) / 32;
+	return field_.read(start_bit, 27);
 }
 
 Longitude Packet::longitude(const size_t start_bit) const {
 	// Shifting and dividing is to sign-extend the source field.
 	// TODO: There's probably a more elegant way to do it.
-	return static_cast<int32_t>(field_.read(start_bit, 28) << 4) / 16;
+	return field_.read(start_bit, 28);
 }
 
 bool Packet::crc_ok() const {

firmware/common/ais_packet.hpp

@@ -40,8 +40,54 @@ struct DateTime {
 	uint8_t second;
 };
 
-using Latitude = int32_t;
-using Longitude = int32_t;
+template<size_t FieldSize, int32_t DegMax, uint32_t NAValue>
+struct LatLonBase {
+	constexpr LatLonBase(
+	) : LatLonBase { raw_not_available }
+	{
+	}
+	
+	constexpr LatLonBase(
+		const int32_t raw
+	) : raw_ { raw }
+	{
+	}
+
+	constexpr LatLonBase(
+		const LatLonBase& other
+	) : raw_ { other.raw_ }
+	{
+	}
+
+	int32_t normalized() const {
+		return static_cast<int32_t>(raw() << sign_extend_shift) / (1 << sign_extend_shift);
+	}
+
+	int32_t raw() const {
+		return raw_;
+	}
+
+	bool is_not_available() const {
+		return raw() == raw_not_available;
+	}
+
+	bool is_valid() const {
+		return (normalized() >= raw_valid_min) && (normalized() <= raw_valid_max);
+	}
+
+private:
+	int32_t raw_;
+
+	static constexpr size_t sign_extend_shift = 32 - FieldSize;
+
+	static constexpr int32_t raw_not_available = NAValue;
+	static constexpr int32_t raw_valid_min = -DegMax * 60 * 10000;
+	static constexpr int32_t raw_valid_max =  DegMax * 60 * 10000;
+};
+
+using Latitude = LatLonBase<27, 90, 0x3412140>;
+using Longitude = LatLonBase<28, 180, 0x6791AC0>;
+
 using RateOfTurn = int8_t;
 using SpeedOverGround = uint16_t;
 using CourseOverGround = uint16_t;