Fixed code smells

protocol/adsb/ra.go

@@ -0,0 +1,137 @@
+package adsb
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+// ResolutionAdvisory is an ACAS message providing information about ResolutionAdvisory
+//
+// Defined at 3.1.2.8.3.1 and 4.3.8.4.2.4
+type ResolutionAdvisory struct {
+	ActiveRA                ActiveResolutionAdvisory
+	RAComplement            RAComplement
+	RATerminatedIndicator   RATerminatedIndicator
+	MultipleThreatEncounter MultipleThreatEncounter
+	ThreatTypeIndicator     ThreatTypeIndicator
+	ThreatIdentityAddress   *ThreatIdentityAddress
+	ThreatIdentityAltitude  *ThreatIdentityAltitude
+	ThreatIdentityRange     *ThreatIdentityRange
+	ThreatIdentityBearing   *ThreatIdentityBearing
+}
+
+// ThreatTypeIndicator indicates the type of information contained in the TID part of the message
+type ThreatTypeIndicator int
+
+// ThreatIdentityAddress is a 3 bytes ICAO Address. The Most Significant Byte of the address
+// is always 0.
+type ThreatIdentityAddress uint32
+
+const (
+	ThreatTypeNoIdentity           ThreatTypeIndicator = iota // No identity data in TID
+	ThreatTypeModeS                                           // Contains a Mode S transponder address
+	ThreatTypeAltitudeRangeBearing                            // Contains altitude, range and bearing data
+	ThreatTypeReserved3                                       // Reserved
+)
+
+// ThreatIdentityAltitude is the altitude of the threat. It is given in 100 feet increment
+type ThreatIdentityAltitude struct {
+	AltitudeValid  bool
+	AltitudeInFeet int32
+}
+
+func parseThreatTypeIndicator(data []byte) ThreatTypeIndicator {
+	return ThreatTypeIndicator((data[2] & 0x0C) >> 2)
+}
+
+// parseThreatIdentityAltitude reads the altitude code from a message
+func parseThreatIdentityAltitude(data []byte) (ThreatIdentityAltitude, error) {
+
+	// Altitude code is a 13 bits fields, so read a uint16
+	// byte         data[2]    |        data[3]        |   data[4]
+	// bit       19 20 21 22 23|24 25 26 27 28 29 30 31|32 33 34 35 36
+	// value     _  _  _  C1 A1|C2 A2 C4 A4  0 B1 D1 B2|D2 B4 D4  _  _
+
+	// Start by D2 B4 D4
+	altitudeCode := uint16(data[4]&0xE0) >> 5
+	// Then pack B1 D1 B2
+	altitudeCode += uint16(data[3]&0x07) << 3
+	// Then C2 A2 C4 A4
+	altitudeCode += uint16(data[3]&0xF0) << 2
+	// Then C1 A1
+	altitudeCode += uint16(data[2]&0x03) << 2
+
+	// Detect invalid altitude
+	if altitudeCode == 0 {
+		return ThreatIdentityAltitude{}, nil
+	}
+
+	c1 := (altitudeCode & 0x0800) != 0
+	a1 := (altitudeCode & 0x0400) != 0
+	c2 := (altitudeCode & 0x0200) != 0
+	a2 := (altitudeCode & 0x0100) != 0
+	c4 := (altitudeCode & 0x0080) != 0
+	a4 := (altitudeCode & 0x0040) != 0
+	b1 := (altitudeCode & 0x0020) != 0
+	d1 := (altitudeCode & 0x0010) != 0
+	b2 := (altitudeCode & 0x0008) != 0
+	d2 := (altitudeCode & 0x0004) != 0
+	b4 := (altitudeCode & 0x0002) != 0
+	d4 := (altitudeCode & 0x0001) != 0
+
+	altitudeFeet, err := gillhamToAltitude(d1, d2, d4, a1, a2, a4, b1, b2, b4, c1, c2, c4)
+	if err != nil {
+		return ThreatIdentityAltitude{}, errors.New("adsb: the altitude field is malformed")
+	}
+
+	return ThreatIdentityAltitude{true, altitudeFeet}, nil
+}
+
+// ThreatIdentityRange is TIDR (threat identity data range subfield). This 7-bit subfield (76-82) shall contain the
+// most recent threat range estimated by ACAS.
+type ThreatIdentityRange byte
+
+func parseThreatIdentityRange(data []byte) ThreatIdentityRange {
+	return ThreatIdentityRange((data[4]&0x1F)<<2 + (data[5]&0xA0)>>6)
+}
+
+// ThreatIdentityBearing is TIDR (threat identity data bearing subfield). This 6-bit subfield (83-88) shall contain the
+// most recent estimated bearing of the threat aircraft, relative to the ACAS aircraft heading.
+type ThreatIdentityBearing byte
+
+func parseThreatIdentityBearing(data []byte) ThreatIdentityBearing {
+	return ThreatIdentityBearing(data[5] & 0x3F)
+}
+
+func parseResolutionAdvisory(data []byte) (*ResolutionAdvisory, error) {
+	if len(data) != 6 {
+		return nil, errors.New("adsb: data for ACAS ResolutionAdvisory must be 6 bytes long")
+	}
+
+	// Format of the message is as follows:
+	//        0                 1                 2                 3                 4                 5
+	//                 |             RAC |  R  R M  T  TID |       TID       |       TID       |       TID       |
+	//       ARA       |   ARA       RAC |  A  A T  T  d d | d d d d d d d d | d d d d d d d d | d d d d d d _ _ |
+	//                 |             RAC |  C  T E  I  TIDA|       TIDA      | TIDA    TIDR    |TIDR     TIDB    |
+	// a a a a a a a a | a a a a a a c c | c c t m i i a a | a a a a a a a a | a a a r r r r r | r r b b b b b b |
+
+	ra := new(ResolutionAdvisory)
+	switch ra.ThreatTypeIndicator = parseThreatTypeIndicator(data); ra.ThreatTypeIndicator {
+	case ThreatTypeModeS:
+		addr := ThreatIdentityAddress((binary.BigEndian.Uint32(data[2:]) >> 2) & 0xFFFFFF)
+		ra.ThreatIdentityAddress = &addr
+
+	case ThreatTypeAltitudeRangeBearing:
+		altitude, err := parseThreatIdentityAltitude(data)
+		if err != nil {
+			return nil, err
+		}
+		ra.ThreatIdentityAltitude = &altitude
+
+		threatRange := parseThreatIdentityRange(data)
+		ra.ThreatIdentityRange = &threatRange
+
+		bearing := parseThreatIdentityBearing(data)
+		ra.ThreatIdentityBearing = &bearing
+	}
+}