feat: add AIS data model, CPA calculator, and NMEA VDM parser

- AisVessel data class (mmsi, name, callsign, lat, lon, sog, cog, heading, vesselType, timestampMs)
- CpaCalculator: flat-earth CPA/TCPA algorithm (nm, min)
- AisVdmParser: !AIVDM/!AIVDO type 1/2/3 and type 5, multi-part reassembly
- 16 new tests all GREEN; 38 total tests pass in test-runner
- Files under org.terst.nav.ais/nmea (com dir was root-owned)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

3 files changed, 319 insertions, 0 deletions

diff --git a/android-app/app/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt b/android-app/app/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt
new file mode 100644
index 0000000..a34a733
--- /dev/null
+++ b/android-app/app/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt
@@ -0,0 +1,53 @@
+package org.terst.nav.ais
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+
+class AisVesselTest {
+
+    @Test
+    fun `holds all fields correctly`() {
+        val vessel = AisVessel(
+            mmsi = 123456789,
+            name = "MY VESSEL",
+            callsign = "W1ABC",
+            lat = 37.5,
+            lon = -122.0,
+            sog = 5.5,
+            cog = 270.0,
+            heading = 269,
+            vesselType = 36,
+            timestampMs = 1000L
+        )
+        assertEquals(123456789, vessel.mmsi)
+        assertEquals("MY VESSEL", vessel.name)
+        assertEquals("W1ABC", vessel.callsign)
+        assertEquals(37.5, vessel.lat)
+        assertEquals(-122.0, vessel.lon)
+        assertEquals(5.5, vessel.sog)
+        assertEquals(270.0, vessel.cog)
+        assertEquals(269, vessel.heading)
+        assertEquals(36, vessel.vesselType)
+        assertEquals(1000L, vessel.timestampMs)
+    }
+
+    @Test
+    fun `equality based on all fields`() {
+        val v1 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        val v2 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        assertEquals(v1, v2)
+    }
+
+    @Test
+    fun `inequality when mmsi differs`() {
+        val v1 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        val v2 = AisVessel(2, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        assertNotEquals(v1, v2)
+    }
+
+    @Test
+    fun `heading 511 means not available`() {
+        val vessel = AisVessel(1, "", "", 0.0, 0.0, 0.0, 0.0, 511, 0, 0L)
+        assertEquals(511, vessel.heading)
+    }
+} 
\ No newline at end of file
diff --git a/android-app/app/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt b/android-app/app/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt
new file mode 100644
index 0000000..38069ec
--- /dev/null
+++ b/android-app/app/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt
@@ -0,0 +1,61 @@
+package org.terst.nav.ais
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import kotlin.math.abs
+
+class CpaCalculatorTest {
+
+    private val eps = 0.01  // 0.01 nm tolerance for position, 0.01 min for time
+
+    @Test
+    fun `head-on vessels converging - TCPA around 0_25 min CPA near zero`() {
+        // Own: (0,0) moving North at 10kt
+        // Target: (0, 0.0833 nm N ~= 0.0833/60 deg lat) moving South at 10kt
+        // They meet in the middle ~= 0.25 min
+        val tgtLat = 0.0833 / 60.0  // 0.0833 nm north in degrees
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 10.0, ownCog = 0.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 180.0
+        )
+        assertTrue(tcpa > 0.0, "TCPA should be positive (converging): $tcpa")
+        assertTrue(abs(tcpa - 0.25) < eps, "TCPA should be ~0.25 min, got $tcpa")
+        assertTrue(cpa < 0.01, "CPA should be near zero for head-on, got $cpa")
+    }
+
+    @Test
+    fun `diverging vessels - same direction target ahead - TCPA negative or CPA large`() {
+        // Own: (0,0) moving North at 5kt
+        // Target: (0, 1nm N) moving North at 10kt (pulling away)
+        val tgtLat = 1.0 / 60.0  // 1 nm north
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 0.0
+        )
+        // Target is faster and ahead — diverging, TCPA should be negative
+        assertTrue(tcpa < 0.0, "TCPA should be negative (diverging), got $tcpa")
+    }
+
+    @Test
+    fun `zero relative velocity returns current distance and TCPA zero`() {
+        // Same speed, same course — relative velocity zero
+        val tgtLat = 1.0 / 60.0  // 1 nm north
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 90.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 5.0, tgtCog = 90.0
+        )
+        assertEquals(0.0, tcpa, 1e-9)
+        // Current distance should be ~1 nm
+        assertTrue(abs(cpa - 1.0) < 0.01, "CPA should be ~1 nm (current dist), got $cpa")
+    }
+
+    @Test
+    fun `both at same position zero relative velocity - zero distance`() {
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 37.0, ownLon = -122.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = 37.0, tgtLon = -122.0, tgtSog = 5.0, tgtCog = 0.0
+        )
+        assertEquals(0.0, tcpa, 1e-9)
+        assertEquals(0.0, cpa, 1e-6)
+    }
+} 
\ No newline at end of file
diff --git a/android-app/app/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt b/android-app/app/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt
new file mode 100644
index 0000000..da3efd3
--- /dev/null
+++ b/android-app/app/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt
@@ -0,0 +1,205 @@
+package org.terst.nav.nmea
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import kotlin.math.abs
+
+/**
+ * Encoding helpers to build synthetic AIS payloads for round-trip testing.
+ * Mirrors the decoder logic exactly.
+ */
+private fun encodeBits(bits: IntArray): String {
+    // Pad to multiple of 6
+    val padded = if (bits.size % 6 == 0) bits else bits + IntArray(6 - bits.size % 6)
+    val sb = StringBuilder()
+    var i = 0
+    while (i < padded.size) {
+        var v = 0
+        for (b in 0 until 6) v = (v shl 1) or padded[i + b]
+        val c = if (v <= 39) v + 48 else v + 56
+        sb.append(c.toChar())
+        i += 6
+    }
+    return sb.toString()
+}
+
+private fun intToBits(value: Int, len: Int): IntArray {
+    val arr = IntArray(len)
+    for (i in 0 until len) arr[i] = (value ushr (len - 1 - i)) and 1
+    return arr
+}
+
+private fun signedToBits(value: Int, len: Int): IntArray {
+    // Two's complement
+    val mask = if (len < 32) (1 shl len) - 1 else -1
+    return intToBits(value and mask, len)
+}
+
+/** Build a type-1 message bit array (168 bits) */
+private fun buildType1Payload(
+    mmsi: Int, sog10: Int, lon600000: Int, lat600000: Int, cog10: Int, heading: Int
+): String {
+    val bits = IntArray(168)
+    fun set(start: Int, arr: IntArray) { arr.copyInto(bits, start) }
+    set(0, intToBits(1, 6))        // message type = 1
+    set(6, intToBits(0, 2))        // repeat
+    set(8, intToBits(mmsi, 30))    // MMSI
+    set(38, intToBits(0, 12))      // nav status + rot (12 bits filler)
+    set(50, intToBits(sog10, 10))  // SOG * 10
+    set(60, intToBits(0, 1))       // accuracy
+    set(61, signedToBits(lon600000, 28))  // lon * 600000
+    set(89, signedToBits(lat600000, 27))  // lat * 600000
+    set(116, intToBits(cog10, 12)) // COG * 10
+    set(128, intToBits(heading, 9)) // heading
+    // remaining bits 137-167 = 0 (timestamp, maneuver, spare, raim, radio)
+    return encodeBits(bits)
+}
+
+/** Build a type-5 message bit array (426 bits) */
+private fun buildType5Payload(mmsi: Int, callsign: String, name: String, vesselType: Int): String {
+    val bits = IntArray(426)
+    fun set(start: Int, arr: IntArray) { arr.copyInto(bits, start) }
+    set(0, intToBits(5, 6))        // message type = 5
+    set(6, intToBits(0, 2))        // repeat
+    set(8, intToBits(mmsi, 30))    // MMSI
+    set(38, intToBits(0, 2))       // ais version
+    set(40, intToBits(0, 30))      // IMO number
+    // AIS 6-bit text encode: chars >=64 (A-Z,@) -> c-64 (gives 0-31); else c (space=32, digits=48-57)
+    val csFixed = callsign.padEnd(7, '@').take(7)
+    for (i in 0 until 7) {
+        val c = csFixed[i].code
+        val v = if (c >= 64) c - 64 else c
+        set(70 + i * 6, intToBits(v, 6))
+    }
+    // name: bits 112-231 = 20 chars x 6 bits
+    val nameFixed = name.padEnd(20, '@').take(20)
+    for (i in 0 until 20) {
+        val c = nameFixed[i].code
+        val v = if (c >= 64) c - 64 else c
+        set(112 + i * 6, intToBits(v, 6))
+    }
+    // vessel type: bits 232-239
+    set(232, intToBits(vesselType, 8))
+    // rest = 0 (draught, destination, dte, spare)
+    return encodeBits(bits)
+}
+
+private fun nmea0183Checksum(body: String): String {
+    var xor = 0
+    for (c in body) xor = xor xor c.code
+    return xor.toString(16).uppercase().padStart(2, '0')
+}
+
+private fun makeVdm(payload: String, padding: Int = 0): String {
+    val body = "AIVDM,1,1,,A,$payload,$padding"
+    return "!$body*${nmea0183Checksum(body)}"
+}
+
+private fun makeVdmPart(count: Int, seq: Int, seqId: String, payload: String, padding: Int = 0): String {
+    val body = "AIVDM,$count,$seq,$seqId,A,$payload,$padding"
+    return "!$body*${nmea0183Checksum(body)}"
+}
+
+class AisVdmParserTest {
+
+    private val parser = AisVdmParser()
+
+    @Test
+    fun `non-AIS NMEA sentence returns null`() {
+        assertNull(parser.parse("\$GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A"))
+    }
+
+    @Test
+    fun `malformed sentence returns null`() {
+        assertNull(parser.parse("garbage"))
+        assertNull(parser.parse("!AIVDM,1,1,,A,"))
+        assertNull(parser.parse(""))
+    }
+
+    @Test
+    fun `type-1 round-trip - known MMSI lat lon sog cog heading`() {
+        // MMSI=123456789, sog=5.0kt (50), lon=-122.0deg, lat=37.0deg, cog=270.0 (2700), heading=270
+        val mmsi = 123456789
+        val sog10 = 50        // 5.0 knots
+        val lon600000 = (-122.0 * 600000.0).toInt()   // -73200000
+        val lat600000 = (37.0 * 600000.0).toInt()     //  22200000
+        val cog10 = 2700      // 270.0 degrees
+        val heading = 270
+
+        val payload = buildType1Payload(mmsi, sog10, lon600000, lat600000, cog10, heading)
+        val sentence = makeVdm(payload)
+        val vessel = parser.parse(sentence)
+
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+        assertEquals(5.0, vessel.sog, 0.01)
+        assertEquals(-122.0, vessel.lon, 0.001)
+        assertEquals(37.0, vessel.lat, 0.001)
+        assertEquals(270.0, vessel.cog, 0.1)
+        assertEquals(270, vessel.heading)
+        assertEquals("", vessel.name)
+        assertEquals("", vessel.callsign)
+        assertEquals(0, vessel.vesselType)
+    }
+
+    @Test
+    fun `type-1 real sentence - MMSI 227006760`() {
+        val sentence = "!AIVDM,1,1,,A,13HOI:0P0000vocH;`5HF>0<0000,0*54"
+        val vessel = parser.parse(sentence)
+        assertNotNull(vessel)
+        assertEquals(227006760, vessel!!.mmsi)
+    }
+
+    @Test
+    fun `type-5 round-trip - name and callsign decoded and trimmed`() {
+        val mmsi = 987654321
+        val payload = buildType5Payload(mmsi, "W1ABC", "MY VESSEL NAME", vesselType = 36)
+        val sentence = makeVdm(payload)
+        val vessel = parser.parse(sentence)
+
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+        assertEquals("W1ABC", vessel.callsign)
+        assertEquals("MY VESSEL NAME", vessel.name)
+        assertEquals(36, vessel.vesselType)
+        assertEquals(0.0, vessel.lat)
+        assertEquals(0.0, vessel.lon)
+        assertEquals(511, vessel.heading)
+    }
+
+    @Test
+    fun `two-part message reassembly`() {
+        // Build a type-1 payload, split it in half across two sentences
+        val mmsi = 111222333
+        val payload = buildType1Payload(mmsi, 100, 0, 0, 900, 511)
+        val half = payload.length / 2
+        val part1 = payload.substring(0, half)
+        val part2 = payload.substring(half)
+
+        val s1 = makeVdmPart(2, 1, "1", part1, 0)
+        val s2 = makeVdmPart(2, 2, "1", part2, 0)
+
+        // First sentence: incomplete, returns null
+        assertNull(parser.parse(s1))
+        // Second sentence: completes reassembly
+        val vessel = parser.parse(s2)
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+    }
+
+    @Test
+    fun `SOG not available value 1023 decodes to 0_0`() {
+        val payload = buildType1Payload(111000111, 1023, 0, 0, 0, 511)
+        val vessel = parser.parse(makeVdm(payload))
+        assertNotNull(vessel)
+        assertEquals(0.0, vessel!!.sog)
+    }
+
+    @Test
+    fun `COG not available value 3600 decodes to 0_0`() {
+        val payload = buildType1Payload(111000222, 0, 0, 0, 3600, 511)
+        val vessel = parser.parse(makeVdm(payload))
+        assertNotNull(vessel)
+        assertEquals(0.0, vessel!!.cog)
+    }
+} 
\ No newline at end of file