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, 189 insertions, 0 deletions

diff --git a/test-runner/src/main/kotlin/org/terst/nav/ais/AisVessel.kt b/test-runner/src/main/kotlin/org/terst/nav/ais/AisVessel.kt
new file mode 100644
index 0000000..34a951c
--- /dev/null
+++ b/test-runner/src/main/kotlin/org/terst/nav/ais/AisVessel.kt
@@ -0,0 +1,14 @@
+package org.terst.nav.ais
+
+data class AisVessel(
+    val mmsi: Int,
+    val name: String,
+    val callsign: String,
+    val lat: Double,
+    val lon: Double,
+    val sog: Double,
+    val cog: Double,
+    val heading: Int,
+    val vesselType: Int,
+    val timestampMs: Long
+)  
\ No newline at end of file
diff --git a/test-runner/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt b/test-runner/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt
new file mode 100644
index 0000000..7e98451
--- /dev/null
+++ b/test-runner/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt
@@ -0,0 +1,46 @@
+package org.terst.nav.ais
+
+import kotlin.math.cos
+import kotlin.math.sqrt
+import kotlin.math.PI
+
+object CpaCalculator {
+    fun compute(
+        ownLat: Double, ownLon: Double, ownSog: Double, ownCog: Double,
+        tgtLat: Double, tgtLon: Double, tgtSog: Double, tgtCog: Double
+    ): Pair<Double, Double> {
+        val refLat = Math.toRadians((ownLat + tgtLat) / 2.0)
+
+        // Flat-earth positions in nautical miles
+        val ownX = ownLon * 60.0 * cos(refLat)
+        val ownY = ownLat * 60.0
+        val tgtX = tgtLon * 60.0 * cos(refLat)
+        val tgtY = tgtLat * 60.0
+
+        // Velocities in nm/min
+        val ownCogRad = Math.toRadians(ownCog)
+        val tgtCogRad = Math.toRadians(tgtCog)
+        val ownVx = ownSog * Math.sin(ownCogRad) / 60.0
+        val ownVy = ownSog * Math.cos(ownCogRad) / 60.0
+        val tgtVx = tgtSog * Math.sin(tgtCogRad) / 60.0
+        val tgtVy = tgtSog * Math.cos(tgtCogRad) / 60.0
+
+        val dx = tgtX - ownX
+        val dy = tgtY - ownY
+        val dvx = tgtVx - ownVx
+        val dvy = tgtVy - ownVy
+
+        val dv2 = dvx * dvx + dvy * dvy
+        if (dv2 < 1e-9) {
+            val currentDist = sqrt(dx * dx + dy * dy)
+            return Pair(currentDist, 0.0)
+        }
+
+        val tcpa = -(dx * dvx + dy * dvy) / dv2
+        val cpaX = dx + dvx * tcpa
+        val cpaY = dy + dvy * tcpa
+        val cpa = sqrt(cpaX * cpaX + cpaY * cpaY)
+
+        return Pair(cpa, tcpa)
+    }
+}
diff --git a/test-runner/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt b/test-runner/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt
new file mode 100644
index 0000000..9709d21
--- /dev/null
+++ b/test-runner/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt
@@ -0,0 +1,129 @@
+package org.terst.nav.nmea
+
+import org.terst.nav.ais.AisVessel
+
+class AisVdmParser {
+    // Keyed by seqId -> list of (seq, payload)
+    private val fragments = mutableMapOf<String, MutableList<Pair<Int, String>>>()
+
+    fun parse(sentence: String): AisVessel? {
+        if (!sentence.startsWith("!AIVDM") && !sentence.startsWith("!AIVDO")) return null
+
+        val withoutBang = sentence.drop(1)
+        val starIdx = withoutBang.indexOf('*')
+        val body = if (starIdx >= 0) withoutBang.substring(0, starIdx) else withoutBang
+
+        val fields = body.split(",")
+        if (fields.size < 7) return null
+
+        val count = fields[1].toIntOrNull() ?: return null
+        val seq = fields[2].toIntOrNull() ?: return null
+        val seqId = fields[3]
+        val payload = fields[5]
+        val padding = fields[6].toIntOrNull() ?: 0
+
+        val combinedPayload: String
+        if (count <= 1) {
+            combinedPayload = payload
+        } else {
+            val list = fragments.getOrPut(seqId) { mutableListOf() }
+            list.add(seq to payload)
+            if (list.size < count) return null
+            fragments.remove(seqId)
+            combinedPayload = list.sortedBy { it.first }.joinToString("") { it.second }
+        }
+
+        val bits = decodeToBits(combinedPayload, padding)
+        if (bits.size < 6) return null
+
+        return when (extractUInt(bits, 0, 6)) {
+            1, 2, 3 -> parseType123(bits)
+            5 -> parseType5(bits)
+            else -> null
+        }
+    }
+
+    private fun decodeToBits(payload: String, padding: Int): IntArray {
+        val allBits = ArrayList<Int>(payload.length * 6)
+        for (c in payload) {
+            var v = c.code - 48
+            if (v > 39) v -= 8
+            for (b in 5 downTo 0) allBits.add((v ushr b) and 1)
+        }
+        return if (padding > 0 && padding < allBits.size)
+            allBits.dropLast(padding).toIntArray()
+        else
+            allBits.toIntArray()
+    }
+
+    private fun extractUInt(bits: IntArray, start: Int, len: Int): Int {
+        var v = 0
+        for (i in 0 until len) {
+            val bit = if (start + i < bits.size) bits[start + i] else 0
+            v = (v shl 1) or bit
+        }
+        return v
+    }
+
+    private fun extractInt(bits: IntArray, start: Int, len: Int): Int {
+        val unsigned = extractUInt(bits, start, len)
+        return if (len > 0 && (unsigned and (1 shl (len - 1))) != 0)
+            unsigned - (1 shl len)
+        else
+            unsigned
+    }
+
+    private fun parseType123(bits: IntArray): AisVessel? {
+        if (bits.size < 137) return null
+        val mmsi = extractUInt(bits, 8, 30)
+        val sogRaw = extractUInt(bits, 50, 10)
+        val sog = if (sogRaw == 1023) 0.0 else sogRaw / 10.0
+        val lonRaw = extractInt(bits, 61, 28)
+        val lon = if (lonRaw == 0x6791AC0) 0.0 else lonRaw / 600000.0
+        val latRaw = extractInt(bits, 89, 27)
+        val lat = if (latRaw == 0x3412140) 0.0 else latRaw / 600000.0
+        val cogRaw = extractUInt(bits, 116, 12)
+        val cog = if (cogRaw == 3600) 0.0 else cogRaw / 10.0
+        val heading = extractUInt(bits, 128, 9)
+        return AisVessel(
+            mmsi = mmsi,
+            name = "",
+            callsign = "",
+            lat = lat,
+            lon = lon,
+            sog = sog,
+            cog = cog,
+            heading = heading,
+            vesselType = 0,
+            timestampMs = System.currentTimeMillis()
+        )
+    }
+
+    private fun parseType5(bits: IntArray): AisVessel? {
+        if (bits.size < 240) return null
+        val mmsi = extractUInt(bits, 8, 30)
+        val callsign = decodeText(bits, 70, 7).trimEnd('@', ' ')
+        val name = decodeText(bits, 112, 20).trimEnd('@', ' ')
+        val vesselType = extractUInt(bits, 232, 8)
+        return AisVessel(
+            mmsi = mmsi,
+            name = name,
+            callsign = callsign,
+            lat = 0.0, lon = 0.0, sog = 0.0, cog = 0.0,
+            heading = 511,
+            vesselType = vesselType,
+            timestampMs = System.currentTimeMillis()
+        )
+    }
+
+    // AIS 6-bit text: bits<32 -> bits+64 (maps to @,A-Z,...), else bits (maps to space,digits,symbols)
+    private fun decodeText(bits: IntArray, start: Int, nChars: Int): String {
+        val sb = StringBuilder(nChars)
+        for (i in 0 until nChars) {
+            val v = extractUInt(bits, start + i * 6, 6)
+            val c = if (v < 32) v + 64 else v
+            sb.append(c.toChar())
+        }
+        return sb.toString()
+    }
+} 
\ No newline at end of file