MicrOBU/app/src/test/java/com/hawhamburg/micr0bu/RecvV2xMessageTest.kt

package com.hawhamburg.micr0bu

import com.hawhamburg.micr0bu.data.mqtt.RecvV2xMessage
import com.hawhamburg.micr0bu.domain.asn1.CamUperCodec
import org.junit.Assert.assertEquals
import org.junit.Assert.assertNotNull
import org.junit.Assert.assertNull
import org.junit.Assert.assertTrue
import org.junit.Test

/**
 * Pins [RecvV2xMessage] to the protobuf wire format of consider it's `RecvV2XMessage`
 * (`v2x_interface.proto`, V2X RX protocol v2.4.2), the envelope the CiT One publishes on its raw
 * `v2x/rx` topics.
 *
 * ## Where the fixtures come from
 * The envelope bytes are written out here by hand from the protobuf encoding rules and the field
 * numbers in that `.proto`, with the derivation in the comments, so a reviewer can check them
 * without running anything. They are deliberately **not** produced by an encoder in this
 * repository: a fixture generated by our own code would agree with our own reader no matter how
 * wrong both were, which is exactly the failure mode the ASN.1 work in this project ran into
 * three times.
 *
 * The CAM payload inside is the golden UPER frame from [CamEncodeGoldenTest], itself verified
 * against `asn1tools` and the real ETSI modules in `asn1/`.
 *
 * ## Why this matters
 * Field numbers are wire-format constants with no self-describing names on the wire. Reading
 * field 2 where the schema says field 3 does not fail loudly, it silently yields a plausible
 * looking byte string that decodes to nothing. These tests are what should fail if the constants
 * in [RecvV2xMessage] are ever "tidied".
 */
class RecvV2xMessageTest {

    /**
     * The golden CAM UPER, 43 bytes, from [CamEncodeGoldenTest]. Its ItsPduHeader reads
     * protocolVersion 2, messageID 2 (CAM), stationID 0x000f423f = 999999.
     */
    private val goldenCam =
        "0202000f423f3700402ab215af6e286477dffffffc23b7743e0027ffc0d0fe0118329337feebfff6000000"

    /**
     * A complete `RecvV2XMessage` carrying [goldenCam], byte by byte:
     *
     * ```
     * 0a 05        field 1 (btpHeader), length-delimited, 5 bytes
     *   08 02        field 1 (type) varint = 2, CAM
     *   10 d1 0f     field 2 (destinationPort) varint = 2001
     * 12 07        field 2 (gnHeader), length-delimited, 7 bytes
     *   42 05        field 8 (dest), length-delimited, 5 bytes
     *     0a 03        field 1 (area), length-delimited, 3 bytes
     *       18 f4 03     field 3 (distA) varint = 500 metres
     * 1a 2b        field 3 (payload), length-delimited, 0x2b = 43 bytes
     * ```
     */
    private val camEnvelope = "0a05080210d10f120742050a0318f4031a2b" + goldenCam

    private fun String.hexToBytes(): ByteArray =
        chunked(2).map { it.toInt(16).toByte() }.toByteArray()

    // ---- the happy path --------------------------------------------------------------------

    @Test
    fun `parses btp header, geo radius and payload from a full envelope`() {
        val msg = RecvV2xMessage.parse(camEnvelope.hexToBytes())
        assertNotNull("envelope should parse", msg)
        msg!!

        assertEquals("btpHeader.type: CAM", 2, msg.pduType)
        assertEquals("btpHeader.destinationPort", 2001, msg.destinationPort)
        assertEquals("gnHeader.dest.area.distA, metres", 500, msg.destAreaRadiusM)
        assertTrue(
            "payload must be the CAM UPER byte for byte",
            msg.payload.contentEquals(goldenCam.hexToBytes()),
        )
    }

    @Test
    fun `extracted payload is decodable UPER, not a mangled copy`() {
        val msg = RecvV2xMessage.parse(camEnvelope.hexToBytes())!!
        // The whole point of carrying bytes rather than a String through the MQTT layer: a UTF-8
        // round trip would replace most of these bytes and this decode would fail.
        val cam = CamUperCodec.decode(msg.payload, receivedAtEpochMs = 1_787_100_000_000L)
        assertNotNull("payload should decode as a CAM", cam)
        assertEquals("stationID from the ItsPduHeader", 999_999L, cam!!.stationId)
    }

    @Test
    fun `reads a DENM envelope's relevance radius`() {
        // Same shape, DENM values: type 1, port 2002, distA 1000 m, a 2-byte stand-in payload.
        // 0a 05 08 01 10 d2 0f | 12 07 42 05 0a 03 18 e8 07 | 1a 02 02 01
        val msg = RecvV2xMessage.parse("0a05080110d20f120742050a0318e8071a020201".hexToBytes())
        assertNotNull(msg)
        assertEquals(1, msg!!.pduType)
        assertEquals(2002, msg.destinationPort)
        assertEquals(1000, msg.destAreaRadiusM)
    }

    // ---- forward compatibility -------------------------------------------------------------

    @Test
    fun `skips unknown fields and does not depend on field order`() {
        // payload first, then an unknown varint (field 7) and an unknown fixed32 (field 6) that
        // this schema revision does not define, then the btpHeader. Protobuf permits all three,
        // and a reader that assumed order or choked on unknowns would break the first time
        // consider it added a field.
        val bytes = ("1a2b" + goldenCam + "38b96035deadbeef0a05080210d10f").hexToBytes()
        val msg = RecvV2xMessage.parse(bytes)
        assertNotNull(msg)
        assertEquals(2, msg!!.pduType)
        assertEquals(2001, msg.destinationPort)
        assertTrue(msg.payload.contentEquals(goldenCam.hexToBytes()))
    }

    @Test
    fun `accepts an envelope carrying nothing but a payload`() {
        val msg = RecvV2xMessage.parse(("1a2b" + goldenCam).hexToBytes())
        assertNotNull(msg)
        assertNull("no btpHeader was sent", msg!!.pduType)
        assertNull("no gnHeader was sent", msg.destAreaRadiusM)
        assertTrue(msg.payload.contentEquals(goldenCam.hexToBytes()))
    }

    // ---- malformed input -------------------------------------------------------------------
    // These arrive off a network topic. A reader that throws takes the MQTT callback thread with
    // it, so every one of these must return null instead.

    @Test
    fun `returns null for a truncated envelope`() {
        val full = camEnvelope.hexToBytes()
        assertNull(RecvV2xMessage.parse(full.copyOfRange(0, full.size / 2)))
    }

    @Test
    fun `returns null when the payload field is present but empty`() {
        assertNull(RecvV2xMessage.parse("1a00".hexToBytes()))
    }

    @Test
    fun `returns null when there is no payload field at all`() {
        assertNull(RecvV2xMessage.parse("0a05080210d10f".hexToBytes()))
    }

    @Test
    fun `returns null for empty input and for bytes that are not protobuf`() {
        assertNull(RecvV2xMessage.parse(ByteArray(0)))
        // A run of continuation bytes: a varint that never terminates, which is what would walk
        // an unguarded reader off the end of the buffer.
        assertNull(RecvV2xMessage.parse(ByteArray(24) { 0xFF.toByte() }))
    }
}