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

package com.hawhamburg.micr0bu

import com.hawhamburg.micr0bu.domain.asn1.CamUperCodec
import com.hawhamburg.micr0bu.domain.cam.Cam
import com.hawhamburg.micr0bu.domain.cam.StationType
import org.junit.Assert.assertEquals
import org.junit.Test

/**
 * Golden-byte test for the CAM this app transmits.
 *
 * ## Why a byte-for-byte fixture
 * This project has now shipped the same class of bug three times: a field encoded with the wrong
 * number of bits, which both ends of this codebase then read back with the *same* wrong number.
 * Phone and ESP32 agree perfectly with each other and with nothing else, so every internal test
 * passes while the frames on air are malformed. It cost a hardware session each time
 * (`CurvatureCalculationMode`, the GeoNetworking reserved bytes, and `yawRateConfidence`).
 *
 * A round-trip test through this codebase's own decoder cannot catch that - it shares the
 * mistake. Only an independent implementation can. So the expected bytes below were produced by
 * `asn1tools` compiled from the real ETSI modules in `asn1/`: it decoded this
 * encoder's output and re-encoded it, and the result was byte-identical to what is asserted here.
 * That is stronger than "it parses" - it means this encoder emits exactly what the reference
 * encoder emits.
 *
 * If a field width is ever "tidied up", this test fails. Do not regenerate the expected value from
 * this encoder's own output - regenerate it through asn1tools, or the test is worthless.
 */
class CamEncodeGoldenTest {

    /**
     * asn1tools-verified encoding of [referenceCam]. The trap this pins down: `YawRateConfidence`
     * has nine enumerands (0..8), so it needs 4 bits and `unavailable` is 8 - not 3 bits and 7.
     */
    private val expectedHex =
        "0202000f423f3700402ab215af6e286477dffffffc23b7743e0027ffc0d0fe0118329337feebfff6000000"

    private val referenceCam = Cam(
        stationId = 999_999L,
        stationType = StationType.CYCLIST,
        latitude = 53.5544955,
        longitude = 10.0225470,
        speedMps = 4.17,
        headingDeg = 63.9,
        yawRateDps = null,
        driveDirection = 0,
        vehicleLengthM = 1.8,
        vehicleWidthM = 0.7,
        accelerationMps2 = 0.4,
        timestamp = 1_787_100_000_000L,
        isOwn = true,
    )

    @Test
    fun `encodes a CAM exactly as the ETSI reference encoder does`() {
        val encoded = CamUperCodec.encode(referenceCam)
            .joinToString("") { "%02x".format(it) }
        assertEquals(expectedHex, encoded)
    }

    @Test
    fun `own decoder agrees with the encoder on every field it reads`() {
        // Self-consistency is necessary but NOT sufficient - see the class KDoc. This guards the
        // decoder against drifting away from the encoder, while the golden bytes above are what
        // guards both of them against drifting away from the standard.
        val round = CamUperCodec.decode(CamUperCodec.encode(referenceCam), referenceCam.timestamp)
        requireNotNull(round)
        assertEquals(referenceCam.stationId, round.stationId)
        assertEquals(referenceCam.stationType, round.stationType)
        assertEquals(referenceCam.latitude, round.latitude, 1e-7)
        assertEquals(referenceCam.longitude, round.longitude, 1e-7)
        assertEquals(referenceCam.speedMps, round.speedMps, 1e-9)
        assertEquals(referenceCam.headingDeg, round.headingDeg, 1e-9)
        assertEquals(referenceCam.vehicleLengthM!!, round.vehicleLengthM!!, 1e-9)
        assertEquals(referenceCam.vehicleWidthM!!, round.vehicleWidthM!!, 1e-9)
        assertEquals(referenceCam.accelerationMps2!!, round.accelerationMps2!!, 1e-9)
    }
}