package com.hawhamburg.micr0bu.data

import android.os.SystemClock
import android.util.Log
import com.hawhamburg.micr0bu.domain.asn1.ItsTime
import java.time.DateTimeException

/**
 * Puts the timestamps this phone transmits on GNSS time instead of its own wall clock.
 *
 * ## Why
 * Every CAM carries a generationDeltaTime and every GeoNetworking header a TST, and receivers use
 * them to judge how fresh a message is and in what order messages came. Both used to come straight
 * from `System.currentTimeMillis()`, so they were only as good as the phone's clock setting. On
 * 2026-09-10 the bench phone was 24 minutes fast: automatic time had no source (no SIM, and the
 * lab Wi-Fi has no internet time), so it had not set the clock once in 69 hours, and every CAM
 * went out stamped 24 minutes in the future. A bike-mounted phone on the road is in exactly that
 * position. GNSS time depends on none of it.
 *
 * ## How
 * [SystemClock.currentGnssTimeClock] (API 29, this app's minSdk) is a UTC clock the platform keeps
 * synchronised from GNSS fixes. One reading of it taken alongside the wall clock gives the wall
 * clock's error, which is then applied to the fix's own timestamp. When GNSS time is unavailable,
 * typically indoors before any satellite fix since boot, the wall clock is used unchanged.
 *
 * Which clock is in use is logged whenever it changes, with the measured error, so a capture shows
 * where a given run's timestamps came from.
 *
 * Only the transmit path uses this. Everything else in the app stays on the wall clock, because
 * received messages, sensor samples and trip records are all stamped with it and must stay
 * comparable with one another.
 */
object GnssTimeSource {
    private const val TAG = "GnssTimeSource"

    /** Whether the last correction used GNSS time; null before the first. For change-only logging. */
    @Volatile private var lastUsedGnss: Boolean? = null

    /** [systemMs], a wall-clock reading, moved onto GNSS time where GNSS time is available. */
    fun correct(systemMs: Long): Long {
        val systemNow = System.currentTimeMillis()
        val gnssNow = try {
            SystemClock.currentGnssTimeClock().millis()
        } catch (e: DateTimeException) {
            null
        }
        noteSource(gnssNow, systemNow)
        return ItsTime.onGnssTime(systemMs, gnssNow, systemNow)
    }

    private fun noteSource(gnssNow: Long?, systemNow: Long) {
        val usingGnss = gnssNow != null
        if (lastUsedGnss == usingGnss) return
        lastUsedGnss = usingGnss
        if (gnssNow != null) {
            Log.i(TAG, "transmit timestamps now on GNSS time; phone clock is " +
                "${"%+.1f".format((systemNow - gnssNow) / 1000.0)} s off")
        } else {
            Log.w(TAG, "GNSS time unavailable, transmit timestamps fall back to the phone clock, " +
                "which has no automatic time source without a SIM or internet")
        }
    }
}